Merge branch 'master' of https://github.com/eugenp/tutorials into BAEL-3860

core-java-modules/core-java-14/README.md

@@ -7,3 +7,4 @@ This module contains articles about Java 14.
 - [Guide to the @Serial Annotation in Java 14](https://www.baeldung.com/java-14-serial-annotation)
 - [Java Text Blocks](https://www.baeldung.com/java-text-blocks)
 - [Pattern Matching for instanceof in Java 14](https://www.baeldung.com/java-pattern-matching-instanceof)
+- [Helpful NullPointerExceptions in Java 14](https://www.baeldung.com/java-14-nullpointerexception)

core-java-modules/core-java-14/src/main/java/com/baeldung/java14/helpfulnullpointerexceptions/HelpfulNullPointerException.java

@@ -0,0 +1,56 @@
+package com.baeldung.java14.helpfulnullpointerexceptions;
+
+public class HelpfulNullPointerException {
+
+    public static void main(String[] args) {
+        Employee employee = null;
+        employee.getName();
+    }
+
+    public String getEmployeeEmailAddress(Employee employee) {
+        String emailAddress = employee.getPersonalDetails().getEmailAddress().toLowerCase();
+        return emailAddress;
+    }
+
+    static class Employee {
+        String name;
+        PersonalDetails personalDetails;
+
+        public String getName() {
+            return name;
+        }
+
+        public void setName(String name) {
+            this.name = name;
+        }
+
+        public PersonalDetails getPersonalDetails() {
+            return personalDetails;
+        }
+
+        public void setPersonalDetails(PersonalDetails personalDetails) {
+            this.personalDetails = personalDetails;
+        }
+    }
+
+    static class PersonalDetails {
+        String emailAddress;
+        String phone;
+
+        public String getEmailAddress() {
+            return emailAddress;
+        }
+
+        public void setEmailAddress(String emailAddress) {
+            this.emailAddress = emailAddress;
+        }
+
+        public String getPhone() {
+            return phone;
+        }
+
+        public void setPhone(String phone) {
+            this.phone = phone;
+        }
+    }
+}

core-java-modules/core-java-14/src/test/java/com/baeldung/java14/helpfulnullpointerexceptions/HelpfulNullPointerExceptionUnitTest.java

@@ -0,0 +1,37 @@
+package com.baeldung.java14.helpfulnullpointerexceptions;
+
+import org.junit.Test;
+
+import static com.baeldung.java14.helpfulnullpointerexceptions.HelpfulNullPointerException.Employee;
+import static com.baeldung.java14.helpfulnullpointerexceptions.HelpfulNullPointerException.PersonalDetails;
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class HelpfulNullPointerExceptionUnitTest {
+
+    @Test (expected = NullPointerException.class)
+    public void givenAnEmptyPersonalDetails_whenEmailAddressIsAccessed_thenThrowNPE() {
+        var helpfulNPE = new HelpfulNullPointerException();
+
+        var employee = new Employee();
+        employee.setName("Eduard");
+        employee.setPersonalDetails(new PersonalDetails());
+        helpfulNPE.getEmployeeEmailAddress(employee);
+    }
+
+    @Test
+    public void givenCompletePersonalDetails_whenEmailAddressIsAccessed_thenSuccess() {
+        var helpfulNPE = new HelpfulNullPointerException();
+        var emailAddress = "eduard@gmx.com";
+
+        var employee = new Employee();
+        employee.setName("Eduard");
+
+        var personalDetails = new PersonalDetails();
+        personalDetails.setEmailAddress(emailAddress.toUpperCase());
+        personalDetails.setPhone("1234");
+        employee.setPersonalDetails(personalDetails);
+
+        assertThat(helpfulNPE.getEmployeeEmailAddress(employee)).isEqualTo(emailAddress);
+    }
+
+}

core-java-modules/core-java-8-2/README.md

@@ -4,7 +4,6 @@ This module contains articles about Java 8 core features
 
 ### Relevant Articles: 
 
-- [How to Delay Code Execution in Java](https://www.baeldung.com/java-delay-code-execution)
 - [Run a Java Application from the Command Line](https://www.baeldung.com/java-run-jar-with-arguments)
 - [Java 8 Stream skip() vs limit()](https://www.baeldung.com/java-stream-skip-vs-limit)
 - [Guide to Java BiFunction Interface](https://www.baeldung.com/java-bifunction-interface)

core-java-modules/core-java-9/README.md

@@ -8,5 +8,4 @@ This module contains articles about Java 9 core features
 - [Introduction to Chronicle Queue](https://www.baeldung.com/java-chronicle-queue)
 - [Iterate Through a Range of Dates in Java](https://www.baeldung.com/java-iterate-date-range)
 - [Initialize a HashMap in Java](https://www.baeldung.com/java-initialize-hashmap)
-- [Immutable Set in Java](https://www.baeldung.com/java-immutable-set)
 - [Immutable ArrayList in Java](https://www.baeldung.com/java-immutable-list)

core-java-modules/core-java-collections-2/README.md

@@ -12,4 +12,3 @@
 - [Sorting in Java](https://www.baeldung.com/java-sorting)
 - [Getting the Size of an Iterable in Java](https://www.baeldung.com/java-iterable-size)
 - [Java Null-Safe Streams from Collections](https://www.baeldung.com/java-null-safe-streams-from-collections)
-- [Operating on and Removing an Item from Stream](https://www.baeldung.com/java-use-remove-item-stream)

core-java-modules/core-java-collections-maps-2/README.md

@@ -0,0 +1,16 @@
+## Java Collections Cookbooks and Examples
+
+This module contains articles about Map data structures in Java.
+
+### Relevant Articles: 
+- [Map of Primitives in Java](https://www.baeldung.com/java-map-primitives)
+- [Copying a HashMap in Java](https://www.baeldung.com/java-copy-hashmap)
+- [A Guide to Java HashMap](https://www.baeldung.com/java-hashmap)
+- [Guide to WeakHashMap in Java](https://www.baeldung.com/java-weakhashmap)
+- [Map to String Conversion in Java](https://www.baeldung.com/java-map-to-string-conversion)
+- [Iterate over a Map in Java](https://www.baeldung.com/java-iterate-map)
+- [Merging Two Maps with Java 8](https://www.baeldung.com/java-merge-maps)
+- [Sort a HashMap in Java](https://www.baeldung.com/java-hashmap-sort)
+- [Finding the Highest Value in a Java Map](https://www.baeldung.com/java-find-map-max)
+- [Initialize a HashMap in Java](https://www.baeldung.com/java-initialize-hashmap)
+- More articles: [[<-- prev]](/core-java-modules/core-java-collections-maps) [[next -->]](/core-java-modules/core-java-collections-maps-3)

core-java-modules/core-java-collections-maps-2/pom.xml

@@ -0,0 +1,79 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>core-java-collections-maps-2</artifactId>
+    <version>0.1.0-SNAPSHOT</version>
+    <name>core-java-collections-maps-2</name>
+    <packaging>jar</packaging>
+
+    <parent>
+        <groupId>com.baeldung</groupId>
+        <artifactId>parent-java</artifactId>
+        <version>0.0.1-SNAPSHOT</version>
+        <relativePath>../../parent-java</relativePath>
+    </parent>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.eclipse.collections</groupId>
+            <artifactId>eclipse-collections</artifactId>
+            <version>${eclipse-collections.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>net.sf.trove4j</groupId>
+            <artifactId>trove4j</artifactId>
+            <version>${trove4j.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>it.unimi.dsi</groupId>
+            <artifactId>fastutil</artifactId>
+            <version>${fastutil.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>colt</groupId>
+            <artifactId>colt</artifactId>
+            <version>${colt.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.commons</groupId>
+            <artifactId>commons-lang3</artifactId>
+            <version>${commons-lang3.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.commons</groupId>
+            <artifactId>commons-collections4</artifactId>
+            <version>${commons-collections4.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>one.util</groupId>
+            <artifactId>streamex</artifactId>
+            <version>${streamex.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>com.jayway.awaitility</groupId>
+            <artifactId>awaitility</artifactId>
+            <version>${avaitility.version}</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.assertj</groupId>
+            <artifactId>assertj-core</artifactId>
+            <version>${assertj.version}</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
+    <properties>
+        <streamex.version>0.6.5</streamex.version>
+        <commons-collections4.version>4.1</commons-collections4.version>
+        <avaitility.version>1.7.0</avaitility.version>
+        <eclipse-collections.version>8.2.0</eclipse-collections.version>
+        <trove4j.version>3.0.2</trove4j.version>
+        <fastutil.version>8.1.0</fastutil.version>
+        <colt.version>1.2.0</colt.version>
+        <assertj.version>3.11.1</assertj.version>
+    </properties>
+
+</project>

core-java-modules/core-java-collections-maps-2/src/main/java/com/baeldung/map/Product.java

@@ -0,0 +1,133 @@
+package com.baeldung.map;
+
+import java.util.*;
+
+public class Product {
+
+    private String name;
+    private String description;
+    private List<String> tags;
+
+    public Product(String name, String description) {
+        this.name = name;
+        this.description = description;
+        this.tags = new ArrayList<>();
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public String getDescription() {
+        return description;
+    }
+
+    public List<String> getTags() {
+        return tags;
+    }
+
+    public Product addTagsOfOtherProdcut(Product product) {
+        this.tags.addAll(product.getTags());
+        return this;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+
+        if (this == o) {
+            return true;
+        }
+        if (o == null || getClass() != o.getClass()) {
+            return false;
+        }
+
+        Product product = (Product) o;
+        return Objects.equals(name, product.name) &&
+                Objects.equals(description, product.description);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(name, description);
+    }
+
+    public static void forEach() {
+
+        HashMap<String, Product> productsByName = new HashMap<>();
+        productsByName.forEach( (key, product)
+                        -> System.out.println("Key: " + key + " Product:" + product.getDescription())
+                //do something with the key and value
+        );
+
+        //Prior to Java 8:
+        for(Map.Entry<String, Product> entry : productsByName.entrySet()) {
+            Product product =  entry.getValue();
+            String key = entry.getKey();
+            //do something with the key and value
+        }
+    }
+
+    public static void getOrDefault() {
+
+        HashMap<String, Product> productsByName = new HashMap<>();
+        Product chocolate = new Product("chocolate", "something sweet");
+        Product defaultProduct = productsByName.getOrDefault("horse carriage", chocolate);
+        Product bike = productsByName.getOrDefault("E-Bike", chocolate);
+
+        //Prior to Java 8:
+        Product bike2 = productsByName.containsKey("E-Bike")
+                ? productsByName.get("E-Bike")
+                : chocolate;
+        Product defaultProduct2 = productsByName.containsKey("horse carriage")
+                ? productsByName.get("horse carriage")
+                : chocolate;
+    }
+
+    public static void putIfAbsent() {
+
+        HashMap<String, Product> productsByName = new HashMap<>();
+        Product chocolate = new Product("chocolate", "something sweet");
+        productsByName.putIfAbsent("E-Bike", chocolate);
+
+        //Prior to Java 8:
+        if(productsByName.containsKey("E-Bike")) {
+            productsByName.put("E-Bike", chocolate);
+        }
+    }
+
+    public static void merge() {
+
+        HashMap<String, Product> productsByName = new HashMap<>();
+        Product eBike2 = new Product("E-Bike", "A bike with a battery");
+        eBike2.getTags().add("sport");
+        productsByName.merge("E-Bike", eBike2, Product::addTagsOfOtherProdcut);
+
+        //Prior to Java 8:
+        if(productsByName.containsKey("E-Bike")) {
+            productsByName.get("E-Bike").addTagsOfOtherProdcut(eBike2);
+        } else {
+            productsByName.put("E-Bike", eBike2);
+        }
+    }
+
+    public static void compute() {
+
+        HashMap<String, Product> productsByName = new HashMap<>();
+        Product eBike2 = new Product("E-Bike", "A bike with a battery");
+
+        productsByName.compute("E-Bike", (k,v) -> {
+            if(v != null) {
+                return v.addTagsOfOtherProdcut(eBike2);
+            } else {
+                return eBike2;
+            }
+        });
+
+        //Prior to Java 8:
+        if(productsByName.containsKey("E-Bike")) {
+            productsByName.get("E-Bike").addTagsOfOtherProdcut(eBike2);
+        } else {
+            productsByName.put("E-Bike", eBike2);
+        }
+    }
+}

core-java-modules/core-java-collections-maps-2/src/main/java/com/baeldung/map/convert/MapToString.java

@@ -0,0 +1,34 @@
+package com.baeldung.map.convert;
+
+import com.google.common.base.Joiner;
+import org.apache.commons.lang3.StringUtils;
+
+import java.util.Map;
+import java.util.stream.Collectors;
+
+public class MapToString {
+
+    public static String convertWithIteration(Map<Integer, ?> map) {
+        StringBuilder mapAsString = new StringBuilder("{");
+        for (Integer key : map.keySet()) {
+            mapAsString.append(key + "=" + map.get(key) + ", ");
+        }
+        mapAsString.delete(mapAsString.length()-2, mapAsString.length()).append("}");
+        return mapAsString.toString();
+    }
+
+    public static String convertWithStream(Map<Integer, ?> map) {
+        String mapAsString = map.keySet().stream()
+                .map(key -> key + "=" + map.get(key))
+                .collect(Collectors.joining(", ", "{", "}"));
+        return mapAsString;
+    }
+
+    public static String convertWithGuava(Map<Integer, ?> map) {
+        return Joiner.on(",").withKeyValueSeparator("=").join(map);
+    }
+
+    public static String convertWithApache(Map map) {
+        return StringUtils.join(map);
+    }
+}

core-java-modules/core-java-collections-maps-2/src/main/java/com/baeldung/map/convert/StringToMap.java

@@ -0,0 +1,21 @@
+package com.baeldung.map.convert;
+
+import com.google.common.base.Splitter;
+
+import java.util.Arrays;
+import java.util.Map;
+import java.util.stream.Collectors;
+
+public class StringToMap {
+
+    public static Map<String, String> convertWithStream(String mapAsString) {
+        Map<String, String> map = Arrays.stream(mapAsString.split(","))
+                .map(entry -> entry.split("="))
+                .collect(Collectors.toMap(entry -> entry[0], entry -> entry[1]));
+        return map;
+    }
+
+    public static Map<String, String> convertWithGuava(String mapAsString) {
+        return Splitter.on(',').withKeyValueSeparator('=').split(mapAsString);
+    }
+}

core-java-modules/core-java-collections-maps-2/src/main/java/com/baeldung/map/copyhashmap/CopyHashMap.java

@@ -0,0 +1,55 @@
+package com.baeldung.map.copyhashmap;
+
+import org.apache.commons.lang3.SerializationUtils;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+public class CopyHashMap {
+    
+    public static <String, Employee> HashMap<String, Employee> copyUsingConstructor(HashMap<String, Employee> originalMap) {
+        return new HashMap<String, Employee>(originalMap);
+    }
+    
+    public static <String, Employee> HashMap<String, Employee> copyUsingClone(HashMap<String, Employee> originalMap) {
+        return (HashMap<String, Employee>) originalMap.clone();
+    }
+
+    public static <String, Employee> HashMap<String, Employee> copyUsingPut(HashMap<String, Employee> originalMap) {
+        HashMap<String, Employee> shallowCopy = new HashMap<String, Employee>();
+        Set<Entry<String, Employee>> entries = originalMap.entrySet();
+        for(Map.Entry<String, Employee> mapEntry: entries) {
+            shallowCopy.put(mapEntry.getKey(), mapEntry.getValue());
+        }
+        
+        return shallowCopy;
+    }
+    
+    public static <String, Employee> HashMap<String, Employee> copyUsingPutAll(HashMap<String, Employee> originalMap) {
+        HashMap<String, Employee> shallowCopy = new HashMap<String, Employee>();
+        shallowCopy.putAll(originalMap);
+        
+        return shallowCopy;
+    }
+    
+    public static <String, Employee> HashMap<String, Employee> copyUsingJava8Stream(HashMap<String, Employee> originalMap) {
+        Set<Entry<String, Employee>> entries = originalMap.entrySet();
+        HashMap<String, Employee> shallowCopy = (HashMap<String, Employee>) entries
+            .stream()
+            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
+        
+        return shallowCopy;
+    }
+    
+    public static <String, Employee> HashMap<String, Employee> shallowCopy(HashMap<String, Employee> originalMap) {
+        return (HashMap<String, Employee>) originalMap.clone();
+    }
+    
+    public static <String, Employee> HashMap<String, Employee> deepCopy(HashMap<String, Employee> originalMap) {
+        return SerializationUtils.clone(originalMap);
+    }
+    
+}

core-java-modules/core-java-collections-maps-2/src/main/java/com/baeldung/map/initialize/MapInitializer.java

@@ -0,0 +1,80 @@
+package com.baeldung.map.initialize;
+
+import java.util.AbstractMap;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+public class MapInitializer {
+
+    public static Map<String, String> articleMapOne;
+    static {
+        articleMapOne = new HashMap<>();
+        articleMapOne.put("ar01", "Intro to Map");
+        articleMapOne.put("ar02", "Some article");
+    }
+
+    public static Map<String, String> createSingletonMap() {
+        Map<String, String> passwordMap = Collections.singletonMap("username1", "password1");
+        return passwordMap;
+
+    }
+
+    public Map<String, String> createEmptyMap() {
+        Map<String, String> emptyMap = Collections.emptyMap();
+        return emptyMap;
+    }
+
+    public Map<String, String> createUsingDoubleBrace() {
+        Map<String, String> doubleBraceMap = new HashMap<String, String>() {
+
+            /**
+             * 
+             */
+            private static final long serialVersionUID = 1L;
+
+            {
+                put("key1", "value1");
+                put("key2", "value2");
+            }
+        };
+        return doubleBraceMap;
+    }
+
+    public Map<String, String> createMapUsingStreamStringArray() {
+        Map<String, String> map = Stream.of(new String[][] { { "Hello", "World" }, { "John", "Doe" }, })
+            .collect(Collectors.toMap(data -> data[0], data -> data[1]));
+
+        return map;
+    }
+
+    public Map<String, Integer> createMapUsingStreamObjectArray() {
+        Map<String, Integer> map = Stream.of(new Object[][] { { "data1", 1 }, { "data2", 2 }, })
+            .collect(Collectors.toMap(data -> (String) data[0], data -> (Integer) data[1]));
+
+        return map;
+    }
+
+    public Map<String, Integer> createMapUsingStreamSimpleEntry() {
+        Map<String, Integer> map = Stream.of(new AbstractMap.SimpleEntry<>("idea", 1), new AbstractMap.SimpleEntry<>("mobile", 2))
+            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
+
+        return map;
+    }
+
+    public Map<String, Integer> createMapUsingStreamSimpleImmutableEntry() {
+        Map<String, Integer> map = Stream.of(new AbstractMap.SimpleImmutableEntry<>("idea", 1), new AbstractMap.SimpleImmutableEntry<>("mobile", 2))
+            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
+
+        return map;
+    }
+
+    public Map<String, String> createImmutableMapWithStreams() {
+        Map<String, String> map = Stream.of(new String[][] { { "Hello", "World" }, { "John", "Doe" }, })
+            .collect(Collectors.collectingAndThen(Collectors.toMap(data -> data[0], data -> data[1]), Collections::<String, String> unmodifiableMap));
+        return map;
+
+    }
+}

core-java-modules/core-java-collections-maps-2/src/main/java/com/baeldung/map/iteration/MapIteration.java

@@ -0,0 +1,74 @@
+package com.baeldung.map.iteration;
+
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.Map;
+
+public class MapIteration {
+
+    public static void main(String[] args) {
+        MapIteration mapIteration = new MapIteration();
+        Map<String, Integer> map = new HashMap<>();
+
+        map.put("One", 1);
+        map.put("Three", 3);
+        map.put("Two", 2);
+
+        System.out.println("Iterating Keys of Map Using KeySet");
+        mapIteration.iterateKeys(map);
+
+        System.out.println("Iterating Map Using Entry Set");
+        mapIteration.iterateUsingEntrySet(map);
+
+        System.out.println("Iterating Using Iterator and Map Entry");
+        mapIteration.iterateUsingIteratorAndEntry(map);
+
+        System.out.println("Iterating Using KeySet and For Each");
+        mapIteration.iterateUsingKeySetAndForeach(map);
+
+        System.out.println("Iterating Map Using Lambda Expression");
+        mapIteration.iterateUsingLambda(map);
+
+        System.out.println("Iterating Using Stream API");
+        mapIteration.iterateUsingStreamAPI(map);
+    }
+
+    public void iterateUsingEntrySet(Map<String, Integer> map) {
+        for (Map.Entry<String, Integer> entry : map.entrySet()) {
+            System.out.println(entry.getKey() + ":" + entry.getValue());
+        }
+    }
+
+    public void iterateUsingLambda(Map<String, Integer> map) {
+        map.forEach((k, v) -> System.out.println((k + ":" + v)));
+    }
+
+    public void iterateUsingIteratorAndEntry(Map<String, Integer> map) {
+        Iterator<Map.Entry<String, Integer>> iterator = map.entrySet()
+            .iterator();
+        while (iterator.hasNext()) {
+            Map.Entry<String, Integer> pair = iterator.next();
+            System.out.println(pair.getKey() + ":" + pair.getValue());
+        }
+    }
+
+    public void iterateUsingKeySetAndForeach(Map<String, Integer> map) {
+        for (String key : map.keySet()) {
+            System.out.println(key + ":" + map.get(key));
+        }
+    }
+
+    public void iterateUsingStreamAPI(Map<String, Integer> map) {
+        map.entrySet()
+            .stream()
+            .forEach(e -> System.out.println(e.getKey() + ":" + e.getValue()));
+    }
+
+    public void iterateKeys(Map<String, Integer> map) {
+        for (String key : map.keySet()) {
+            System.out.println(key);
+        }
+
+    }
+
+}

core-java-modules/core-java-collections-maps-2/src/main/java/com/baeldung/map/mapmax/MapMax.java

@@ -0,0 +1,92 @@
+package com.baeldung.map.mapmax;
+
+import java.util.*;
+import java.util.Map.Entry;
+
+public class MapMax {
+
+    public <K, V extends Comparable<V>> V maxUsingIteration(Map<K, V> map) {
+
+        Map.Entry<K, V> maxEntry = null;
+
+        for (Map.Entry<K, V> entry : map.entrySet()) {
+
+            if (maxEntry == null || entry.getValue()
+                .compareTo(maxEntry.getValue()) > 0) {
+                maxEntry = entry;
+            }
+        }
+
+        return maxEntry.getValue();
+    }
+
+    public <K, V extends Comparable<V>> V maxUsingCollectionsMax(Map<K, V> map) {
+
+        Entry<K, V> maxEntry = Collections.max(map.entrySet(), new Comparator<Entry<K, V>>() {
+            public int compare(Entry<K, V> e1, Entry<K, V> e2) {
+                return e1.getValue()
+                    .compareTo(e2.getValue());
+            }
+        });
+
+        return maxEntry.getValue();
+    }
+
+    public <K, V extends Comparable<V>> V maxUsingCollectionsMaxAndLambda(Map<K, V> map) {
+
+        Entry<K, V> maxEntry = Collections.max(map.entrySet(), (Entry<K, V> e1, Entry<K, V> e2) -> e1.getValue()
+            .compareTo(e2.getValue()));
+
+        return maxEntry.getValue();
+    }
+
+    public <K, V extends Comparable<V>> V maxUsingCollectionsMaxAndMethodReference(Map<K, V> map) {
+
+        Entry<K, V> maxEntry = Collections.max(map.entrySet(), Comparator.comparing(Map.Entry::getValue));
+
+        return maxEntry.getValue();
+    }
+
+    public <K, V extends Comparable<V>> V maxUsingStreamAndLambda(Map<K, V> map) {
+
+        Optional<Entry<K, V>> maxEntry = map.entrySet()
+            .stream()
+            .max((Entry<K, V> e1, Entry<K, V> e2) -> e1.getValue()
+                .compareTo(e2.getValue()));
+
+        return maxEntry.get()
+            .getValue();
+    }
+
+    public <K, V extends Comparable<V>> V maxUsingStreamAndMethodReference(Map<K, V> map) {
+
+        Optional<Entry<K, V>> maxEntry = map.entrySet()
+            .stream()
+            .max(Comparator.comparing(Map.Entry::getValue));
+
+        return maxEntry.get()
+            .getValue();
+    }
+
+    public static void main(String[] args) {
+
+        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
+
+        map.put(1, 3);
+        map.put(2, 4);
+        map.put(3, 5);
+        map.put(4, 6);
+        map.put(5, 7);
+        
+        MapMax mapMax = new MapMax();
+
+        System.out.println(mapMax.maxUsingIteration(map));
+        System.out.println(mapMax.maxUsingCollectionsMax(map));
+        System.out.println(mapMax.maxUsingCollectionsMaxAndLambda(map));
+        System.out.println(mapMax.maxUsingCollectionsMaxAndMethodReference(map));
+        System.out.println(mapMax.maxUsingStreamAndLambda(map));
+        System.out.println(mapMax.maxUsingStreamAndMethodReference(map));
+
+    }
+
+}

core-java-modules/core-java-collections-maps-2/src/main/java/com/baeldung/map/mergemaps/Employee.java

@@ -0,0 +1,60 @@
+package com.baeldung.map.mergemaps;
+
+public class Employee implements Comparable<Employee> {
+
+    private Long id;
+    private String name;
+
+    public Employee(Long id, String name) {
+        this.name = name;
+        this.id = id;
+    }
+
+    public Long getId() {
+        return id;
+    }
+
+    public void setId(Long id) {
+        this.id = id;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+
+        Employee employee = (Employee) o;
+
+        if (!id.equals(employee.id)) return false;
+        return name.equals(employee.name);
+
+    }
+
+    @Override
+    public int hashCode() {
+        int result = id.hashCode();
+        result = 31 * result + name.hashCode();
+        return result;
+    }
+
+    @Override
+    public String toString() {
+        return "Employee{" +
+                "id=" + id +
+                ", name='" + name + '\'' +
+                '}';
+    }
+
+    @Override
+    public int compareTo(Employee employee) {
+        return (int)(this.id - employee.getId());
+    }
+}

core-java-modules/core-java-collections-maps-2/src/main/java/com/baeldung/map/mergemaps/MergeMaps.java

@@ -0,0 +1,105 @@
+package com.baeldung.map.mergemaps;
+
+import one.util.streamex.EntryStream;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+public class MergeMaps {
+
+    private static Map<String, Employee> map1 = new HashMap<>();
+    private static Map<String, Employee> map2 = new HashMap<>();
+
+    public static void main(String[] args) {
+
+        initialize();
+
+        mergeFunction();
+
+        streamConcat();
+
+        streamOf();
+
+        streamEx();
+
+        streamMerge();
+    }
+
+    private static void streamMerge() {
+
+        Map<String, Employee> map3 = map2.entrySet()
+                .stream()
+                .collect(
+                        Collectors.toMap(
+                                Map.Entry::getKey,
+                                Map.Entry::getValue,
+                                (v1, v2) -> new Employee(v1.getId(), v2.getName()),
+                                () -> new HashMap<>(map1)
+                        )
+                );
+
+        System.out.println(map3);
+    }
+
+    private static void streamEx() {
+        Map<String, Employee> map3 = EntryStream.of(map1)
+                .append(EntryStream.of(map2))
+                .toMap((e1, e2) -> e1);
+
+        System.out.println(map3);
+
+    }
+
+    private static void streamOf() {
+        Map<String, Employee> map3 = Stream.of(map1, map2)
+                .flatMap(map -> map.entrySet().stream())
+                .collect(
+                        Collectors.toMap(
+                                Map.Entry::getKey,
+                                Map.Entry::getValue,
+                                (v1, v2) -> new Employee(v1.getId(), v2.getName())
+                        )
+                );
+
+        map3.entrySet().forEach(System.out::println);
+    }
+
+    private static void streamConcat() {
+        Map<String, Employee> result = Stream.concat(map1.entrySet().stream(), map2.entrySet().stream()).collect(Collectors.toMap(
+                Map.Entry::getKey,
+                Map.Entry::getValue,
+                (value1, value2) -> new Employee(value2.getId(), value1.getName())
+        ));
+
+        result.entrySet().forEach(System.out::println);
+    }
+
+    private static void mergeFunction() {
+        Map<String, Employee> map3 = new HashMap<>(map1);
+
+        map2.forEach(
+                (key, value) -> map3.merge(key, value, (v1, v2) ->
+                        new Employee(v1.getId(), v2.getName()))
+        );
+
+        map3.entrySet().forEach(System.out::println);
+    }
+
+
+    private static void initialize() {
+        Employee employee1 = new Employee(1L, "Henry");
+        map1.put(employee1.getName(), employee1);
+        Employee employee2 = new Employee(22L, "Annie");
+        map1.put(employee2.getName(), employee2);
+        Employee employee3 = new Employee(8L, "John");
+        map1.put(employee3.getName(), employee3);
+
+        Employee employee4 = new Employee(2L, "George");
+        map2.put(employee4.getName(), employee4);
+        Employee employee5 = new Employee(3L, "Henry");
+        map2.put(employee5.getName(), employee5);
+    }
+
+}

core-java-modules/core-java-collections-maps-2/src/main/java/com/baeldung/map/primitives/PrimitiveMaps.java

@@ -0,0 +1,69 @@
+package com.baeldung.map.primitives;
+
+import cern.colt.map.AbstractIntDoubleMap;
+import cern.colt.map.OpenIntDoubleHashMap;
+import gnu.trove.map.TDoubleIntMap;
+import gnu.trove.map.hash.TDoubleIntHashMap;
+import it.unimi.dsi.fastutil.ints.Int2BooleanMap;
+import it.unimi.dsi.fastutil.ints.Int2BooleanOpenHashMap;
+import it.unimi.dsi.fastutil.ints.Int2BooleanSortedMap;
+import it.unimi.dsi.fastutil.ints.Int2BooleanSortedMaps;
+import org.eclipse.collections.api.map.primitive.ImmutableIntIntMap;
+import org.eclipse.collections.api.map.primitive.MutableIntIntMap;
+import org.eclipse.collections.api.map.primitive.MutableObjectDoubleMap;
+import org.eclipse.collections.impl.factory.primitive.IntIntMaps;
+import org.eclipse.collections.impl.factory.primitive.ObjectDoubleMaps;
+
+public class PrimitiveMaps {
+
+    public static void main(String[] args) {
+
+        eclipseCollectionsMap();
+        troveMap();
+        coltMap();
+        fastutilMap();
+    }
+
+    private static void fastutilMap() {
+        Int2BooleanMap int2BooleanMap = new Int2BooleanOpenHashMap();
+        int2BooleanMap.put(1, true);
+        int2BooleanMap.put(7, false);
+        int2BooleanMap.put(4, true);
+
+        boolean value = int2BooleanMap.get(1);
+
+        Int2BooleanSortedMap int2BooleanSorted = Int2BooleanSortedMaps.EMPTY_MAP;
+    }
+
+    private static void coltMap() {
+        AbstractIntDoubleMap map = new OpenIntDoubleHashMap();
+        map.put(1, 4.5);
+        double value = map.get(1);
+    }
+
+    private static void eclipseCollectionsMap() {
+        MutableIntIntMap mutableIntIntMap = IntIntMaps.mutable.empty();
+        mutableIntIntMap.addToValue(1, 1);
+
+        ImmutableIntIntMap immutableIntIntMap = IntIntMaps.immutable.empty();
+
+        MutableObjectDoubleMap<String> dObject = ObjectDoubleMaps.mutable.empty();
+        dObject.addToValue("price", 150.5);
+        dObject.addToValue("quality", 4.4);
+        dObject.addToValue("stability", 0.8);
+    }
+
+    private static void troveMap() {
+        double[] doubles = new double[] {1.2, 4.5, 0.3};
+        int[] ints = new int[] {1, 4, 0};
+
+        TDoubleIntMap doubleIntMap = new TDoubleIntHashMap(doubles, ints);
+
+        doubleIntMap.put(1.2, 22);
+        doubleIntMap.put(4.5, 16);
+
+        doubleIntMap.adjustValue(1.2, 1);
+        doubleIntMap.adjustValue(4.5, 4);
+        doubleIntMap.adjustValue(0.3, 7);
+    }
+}

core-java-modules/core-java-collections-maps-2/src/main/java/com/baeldung/map/sort/SortHashMap.java

@@ -0,0 +1,104 @@
+package com.baeldung.map.sort;
+
+import com.baeldung.map.mergemaps.Employee;
+import com.google.common.base.Functions;
+import com.google.common.collect.ImmutableSortedMap;
+import com.google.common.collect.Ordering;
+
+import java.util.*;
+import java.util.stream.Collectors;
+
+public class SortHashMap {
+
+    private static Map<String, Employee> map = new HashMap<>();
+
+    public static void main(String[] args) {
+
+        initialize();
+
+        treeMapSortByKey();
+
+        arrayListSortByValue();
+        arrayListSortByKey();
+
+        sortStream();
+
+        sortGuava();
+
+        addDuplicates();
+
+        treeSetByKey();
+        treeSetByValue();
+
+    }
+
+    private static void sortGuava() {
+        final Ordering naturalOrdering =
+        Ordering.natural().onResultOf(Functions.forMap(map, null));
+
+        System.out.println(ImmutableSortedMap.copyOf(map, naturalOrdering));
+    }
+
+    private static void sortStream() {
+        map.entrySet().stream()
+                .sorted(Map.Entry.<String, Employee>comparingByKey().reversed())
+                .forEach(System.out::println);
+
+        Map<String, Employee> result = map.entrySet().stream()
+                .sorted(Map.Entry.comparingByValue())
+                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,
+                (oldValue, newValue) -> oldValue, LinkedHashMap::new));
+
+        result.entrySet().forEach(System.out::println);
+    }
+
+    private static void treeSetByValue() {
+        SortedSet<Employee> values = new TreeSet<>(map.values());
+        System.out.println(values);
+    }
+
+    private static void treeSetByKey() {
+        SortedSet<String> keysSet = new TreeSet<>(map.keySet());
+        System.out.println(keysSet);
+    }
+
+    private static void treeMapSortByKey() {
+        TreeMap<String, Employee> sorted = new TreeMap<>(map);
+        sorted.putAll(map);
+
+        sorted.entrySet().forEach(System.out::println);
+
+    }
+
+    private static void arrayListSortByValue() {
+        List<Employee> employeeById = new ArrayList<>(map.values());
+
+        Collections.sort(employeeById);
+
+        System.out.println(employeeById);
+    }
+
+    private static void arrayListSortByKey() {
+        List<String> employeeByKey = new ArrayList<>(map.keySet());
+        Collections.sort(employeeByKey);
+        System.out.println(employeeByKey);
+    }
+
+    private static void initialize() {
+        Employee employee1 = new Employee(1L, "Mher");
+        map.put(employee1.getName(), employee1);
+        Employee employee2 = new Employee(22L, "Annie");
+        map.put(employee2.getName(), employee2);
+        Employee employee3 = new Employee(8L, "John");
+        map.put(employee3.getName(), employee3);
+        Employee employee4 = new Employee(2L, "George");
+        map.put(employee4.getName(), employee4);
+    }
+
+    private static void addDuplicates() {
+        Employee employee5 = new Employee(1L, "Mher");
+        map.put(employee5.getName(), employee5);
+        Employee employee6 = new Employee(22L, "Annie");
+        map.put(employee6.getName(), employee6);
+    }
+}

core-java-modules/core-java-collections-maps-2/src/test/java/com/baeldung/map/ProductUnitTest.java

@@ -0,0 +1,174 @@
+package com.baeldung.map;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Objects;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class ProductUnitTest {
+
+
+    @Test
+    public void getExistingValue() {
+        HashMap<String, Product> productsByName = new HashMap<>();
+
+        Product eBike = new Product("E-Bike", "A bike with a battery");
+        Product roadBike = new Product("Road bike", "A bike for competition");
+
+        productsByName.put(eBike.getName(), eBike);
+        productsByName.put(roadBike.getName(), roadBike);
+
+        Product nextPurchase = productsByName.get("E-Bike");
+
+        assertEquals("A bike with a battery", nextPurchase.getDescription());
+    }
+
+    @Test
+    public void getNonExistingValue() {
+        HashMap<String, Product> productsByName = new HashMap<>();
+
+        Product eBike = new Product("E-Bike", "A bike with a battery");
+        Product roadBike = new Product("Road bike", "A bike for competition");
+
+        productsByName.put(eBike.getName(), eBike);
+        productsByName.put(roadBike.getName(), roadBike);
+
+        Product nextPurchase = productsByName.get("Car");
+
+        assertNull(nextPurchase);
+    }
+
+    @Test
+    public void getExistingValueAfterSameKeyInsertedTwice() {
+        HashMap<String, Product> productsByName = new HashMap<>();
+
+        Product eBike = new Product("E-Bike", "A bike with a battery");
+        Product roadBike = new Product("Road bike", "A bike for competition");
+        Product newEBike = new Product("E-Bike", "A bike with a better battery");
+
+        productsByName.put(eBike.getName(), eBike);
+        productsByName.put(roadBike.getName(), roadBike);
+        productsByName.put(newEBike.getName(), newEBike);
+
+        Product nextPurchase = productsByName.get("E-Bike");
+
+        assertEquals("A bike with a better battery", nextPurchase.getDescription());
+    }
+
+    @Test
+    public void getExistingValueWithNullKey() {
+        HashMap<String, Product> productsByName = new HashMap<>();
+
+        Product defaultProduct = new Product("Chocolate", "At least buy chocolate");
+
+        productsByName.put(null, defaultProduct);
+        productsByName.put(defaultProduct.getName(), defaultProduct);
+
+        Product nextPurchase = productsByName.get(null);
+        assertEquals("At least buy chocolate", nextPurchase.getDescription());
+
+        nextPurchase = productsByName.get("Chocolate");
+        assertEquals("At least buy chocolate", nextPurchase.getDescription());
+    }
+
+    @Test
+    public void insertSameObjectWithDifferentKey() {
+        HashMap<String, Product> productsByName = new HashMap<>();
+
+        Product defaultProduct = new Product("Chocolate", "At least buy chocolate");
+
+        productsByName.put(null, defaultProduct);
+        productsByName.put(defaultProduct.getName(), defaultProduct);
+
+        assertSame(productsByName.get(null), productsByName.get("Chocolate"));
+    }
+
+    @Test
+    public void checkIfKeyExists() {
+        HashMap<String, Product> productsByName = new HashMap<>();
+
+        Product eBike = new Product("E-Bike", "A bike with a battery");
+
+        productsByName.put(eBike.getName(), eBike);
+
+        assertTrue(productsByName.containsKey("E-Bike"));
+    }
+
+    @Test
+    public void checkIfValueExists() {
+        HashMap<String, Product> productsByName = new HashMap<>();
+
+        Product eBike = new Product("E-Bike", "A bike with a battery");
+
+        productsByName.put(eBike.getName(), eBike);
+
+        assertTrue(productsByName.containsValue(eBike));
+    }
+
+    @Test
+    public void removeExistingKey() {
+        HashMap<String, Product> productsByName = new HashMap<>();
+
+        Product eBike = new Product("E-Bike", "A bike with a battery");
+        Product roadBike = new Product("Road bike", "A bike for competition");
+
+        productsByName.put(eBike.getName(), eBike);
+        productsByName.put(roadBike.getName(), roadBike);
+
+        productsByName.remove("E-Bike");
+
+        assertNull(productsByName.get("E-Bike"));
+    }
+
+    @Test
+    public void givenMutableKeyWhenKeyChangeThenValueNotFound() {
+        // Given
+        MutableKey key = new MutableKey("initial");
+
+        Map<MutableKey, String> items = new HashMap<>();
+        items.put(key, "success");
+
+        // When
+        key.setName("changed");
+
+        // Then
+        assertNull(items.get(key));
+    }
+
+    static class MutableKey {
+        private String name;
+
+        public MutableKey(String name) {
+            this.name = name;
+        }
+
+        public String getName() {
+            return name;
+        }
+
+        public void setName(String name) {
+            this.name = name;
+        }
+
+        @Override
+        public boolean equals(Object o) {
+            if (this == o) {
+                return true;
+            }
+            if (o == null || getClass() != o.getClass()) {
+                return false;
+            }
+            MutableKey that = (MutableKey) o;
+            return Objects.equals(name, that.name);
+        }
+
+        @Override
+        public int hashCode() {
+            return Objects.hash(name);
+        }
+    }
+
+}

core-java-modules/core-java-collections-maps-2/src/test/java/com/baeldung/map/convert/MapToStringUnitTest.java

@@ -0,0 +1,48 @@
+package com.baeldung.map.convert;
+
+import org.apache.commons.collections4.MapUtils;
+import org.junit.Assert;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class MapToStringUnitTest {
+
+    private Map<Integer, String> wordsByKey = new HashMap<>();
+
+    @BeforeEach
+    public void setup() {
+        wordsByKey.clear();
+        wordsByKey.put(1, "one");
+        wordsByKey.put(2, "two");
+        wordsByKey.put(3, "three");
+        wordsByKey.put(4, "four");
+    }
+
+    @Test
+    public void givenMap_WhenUsingIteration_ThenResultingMapIsCorrect() {
+        String mapAsString = MapToString.convertWithIteration(wordsByKey);
+        Assert.assertEquals("{1=one, 2=two, 3=three, 4=four}", mapAsString);
+    }
+
+    @Test
+    public void givenMap_WhenUsingStream_ThenResultingMapIsCorrect() {
+        String mapAsString = MapToString.convertWithStream(wordsByKey);
+        Assert.assertEquals("{1=one, 2=two, 3=three, 4=four}", mapAsString);
+    }
+
+    @Test
+    public void givenMap_WhenUsingGuava_ThenResultingMapIsCorrect() {
+        String mapAsString = MapToString.convertWithGuava(wordsByKey);
+        Assert.assertEquals("1=one,2=two,3=three,4=four", mapAsString);
+    }
+
+    @Test
+    public void givenMap_WhenUsingApache_ThenResultingMapIsCorrect() {
+        String mapAsString = MapToString.convertWithApache(wordsByKey);
+        Assert.assertEquals("{1=one, 2=two, 3=three, 4=four}", mapAsString);
+        MapUtils.debugPrint(System.out, "Map as String", wordsByKey);
+    }
+}

core-java-modules/core-java-collections-maps-2/src/test/java/com/baeldung/map/convert/StringToMapUnitTest.java

@@ -0,0 +1,23 @@
+package com.baeldung.map.convert;
+
+import org.junit.Assert;
+import org.junit.jupiter.api.Test;
+
+import java.util.Map;
+
+public class StringToMapUnitTest {
+
+    @Test
+    public void givenString_WhenUsingStream_ThenResultingStringIsCorrect() {
+        Map<String, String> wordsByKey = StringToMap.convertWithStream("1=one,2=two,3=three,4=four");
+        Assert.assertEquals(4, wordsByKey.size());
+        Assert.assertEquals("one", wordsByKey.get("1"));
+    }
+
+    @Test
+    void givenString_WhenUsingGuava_ThenResultingStringIsCorrect() {
+        Map<String, String> wordsByKey = StringToMap.convertWithGuava("1=one,2=two,3=three,4=four");
+        Assert.assertEquals(4, wordsByKey.size());
+        Assert.assertEquals("one", wordsByKey.get("1"));
+    }
+}

core-java-modules/core-java-collections-maps-2/src/test/java/com/baeldung/map/copyhashmap/CopyHashMapUnitTest.java

@@ -0,0 +1,76 @@
+package com.baeldung.map.copyhashmap;
+
+import com.google.common.collect.ImmutableMap;
+import org.junit.Test;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class CopyHashMapUnitTest {
+    
+    @Test
+    public void givenHashMap_whenShallowCopy_thenCopyisNotSameAsOriginal() {
+        
+        HashMap<String, Employee> map = new HashMap<>();
+        Employee emp1 = new Employee("John");
+        Employee emp2 = new Employee("Norman");
+        map.put("emp1",emp1);
+        map.put("emp2",emp2);
+    
+        HashMap<String, Employee> shallowCopy = CopyHashMap.shallowCopy(map);
+        
+        assertThat(shallowCopy).isNotSameAs(map);
+        
+    }
+    
+    @Test
+    public void givenHashMap_whenShallowCopyModifyingOriginalObject_thenCopyShouldChange() {
+        
+        HashMap<String, Employee> map = new HashMap<>();
+        Employee emp1 = new Employee("John");
+        Employee emp2 = new Employee("Norman");
+        map.put("emp1",emp1);
+        map.put("emp2",emp2);
+        
+        HashMap<String, Employee> shallowCopy = CopyHashMap.shallowCopy(map);
+        
+        emp1.setName("Johny");
+        
+        assertThat(shallowCopy.get("emp1")).isEqualTo(map.get("emp1"));
+        
+    }
+    
+    @Test
+    public void givenHashMap_whenDeepCopyModifyingOriginalObject_thenCopyShouldNotChange() {
+        
+        HashMap<String, Employee> map = new HashMap<>();
+        Employee emp1 = new Employee("John");
+        Employee emp2 = new Employee("Norman");
+        map.put("emp1",emp1);
+        map.put("emp2",emp2);
+        HashMap<String, Employee> deepCopy = CopyHashMap.deepCopy(map);
+        
+        emp1.setName("Johny");
+        
+        assertThat(deepCopy.get("emp1")).isNotEqualTo(map.get("emp1"));
+        
+    }
+    
+    @Test
+    public void givenImmutableMap_whenCopyUsingGuava_thenCopyShouldNotChange() {
+        Employee emp1 = new Employee("John");
+        Employee emp2 = new Employee("Norman");
+        
+        Map<String, Employee> map = ImmutableMap.<String, Employee> builder()
+            .put("emp1",emp1)
+            .put("emp2",emp2)
+            .build();
+        Map<String, Employee> shallowCopy = ImmutableMap.copyOf(map);
+        
+        assertThat(shallowCopy).isSameAs(map);
+        
+    }
+
+}

core-java-modules/core-java-collections-maps-2/src/test/java/com/baeldung/map/copyhashmap/Employee.java

@@ -0,0 +1,28 @@
+package com.baeldung.map.copyhashmap;
+
+import java.io.Serializable;
+
+public class Employee implements Serializable{
+    
+    private String name;
+    
+    public Employee(String name) {
+        super();
+        this.name = name;
+    }
+    public String getName() {
+        return name;
+    }
+    
+    public void setName(String name) {
+        this.name = name;
+    }
+    
+    @Override
+    public String toString() {
+        return this.name;
+    }
+ 
+}
+
+

core-java-modules/core-java-collections-maps-2/src/test/java/com/baeldung/map/initialize/MapInitializerUnitTest.java

@@ -0,0 +1,27 @@
+package com.baeldung.map.initialize;
+
+import org.junit.Test;
+
+import java.util.Map;
+
+import static org.junit.Assert.assertEquals;
+
+public class MapInitializerUnitTest {
+    
+    @Test
+    public void givenStaticMap_whenUpdated_thenCorrect() {
+        
+        MapInitializer.articleMapOne.put("NewArticle1", "Convert array to List");
+        
+        assertEquals(MapInitializer.articleMapOne.get("NewArticle1"), "Convert array to List");
+        
+    }
+    
+    @Test(expected=UnsupportedOperationException.class)
+    public void givenSingleTonMap_whenEntriesAdded_throwsException() {
+        
+        Map<String, String> map = MapInitializer.createSingletonMap();
+        map.put("username2", "password2");
+    }
+
+}

core-java-modules/core-java-collections-maps-2/src/test/java/com/baeldung/map/mapmax/MapMaxUnitTest.java

@@ -0,0 +1,58 @@
+package com.baeldung.map.mapmax;
+
+
+import org.junit.Before;
+import org.junit.Test;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import static org.junit.Assert.assertEquals;
+
+public class MapMaxUnitTest {
+    
+    Map<Integer, Integer> map = null;
+    MapMax mapMax = null;
+        
+    
+    @Before
+    public void setupTestData() {
+        map = new HashMap<Integer, Integer>();
+        map.put(23, 12);
+        map.put(46, 24);
+        map.put(27, 38); 
+        mapMax = new MapMax();
+    }
+    
+    @Test
+    public void givenMap_whenIterated_thenReturnMaxValue() {
+        assertEquals(new Integer(38), mapMax.maxUsingIteration(map));
+    }
+    
+    @Test
+    public void givenMap_whenUsingCollectionsMax_thenReturnMaxValue() {
+        assertEquals(new Integer(38), mapMax.maxUsingCollectionsMax(map));
+    }
+    
+    @Test
+    public void givenMap_whenUsingCollectionsMaxAndLambda_thenReturnMaxValue() {
+        assertEquals(new Integer(38), mapMax.maxUsingCollectionsMaxAndLambda(map));
+    }
+    
+    @Test
+    public void givenMap_whenUsingCollectionsMaxAndMethodReference_thenReturnMaxValue() {
+        assertEquals(new Integer(38), mapMax.maxUsingCollectionsMaxAndMethodReference(map));
+    }
+    
+    @Test
+    public void givenMap_whenUsingStreamAndLambda_thenReturnMaxValue() {
+        assertEquals(new Integer(38), mapMax.maxUsingStreamAndLambda(map));
+    }
+    
+    @Test
+    public void givenMap_whenUsingStreamAndMethodReference_thenReturnMaxValue() {
+        assertEquals(new Integer(38), mapMax.maxUsingStreamAndMethodReference   (map));
+    }
+
+
+}

core-java-modules/core-java-collections-maps-2/src/test/java/com/baeldung/map/weakhashmap/WeakHashMapUnitTest.java

@@ -0,0 +1,72 @@
+package com.baeldung.map.weakhashmap;
+
+
+import org.junit.Test;
+
+import java.util.WeakHashMap;
+import java.util.concurrent.TimeUnit;
+
+import static com.jayway.awaitility.Awaitility.await;
+import static org.junit.Assert.assertTrue;
+
+public class WeakHashMapUnitTest {
+
+    @Test
+    public void givenWeakHashMap_whenCacheValueThatHasNoReferenceToIt_GCShouldReclaimThatObject() {
+        //given
+        WeakHashMap<UniqueImageName, BigImage> map = new WeakHashMap<>();
+        BigImage bigImage = new BigImage("image_id");
+        UniqueImageName imageName = new UniqueImageName("name_of_big_image");
+
+        map.put(imageName, bigImage);
+        assertTrue(map.containsKey(imageName));
+
+        //when big image key is not reference anywhere
+        imageName = null;
+        System.gc();
+
+        //then GC will finally reclaim that object
+        await().atMost(10, TimeUnit.SECONDS).until(map::isEmpty);
+    }
+
+    @Test
+    public void givenWeakHashMap_whenCacheValueThatHasNoReferenceToIt_GCShouldReclaimThatObjectButLeaveReferencedObject() {
+        //given
+        WeakHashMap<UniqueImageName, BigImage> map = new WeakHashMap<>();
+        BigImage bigImageFirst = new BigImage("foo");
+        UniqueImageName imageNameFirst = new UniqueImageName("name_of_big_image");
+
+        BigImage bigImageSecond = new BigImage("foo_2");
+        UniqueImageName imageNameSecond = new UniqueImageName("name_of_big_image_2");
+
+        map.put(imageNameFirst, bigImageFirst);
+        map.put(imageNameSecond, bigImageSecond);
+        assertTrue(map.containsKey(imageNameFirst));
+        assertTrue(map.containsKey(imageNameSecond));
+
+        //when
+        imageNameFirst = null;
+        System.gc();
+
+        //then
+        await().atMost(10, TimeUnit.SECONDS).until(() -> map.size() == 1);
+        await().atMost(10, TimeUnit.SECONDS).until(() -> map.containsKey(imageNameSecond));
+    }
+
+
+    class BigImage {
+        public final String imageId;
+
+        BigImage(String imageId) {
+            this.imageId = imageId;
+        }
+    }
+
+    class UniqueImageName {
+        public final String imageName;
+
+        UniqueImageName(String imageName) {
+            this.imageName = imageName;
+        }
+    }
+}

core-java-modules/core-java-collections-maps-3/README.md

@@ -0,0 +1,8 @@
+## Java Collections Cookbooks and Examples
+
+This module contains articles about Map data structures in Java.
+
+### Relevant Articles: 
+- [Java TreeMap vs HashMap](https://www.baeldung.com/java-treemap-vs-hashmap)
+- [Comparing Two HashMaps in Java](https://www.baeldung.com/java-compare-hashmaps)
+- More articles: [[<-- prev]](/core-java-modules/core-java-collections-maps-2)

core-java-modules/core-java-collections-maps-3/pom.xml

@@ -0,0 +1,26 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>core-java-collections-maps-3</artifactId>
+    <version>0.1.0-SNAPSHOT</version>
+    <name>core-java-collections-maps-3</name>
+    <packaging>jar</packaging>
+
+    <parent>
+        <groupId>com.baeldung</groupId>
+        <artifactId>parent-java</artifactId>
+        <version>0.0.1-SNAPSHOT</version>
+        <relativePath>../../parent-java</relativePath>
+    </parent>
+
+    <dependencies>
+        
+    </dependencies>
+
+    <properties>
+        
+    </properties>
+
+</project>

core-java-modules/core-java-collections-maps-3/src/test/java/com/baeldung/map/compare/HashMapComparisonUnitTest.java

@@ -0,0 +1,225 @@
+package com.baeldung.map.compare;
+
+import com.google.common.base.Equivalence;
+import com.google.common.collect.MapDifference;
+import com.google.common.collect.MapDifference.ValueDifference;
+import com.google.common.collect.Maps;
+import org.junit.Before;
+import org.junit.Test;
+
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.stream.Collectors;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.collection.IsMapContaining.hasEntry;
+import static org.hamcrest.collection.IsMapContaining.hasKey;
+import static org.junit.Assert.*;
+
+public class HashMapComparisonUnitTest {
+    
+    Map<String, String> asiaCapital1;
+    Map<String, String> asiaCapital2;
+    Map<String, String> asiaCapital3;
+    
+    Map<String, String[]> asiaCity1;
+    Map<String, String[]> asiaCity2;
+    Map<String, String[]> asiaCity3;
+    
+    @Before
+    public void setup(){
+        asiaCapital1 = new HashMap<String, String>();
+        asiaCapital1.put("Japan", "Tokyo");
+        asiaCapital1.put("South Korea", "Seoul");
+        
+        asiaCapital2 = new HashMap<String, String>();
+        asiaCapital2.put("South Korea", "Seoul");
+        asiaCapital2.put("Japan", "Tokyo");
+
+        asiaCapital3 = new HashMap<String, String>();
+        asiaCapital3.put("Japan", "Tokyo");
+        asiaCapital3.put("China", "Beijing");
+        
+        asiaCity1 = new HashMap<String, String[]>();
+        asiaCity1.put("Japan", new String[] { "Tokyo", "Osaka" });
+        asiaCity1.put("South Korea", new String[] { "Seoul", "Busan" });
+
+        asiaCity2 = new HashMap<String, String[]>();
+        asiaCity2.put("South Korea", new String[] { "Seoul", "Busan" });
+        asiaCity2.put("Japan", new String[] { "Tokyo", "Osaka" });
+
+        asiaCity3 = new HashMap<String, String[]>();
+        asiaCity3.put("Japan", new String[] { "Tokyo", "Osaka" });
+        asiaCity3.put("China", new String[] { "Beijing", "Hong Kong" });
+    }
+
+    @Test
+    public void whenCompareTwoHashMapsUsingEquals_thenSuccess() {
+        assertTrue(asiaCapital1.equals(asiaCapital2));
+        assertFalse(asiaCapital1.equals(asiaCapital3));
+    }
+
+    @Test
+    public void whenCompareTwoHashMapsWithArrayValuesUsingEquals_thenFail() {
+        assertFalse(asiaCity1.equals(asiaCity2));
+    }
+
+    @Test
+    public void whenCompareTwoHashMapsUsingStreamAPI_thenSuccess() {
+        assertTrue(areEqual(asiaCapital1, asiaCapital2));
+        assertFalse(areEqual(asiaCapital1, asiaCapital3));
+    }
+
+    @Test
+    public void whenCompareTwoHashMapsWithArrayValuesUsingStreamAPI_thenSuccess() {
+        assertTrue(areEqualWithArrayValue(asiaCity1, asiaCity2));
+        assertFalse(areEqualWithArrayValue(asiaCity1, asiaCity3));
+    }
+
+    @Test
+    public void whenCompareTwoHashMapKeys_thenSuccess() {
+        assertTrue(asiaCapital1.keySet().equals(asiaCapital2.keySet()));
+        assertFalse(asiaCapital1.keySet().equals(asiaCapital3.keySet()));
+    }
+
+    @Test
+    public void whenCompareTwoHashMapKeyValuesUsingStreamAPI_thenSuccess() {
+        Map<String, String> asiaCapital3 = new HashMap<String, String>();
+        asiaCapital3.put("Japan", "Tokyo");
+        asiaCapital3.put("South Korea", "Seoul");
+        asiaCapital3.put("China", "Beijing");
+
+        Map<String, String> asiaCapital4 = new HashMap<String, String>();
+        asiaCapital4.put("South Korea", "Seoul");
+        asiaCapital4.put("Japan", "Osaka");
+        asiaCapital4.put("China", "Beijing");
+
+        Map<String, Boolean> result = areEqualKeyValues(asiaCapital3, asiaCapital4);
+        
+        assertEquals(3, result.size());
+        assertThat(result, hasEntry("Japan", false));
+        assertThat(result, hasEntry("South Korea", true));
+        assertThat(result, hasEntry("China", true));
+    }
+
+    @Test
+    public void givenDifferentMaps_whenGetDiffUsingGuava_thenSuccess() {
+        Map<String, String> asia1 = new HashMap<String, String>();
+        asia1.put("Japan", "Tokyo");
+        asia1.put("South Korea", "Seoul");
+        asia1.put("India", "New Delhi");
+
+        Map<String, String> asia2 = new HashMap<String, String>();
+        asia2.put("Japan", "Tokyo");
+        asia2.put("China", "Beijing");
+        asia2.put("India", "Delhi");
+
+        MapDifference<String, String> diff = Maps.difference(asia1, asia2);
+        Map<String, ValueDifference<String>> entriesDiffering = diff.entriesDiffering();
+        
+        assertFalse(diff.areEqual());
+        assertEquals(1, entriesDiffering.size());
+        assertThat(entriesDiffering, hasKey("India"));
+        assertEquals("New Delhi", entriesDiffering.get("India").leftValue());
+        assertEquals("Delhi", entriesDiffering.get("India").rightValue());
+    }
+
+    @Test
+    public void givenDifferentMaps_whenGetEntriesOnOneSideUsingGuava_thenSuccess() {
+        Map<String, String> asia1 = new HashMap<String, String>();
+        asia1.put("Japan", "Tokyo");
+        asia1.put("South Korea", "Seoul");
+        asia1.put("India", "New Delhi");
+
+        Map<String, String> asia2 = new HashMap<String, String>();
+        asia2.put("Japan", "Tokyo");
+        asia2.put("China", "Beijing");
+        asia2.put("India", "Delhi");
+
+        MapDifference<String, String> diff = Maps.difference(asia1, asia2);
+        Map<String, String> entriesOnlyOnRight = diff.entriesOnlyOnRight();
+        Map<String, String> entriesOnlyOnLeft = diff.entriesOnlyOnLeft();
+
+        assertEquals(1, entriesOnlyOnRight.size());
+        assertThat(entriesOnlyOnRight, hasEntry("China", "Beijing"));
+        assertEquals(1, entriesOnlyOnLeft.size());
+        assertThat(entriesOnlyOnLeft, hasEntry("South Korea", "Seoul"));
+    }
+
+    @Test
+    public void givenDifferentMaps_whenGetCommonEntriesUsingGuava_thenSuccess() {
+        Map<String, String> asia1 = new HashMap<String, String>();
+        asia1.put("Japan", "Tokyo");
+        asia1.put("South Korea", "Seoul");
+        asia1.put("India", "New Delhi");
+
+        Map<String, String> asia2 = new HashMap<String, String>();
+        asia2.put("Japan", "Tokyo");
+        asia2.put("China", "Beijing");
+        asia2.put("India", "Delhi");
+
+        MapDifference<String, String> diff = Maps.difference(asia1, asia2);
+        Map<String, String> entriesInCommon = diff.entriesInCommon();
+
+        assertEquals(1, entriesInCommon.size());
+        assertThat(entriesInCommon, hasEntry("Japan", "Tokyo"));
+    }
+
+    @Test
+    public void givenSimilarMapsWithArrayValue_whenCompareUsingGuava_thenFail() {
+        MapDifference<String, String[]> diff = Maps.difference(asiaCity1, asiaCity2);
+        assertFalse(diff.areEqual());
+    }
+
+    @Test
+    public void givenSimilarMapsWithArrayValue_whenCompareUsingGuavaEquivalence_thenSuccess() {
+        Equivalence<String[]> eq = new Equivalence<String[]>() {
+            @Override
+            protected boolean doEquivalent(String[] a, String[] b) {
+                return Arrays.equals(a, b);
+            }
+
+            @Override
+            protected int doHash(String[] value) {
+                return value.hashCode();
+            }
+        };
+
+        MapDifference<String, String[]> diff = Maps.difference(asiaCity1, asiaCity2, eq);
+        assertTrue(diff.areEqual());
+        
+        diff = Maps.difference(asiaCity1, asiaCity3, eq);
+        assertFalse(diff.areEqual());
+    }
+
+    // ===========================================================================
+
+    private boolean areEqual(Map<String, String> first, Map<String, String> second) {
+        if (first.size() != second.size()) {
+            return false;
+        }
+
+        return first.entrySet()
+            .stream()
+            .allMatch(e -> e.getValue()
+                .equals(second.get(e.getKey())));
+    }
+
+    private boolean areEqualWithArrayValue(Map<String, String[]> first, Map<String, String[]> second) {
+        if (first.size() != second.size()) {
+            return false;
+        }
+
+        return first.entrySet()
+            .stream()
+            .allMatch(e -> Arrays.equals(e.getValue(), second.get(e.getKey())));
+    }
+
+    private Map<String, Boolean> areEqualKeyValues(Map<String, String> first, Map<String, String> second) {
+        return first.entrySet()
+            .stream()
+            .collect(Collectors.toMap(e -> e.getKey(), e -> e.getValue().equals(second.get(e.getKey()))));
+    }
+
+}

core-java-modules/core-java-collections-maps-3/src/test/java/com/baeldung/map/treemaphashmap/TreeMapVsHashMapUnitTest.java

@@ -0,0 +1,58 @@
+package com.baeldung.map.treemaphashmap;
+
+import org.hamcrest.Matchers;
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.TreeMap;
+
+public class TreeMapVsHashMapUnitTest {
+
+    @Test
+    public void whenInsertObjectsTreeMap_thenNaturalOrder() {
+        Map<Integer, String> treemap = new TreeMap<>();
+        treemap.put(3, "TreeMap");
+        treemap.put(2, "vs");
+        treemap.put(1, "HashMap");
+        Assert.assertThat(treemap.keySet(), Matchers.contains(1, 2, 3));
+    }
+
+    @Test(expected = NullPointerException.class)
+    public void whenInsertNullInTreeMap_thenException() {
+        Map<Integer, String> treemap = new TreeMap<>();
+        treemap.put(null, "NullPointerException");
+    }
+
+    @Test
+    public void whenInsertObjectsHashMap_thenRandomOrder() {
+        Map<Integer, String> hashmap = new HashMap<>();
+        hashmap.put(3, "TreeMap");
+        hashmap.put(2, "vs");
+        hashmap.put(1, "HashMap");
+        Assert.assertThat(hashmap.keySet(), Matchers.containsInAnyOrder(1, 2, 3));
+    }
+
+    @Test
+    public void whenInsertNullInHashMap_thenInsertsNull() {
+        Map<Integer, String> hashmap = new HashMap<>();
+        hashmap.put(null, null);
+        Assert.assertNull(hashmap.get(null));
+    }
+
+    @Test
+    public void givenHashMapAndTreeMap_whenputDuplicates_thenOnlyUnique() {
+        Map<Integer, String> treeMap = new HashMap<>();
+        treeMap.put(1, "Baeldung");
+        treeMap.put(1, "Baeldung");
+
+        Assert.assertTrue(treeMap.size() == 1);
+
+        Map<Integer, String> treeMap2 = new TreeMap<>();
+        treeMap2.put(1, "Baeldung");
+        treeMap2.put(1, "Baeldung");
+
+        Assert.assertTrue(treeMap2.size() == 1);
+    }
+}

core-java-modules/core-java-collections-maps/README.md

@@ -0,0 +1,15 @@
+## Java Collections Cookbooks and Examples
+
+This module contains articles about Map data structures in Java.
+
+### Relevant Articles: 
+- [Guide to the Guava BiMap](https://www.baeldung.com/guava-bimap)
+- [A Guide to LinkedHashMap in Java](https://www.baeldung.com/java-linked-hashmap)
+- [A Guide to TreeMap in Java](https://www.baeldung.com/java-treemap)
+- [How to Store Duplicate Keys in a Map in Java?](https://www.baeldung.com/java-map-duplicate-keys)
+- [Get the Key for a Value from a Java Map](https://www.baeldung.com/java-map-key-from-value)
+- [How to Check If a Key Exists in a Map](https://www.baeldung.com/java-map-key-exists)
+- [Immutable Map Implementations in Java](https://www.baeldung.com/java-immutable-maps) 
+- [Guide to Apache Commons MultiValuedMap](https://www.baeldung.com/apache-commons-multi-valued-map)
+- [The Java HashMap Under the Hood](https://www.baeldung.com/java-hashmap-advanced)
+- More articles: [[next -->]](/core-java-modules/core-java-collections-maps-2)

core-java-modules/core-java-collections-maps/pom.xml

@@ -0,0 +1,35 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>core-java-collections-maps</artifactId>
+    <version>0.1.0-SNAPSHOT</version>
+    <name>core-java-collections-maps</name>
+    <packaging>jar</packaging>
+
+   <parent>
+        <groupId>com.baeldung</groupId>
+        <artifactId>parent-java</artifactId>
+        <version>0.0.1-SNAPSHOT</version>
+        <relativePath>../../parent-java</relativePath>
+    </parent>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.apache.commons</groupId>
+            <artifactId>commons-collections4</artifactId>
+            <version>${commons-collections4.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.assertj</groupId>
+            <artifactId>assertj-core</artifactId>
+            <version>${assertj.version}</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
+    <properties>
+        <commons-collections4.version>4.1</commons-collections4.version>
+        <assertj.version>3.6.1</assertj.version>
+    </properties>
+</project>

core-java-modules/core-java-collections-maps/src/main/java/com/baeldung/map/MapUtil.java

@@ -0,0 +1,44 @@
+/**
+ * 
+ */
+package com.baeldung.map;
+
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.Set;
+import java.util.stream.Stream;
+
+/**
+ * @author swpraman
+ *
+ */
+public class MapUtil {
+    
+    public static <K, V> Stream<K> keys(Map<K, V> map, V value) {
+        return map.entrySet()
+                .stream()
+                .filter(entry -> value.equals(entry.getValue()))
+                .map(Map.Entry::getKey);
+    }
+    
+    public static <K, V> Set<K> getKeys(Map<K, V> map, V value) {
+        Set<K> keys = new HashSet<>();
+        for (Entry<K, V> entry : map.entrySet()) {
+            if (entry.getValue().equals(value)) {
+                keys.add(entry.getKey());
+            }
+        }
+        return keys;
+    }
+    
+    public static <K, V> K getKey(Map<K, V> map, V value) {
+        for (Entry<K, V> entry : map.entrySet()) {
+            if (entry.getValue().equals(value)) {
+                return entry.getKey();
+            }
+        }
+        return null;
+    }
+
+}

core-java-modules/core-java-collections-maps/src/main/java/com/baeldung/map/MyKey.java

@@ -0,0 +1,64 @@
+package com.baeldung.map;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class MyKey {
+    private static final Logger LOG = LoggerFactory.getLogger(MyKey.class);
+    
+    private String name;
+    private int id;
+
+    public MyKey(int id, String name) {
+        this.id = id;
+        this.name = name;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public int getId() {
+        return id;
+    }
+
+    public void setId(int id) {
+        this.id = id;
+    }
+
+    @Override
+    public int hashCode() {
+        LOG.debug("Calling hashCode()");
+        return id;
+    }
+
+    @Override
+    public String toString() {
+        return "MyKey [name=" + name + ", id=" + id + "]";
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        LOG.debug("Calling equals() for key: " + obj);
+        if (this == obj)
+            return true;
+        if (obj == null)
+            return false;
+        if (getClass() != obj.getClass())
+            return false;
+        MyKey other = (MyKey) obj;
+        if (id != other.id)
+            return false;
+        if (name == null) {
+            if (other.name != null)
+                return false;
+        } else if (!name.equals(other.name))
+            return false;
+        return true;
+    }
+
+}

core-java-modules/core-java-collections-maps/src/main/java/com/baeldung/map/MyLinkedHashMap.java

@@ -0,0 +1,23 @@
+package com.baeldung.map;
+
+import java.util.LinkedHashMap;
+import java.util.Map;
+
+public class MyLinkedHashMap<K, V> extends LinkedHashMap<K, V> {
+
+    /**
+     * 
+     */
+    private static final long serialVersionUID = 1L;
+    private static final int MAX_ENTRIES = 5;
+
+    public MyLinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder) {
+        super(initialCapacity, loadFactor, accessOrder);
+    }
+
+    @Override
+    protected boolean removeEldestEntry(Map.Entry eldest) {
+        return size() > MAX_ENTRIES;
+    }
+
+}

core-java-modules/core-java-collections-maps/src/main/resources/logback.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<configuration>
+    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
+        <encoder>
+            <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n
+            </pattern>
+        </encoder>
+    </appender>
+
+    <root level="INFO">
+        <appender-ref ref="STDOUT" />
+    </root>
+</configuration>

core-java-modules/core-java-collections-maps/src/test/java/com/baeldung/guava/GuavaBiMapUnitTest.java

@@ -0,0 +1,120 @@
+package com.baeldung.guava;
+
+import static org.junit.Assert.*;
+import java.util.HashMap;
+import java.util.Map;
+import org.junit.Test;
+import com.google.common.collect.BiMap;
+import com.google.common.collect.EnumHashBiMap;
+import com.google.common.collect.HashBiMap;
+import com.google.common.collect.ImmutableBiMap;
+
+public class GuavaBiMapUnitTest {
+    @Test
+    public void whenQueryByValue_returnsKey() {
+        final BiMap<String, String> capitalCountryBiMap = HashBiMap.create();
+        capitalCountryBiMap.put("New Delhi", "India");
+        capitalCountryBiMap.put("Washingon, D.C.", "USA");
+        capitalCountryBiMap.put("Moscow", "Russia");
+
+        final String countryCapitalName = capitalCountryBiMap.inverse().get("India");
+
+        assertEquals("New Delhi", countryCapitalName);
+    }
+
+    @Test
+    public void whenCreateBiMapFromExistingMap_returnsKey() {
+        final Map<String, String> capitalCountryMap = new HashMap<>();
+        capitalCountryMap.put("New Delhi", "India");
+        capitalCountryMap.put("Washingon, D.C.", "USA");
+        capitalCountryMap.put("Moscow", "Russia");
+        final BiMap<String, String> capitalCountryBiMap = HashBiMap.create(capitalCountryMap);
+
+        final String countryCapitalName = capitalCountryBiMap.inverse().get("India");
+
+        assertEquals("New Delhi", countryCapitalName);
+    }
+
+    @Test
+    public void whenQueryByKey_returnsValue() {
+        final BiMap<String, String> capitalCountryBiMap = HashBiMap.create();
+
+        capitalCountryBiMap.put("New Delhi", "India");
+        capitalCountryBiMap.put("Washingon, D.C.", "USA");
+        capitalCountryBiMap.put("Moscow", "Russia");
+
+        assertEquals("USA", capitalCountryBiMap.get("Washingon, D.C."));
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void whenSameValueIsPresent_throwsException() {
+        final BiMap<String, String> capitalCountryBiMap = HashBiMap.create();
+
+        capitalCountryBiMap.put("New Delhi", "India");
+        capitalCountryBiMap.put("Washingon, D.C.", "USA");
+        capitalCountryBiMap.put("Moscow", "Russia");
+        capitalCountryBiMap.put("Trump", "USA");
+    }
+
+    @Test
+    public void givenSameValueIsPresent_whenForcePut_completesSuccessfully() {
+        final BiMap<String, String> capitalCountryBiMap = HashBiMap.create();
+
+        capitalCountryBiMap.put("New Delhi", "India");
+        capitalCountryBiMap.put("Washingon, D.C.", "USA");
+        capitalCountryBiMap.put("Moscow", "Russia");
+        capitalCountryBiMap.forcePut("Trump", "USA");
+
+        assertEquals("USA", capitalCountryBiMap.get("Trump"));
+        assertEquals("Trump", capitalCountryBiMap.inverse().get("USA"));
+    }
+
+    @Test
+    public void whenSameKeyIsPresent_replacesAlreadyPresent() {
+        final BiMap<String, String> capitalCountryBiMap = HashBiMap.create();
+
+        capitalCountryBiMap.put("New Delhi", "India");
+        capitalCountryBiMap.put("Washingon, D.C.", "USA");
+        capitalCountryBiMap.put("Moscow", "Russia");
+        capitalCountryBiMap.put("Washingon, D.C.", "HongKong");
+
+        assertEquals("HongKong", capitalCountryBiMap.get("Washingon, D.C."));
+    }
+
+    @Test
+    public void whenUsingImmutableBiMap_allowsPutSuccessfully() {
+        final BiMap<String, String> capitalCountryBiMap = new ImmutableBiMap.Builder<String, String>().put("New Delhi", "India").put("Washingon, D.C.", "USA").put("Moscow", "Russia").build();
+
+        assertEquals("USA", capitalCountryBiMap.get("Washingon, D.C."));
+    }
+
+    @Test(expected = UnsupportedOperationException.class)
+    public void whenUsingImmutableBiMap_doesntAllowRemove() {
+        final BiMap<String, String> capitalCountryBiMap = new ImmutableBiMap.Builder<String, String>().put("New Delhi", "India").put("Washingon, D.C.", "USA").put("Moscow", "Russia").build();
+
+        capitalCountryBiMap.remove("New Delhi");
+    }
+
+    @Test(expected = UnsupportedOperationException.class)
+    public void whenUsingImmutableBiMap_doesntAllowPut() {
+        final BiMap<String, String> capitalCountryBiMap = new ImmutableBiMap.Builder<String, String>().put("New Delhi", "India").put("Washingon, D.C.", "USA").put("Moscow", "Russia").build();
+
+        capitalCountryBiMap.put("New York", "USA");
+    }
+
+    private enum Operation {
+        ADD, SUBTRACT, MULTIPLY, DIVIDE
+    }
+
+    @Test
+    public void whenUsingEnumAsKeyInMap_replacesAlreadyPresent() {
+        final BiMap<Operation, String> operationStringBiMap = EnumHashBiMap.create(Operation.class);
+
+        operationStringBiMap.put(Operation.ADD, "Add");
+        operationStringBiMap.put(Operation.SUBTRACT, "Subtract");
+        operationStringBiMap.put(Operation.MULTIPLY, "Multiply");
+        operationStringBiMap.put(Operation.DIVIDE, "Divide");
+
+        assertEquals("Divide", operationStringBiMap.get(Operation.DIVIDE));
+    }
+}

core-java-modules/core-java-collections-maps/src/test/java/com/baeldung/map/ImmutableMapUnitTest.java

@@ -0,0 +1,84 @@
+package com.baeldung.map;
+
+import com.google.common.collect.ImmutableMap;
+import org.junit.jupiter.api.Test;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+
+public class ImmutableMapUnitTest {
+
+	@Test
+	public void whenCollectionsUnModifiableMapMethod_thenOriginalCollectionChangesReflectInUnmodifiableMap() {
+
+		Map<String, String> mutableMap = new HashMap<>();
+		mutableMap.put("USA", "North America");
+
+		Map<String, String> unmodifiableMap = Collections.unmodifiableMap(mutableMap);
+		assertThrows(UnsupportedOperationException.class, () -> unmodifiableMap.put("Canada", "North America"));
+		
+		mutableMap.remove("USA");
+		assertFalse(unmodifiableMap.containsKey("USA"));
+		
+		mutableMap.put("Mexico", "North America");
+		assertTrue(unmodifiableMap.containsKey("Mexico"));
+	}
+	
+	@Test
+	@SuppressWarnings("deprecation")
+	public void whenGuavaImmutableMapFromCopyOfMethod_thenOriginalCollectionChangesDoNotReflectInImmutableMap() {
+
+		Map<String, String> mutableMap = new HashMap<>();
+		mutableMap.put("USA", "North America");
+
+		ImmutableMap<String, String> immutableMap = ImmutableMap.copyOf(mutableMap);
+		assertTrue(immutableMap.containsKey("USA"));
+		
+		assertThrows(UnsupportedOperationException.class, () -> immutableMap.put("Canada", "North America"));
+		
+		mutableMap.remove("USA");
+		assertTrue(immutableMap.containsKey("USA"));
+		
+		mutableMap.put("Mexico", "North America");
+		assertFalse(immutableMap.containsKey("Mexico"));
+	}
+	
+	@Test
+	@SuppressWarnings("deprecation")
+	public void whenGuavaImmutableMapFromBuilderMethod_thenOriginalCollectionChangesDoNotReflectInImmutableMap() {
+
+		Map<String, String> mutableMap = new HashMap<>();
+		mutableMap.put("USA", "North America");
+
+		ImmutableMap<String, String> immutableMap = ImmutableMap.<String, String>builder()
+				.putAll(mutableMap)
+				.put("Costa Rica", "North America")
+				.build();
+		assertTrue(immutableMap.containsKey("USA"));
+		assertTrue(immutableMap.containsKey("Costa Rica"));
+		
+		assertThrows(UnsupportedOperationException.class, () -> immutableMap.put("Canada", "North America"));
+		
+		mutableMap.remove("USA");
+		assertTrue(immutableMap.containsKey("USA"));
+		
+		mutableMap.put("Mexico", "North America");
+		assertFalse(immutableMap.containsKey("Mexico"));
+	}
+	
+	@Test
+	@SuppressWarnings("deprecation")
+	public void whenGuavaImmutableMapFromOfMethod_thenOriginalCollectionChangesDoNotReflectInImmutableMap() {
+
+		ImmutableMap<String, String> immutableMap = ImmutableMap.of("USA", "North America", "Costa Rica", "North America");
+		assertTrue(immutableMap.containsKey("USA"));
+		assertTrue(immutableMap.containsKey("Costa Rica"));
+
+		assertThrows(UnsupportedOperationException.class, () -> immutableMap.put("Canada", "North America"));
+	}
+	
+}

core-java-modules/core-java-collections-maps/src/test/java/com/baeldung/map/KeyCheckUnitTest.java

@@ -0,0 +1,27 @@
+package com.baeldung.map;
+
+import org.junit.Test;
+
+import java.util.Collections;
+import java.util.Map;
+
+import static org.junit.Assert.*;
+
+public class KeyCheckUnitTest {
+
+    @Test
+    public void whenKeyIsPresent_thenContainsKeyReturnsTrue() {
+        Map<String, String> map = Collections.singletonMap("key", "value");
+
+        assertTrue(map.containsKey("key"));
+        assertFalse(map.containsKey("missing"));
+    }
+
+    @Test
+    public void whenKeyHasNullValue_thenGetStillWorks() {
+        Map<String, String> map = Collections.singletonMap("nothing", null);
+
+        assertTrue(map.containsKey("nothing"));
+        assertNull(map.get("nothing"));
+    }
+}

core-java-modules/core-java-collections-maps/src/test/java/com/baeldung/map/MapMultipleValuesUnitTest.java

@@ -0,0 +1,119 @@
+package com.baeldung.map;
+
+import com.google.common.collect.ArrayListMultimap;
+import com.google.common.collect.LinkedHashMultimap;
+import com.google.common.collect.Multimap;
+import com.google.common.collect.TreeMultimap;
+import org.apache.commons.collections4.MultiMap;
+import org.apache.commons.collections4.MultiMapUtils;
+import org.apache.commons.collections4.MultiValuedMap;
+import org.apache.commons.collections4.map.MultiValueMap;
+import org.apache.commons.collections4.multimap.ArrayListValuedHashMap;
+import org.apache.commons.collections4.multimap.HashSetValuedHashMap;
+import org.junit.Test;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.*;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class MapMultipleValuesUnitTest {
+    private static final Logger LOG = LoggerFactory.getLogger(MapMultipleValuesUnitTest.class);
+
+    @Test
+    public void givenHashMap_whenPuttingTwice_thenReturningFirstValue() {
+        Map<String, String> map = new HashMap<>();
+        assertThat(map.put("key1", "value1")).isEqualTo(null);
+        assertThat(map.put("key1", "value2")).isEqualTo("value1");
+        assertThat(map.get("key1")).isEqualTo("value2");
+    }
+
+    @Test
+    public void givenCollectionAsValue_whenPuttingTwice_thenReturningCollection() {
+        Map<String, List<String>> map = new HashMap<>();
+        List<String> list = new ArrayList<>();
+        map.put("key1", list);
+        map.get("key1").add("value1");
+        map.get("key1").add("value2");
+        assertThat(map.get("key1").get(0)).isEqualTo("value1");
+        assertThat(map.get("key1").get(1)).isEqualTo("value2");
+    }
+
+    @Test
+    public void givenCollectionAsValueAndJava8_whenPuttingTwice_thenReturningCollection() {
+        Map<String, List<String>> map = new HashMap<>();
+        map.computeIfAbsent("key1", k -> new ArrayList<>()).add("value1");
+        map.computeIfAbsent("key1", k -> new ArrayList<>()).add("value2");
+        assertThat(map.get("key1").get(0)).isEqualTo("value1");
+        assertThat(map.get("key1").get(1)).isEqualTo("value2");
+    }
+
+    @Test
+    public void givenMultiValueMap_whenPuttingTwice_thenReturningValues() {
+        MultiMap<String, String> map = new MultiValueMap<>();
+        map.put("key1", "value1");
+        map.put("key1", "value2");
+        assertThat((Collection<String>) map.get("key1"))
+          .contains("value1", "value2");
+    }
+
+    @Test
+    public void givenArrayListValuedHashMap_whenPuttingDoubleValues_thenReturningAllValues() {
+        MultiValuedMap<String, String> map = new ArrayListValuedHashMap<>();
+        map.put("key1", "value1");
+        map.put("key1", "value2");
+        map.put("key1", "value2");
+        assertThat((Collection<String>) map.get("key1"))
+          .containsExactly("value1", "value2", "value2");
+    }
+    
+    @Test
+    public void givenHashSetValuedHashMap_whenPuttingTwiceTheSame_thenReturningOneValue() {
+        MultiValuedMap<String, String> map = new HashSetValuedHashMap<>();
+        map.put("key1", "value1");
+        map.put("key1", "value1");
+        assertThat((Collection<String>) map.get("key1"))
+          .containsExactly("value1");
+    }
+
+    @Test(expected = UnsupportedOperationException.class)
+    public void givenUnmodifiableMultiValuedMap_whenInserting_thenThrowingException() {
+        MultiValuedMap<String, String> map = new ArrayListValuedHashMap<>();
+        map.put("key1", "value1");
+        map.put("key1", "value2");
+        MultiValuedMap<String, String> immutableMap =
+          MultiMapUtils.unmodifiableMultiValuedMap(map);
+        immutableMap.put("key1", "value3");
+    }
+    
+    @Test
+    public void givenArrayListMultiMap_whenInserting_thenCorrectOutput() {
+        Multimap<String, String> map = ArrayListMultimap.create();
+        map.put("key1", "value2");
+        map.put("key1", "value1");
+        assertThat((Collection<String>) map.get("key1"))
+          .containsExactly("value2", "value1");
+    }   
+    
+    @Test
+    public void givenLinkedHashMultiMap_whenInserting_thenReturningValuesInInsertionOrder() {
+        Multimap<String, String> map = LinkedHashMultimap.create();
+        map.put("key1", "value3");
+        map.put("key1", "value1");
+        map.put("key1", "value2");
+        assertThat((Collection<String>) map.get("key1"))
+          .containsExactly("value3", "value1", "value2");
+    }
+
+    @Test
+    public void givenTreeMultimap_whenInserting_thenReturningValuesInNaturalOrder() {
+        Multimap<String, String> map = TreeMultimap.create();
+        map.put("key1", "value3");
+        map.put("key1", "value1");
+        map.put("key1", "value2");
+        assertThat((Collection<String>) map.get("key1"))
+          .containsExactly("value1", "value2", "value3");
+    }
+    
+}

core-java-modules/core-java-collections-maps/src/test/java/com/baeldung/map/MapUnitTest.java

@@ -0,0 +1,336 @@
+package com.baeldung.map;
+
+import org.junit.Test;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.*;
+import java.util.Map.Entry;
+
+import static org.junit.Assert.*;
+
+public class MapUnitTest {
+    private static final Logger LOG = LoggerFactory.getLogger(MapUnitTest.class);
+
+    
+    @Test
+    public void givenHashMap_whenRetrievesKeyset_thenCorrect() {
+        Map<String, String> map = new HashMap<>();
+        map.put("name", "baeldung");
+        map.put("type", "blog");
+
+        Set<String> keys = map.keySet();
+
+        assertEquals(2, keys.size());
+        assertTrue(keys.contains("name"));
+        assertTrue(keys.contains("type"));
+    }
+
+    @Test
+    public void givenHashMap_whenRetrievesValues_thenCorrect() {
+        Map<String, String> map = new HashMap<>();
+        map.put("name", "baeldung");
+        map.put("type", "blog");
+
+        Collection<String> values = map.values();
+
+        assertEquals(2, values.size());
+        assertTrue(values.contains("baeldung"));
+        assertTrue(values.contains("blog"));
+    }
+
+    @Test
+    public void givenHashMap_whenRetrievesEntries_thenCorrect() {
+        Map<String, String> map = new HashMap<>();
+        map.put("name", "baeldung");
+        map.put("type", "blog");
+
+        Set<Entry<String, String>> entries = map.entrySet();
+
+        assertEquals(2, entries.size());
+        for (Entry<String, String> e : entries) {
+            String key = e.getKey();
+            String val = e.getValue();
+            assertTrue(key.equals("name") || key.equals("type"));
+            assertTrue(val.equals("baeldung") || val.equals("blog"));
+        }
+    }
+
+    @Test
+    public void givenKeySet_whenChangeReflectsInMap_thenCorrect() {
+        Map<String, String> map = new HashMap<>();
+        map.put("name", "baeldung");
+        map.put("type", "blog");
+
+        assertEquals(2, map.size());
+
+        Set<String> keys = map.keySet();
+
+        keys.remove("name");
+        assertEquals(1, map.size());
+    }
+
+    @Test(expected = ConcurrentModificationException.class)
+    public void givenIterator_whenFailsFastOnModification_thenCorrect() {
+        Map<String, String> map = new HashMap<>();
+        map.put("name", "baeldung");
+        map.put("type", "blog");
+
+        Set<String> keys = map.keySet();
+        Iterator<String> it = keys.iterator();
+        map.remove("type");
+        while (it.hasNext()) {
+            String key = it.next();
+        }
+    }
+
+    public void givenIterator_whenRemoveWorks_thenCorrect() {
+        Map<String, String> map = new HashMap<>();
+        map.put("name", "baeldung");
+        map.put("type", "blog");
+
+        Set<String> keys = map.keySet();
+        Iterator<String> it = keys.iterator();
+        while (it.hasNext()) {
+            it.next();
+            it.remove();
+        }
+        assertEquals(0, map.size());
+    }
+
+    @Test
+    public void whenHashCodeIsCalledOnPut_thenCorrect() {
+        MyKey key = new MyKey(1, "name");
+        Map<MyKey, String> map = new HashMap<>();
+        map.put(key, "val");
+    }
+
+    @Test
+    public void whenHashCodeIsCalledOnGet_thenCorrect() {
+        MyKey key = new MyKey(1, "name");
+        Map<MyKey, String> map = new HashMap<>();
+        map.put(key, "val");
+        map.get(key);
+    }
+
+    @Test
+    public void whenGetWorks_thenCorrect() {
+        Map<String, String> map = new HashMap<>();
+        map.put("key", "val");
+
+        String val = map.get("key");
+
+        assertEquals("val", val);
+    }
+
+    @Test
+    public void givenNewKey_whenPutReturnsNull_thenCorrect() {
+        Map<String, String> map = new HashMap<>();
+
+        String rtnVal = map.put("key1", "val1");
+
+        assertNull(rtnVal);
+    }
+
+    @Test
+    public void givenUnmappedKey_whenGetReturnsNull_thenCorrect() {
+        Map<String, String> map = new HashMap<>();
+
+        String rtnVal = map.get("key1");
+
+        assertNull(rtnVal);
+    }
+
+    @Test
+    public void givenNullVal_whenPutReturnsNull_thenCorrect() {
+        Map<String, String> map = new HashMap<>();
+
+        String rtnVal = map.put("key1", null);
+
+        assertNull(rtnVal);
+    }
+
+    @Test
+    public void givenNullKeyAndVal_whenAccepts_thenCorrect() {
+        Map<String, String> map = new HashMap<>();
+
+        map.put(null, null);
+    }
+
+    @Test
+    public void givenNullVal_whenRetrieves_thenCorrect() {
+        Map<String, String> map = new HashMap<>();
+        map.put("key", null);
+
+        String val = map.get("key");
+
+        assertNull(val);
+    }
+
+    @Test
+    public void whenContainsDistinguishesNullValues_thenCorrect() {
+        Map<String, String> map = new HashMap<>();
+
+        String val1 = map.get("key");
+        boolean valPresent = map.containsKey("key");
+
+        assertNull(val1);
+        assertFalse(valPresent);
+
+        map.put("key", null);
+        String val = map.get("key");
+        valPresent = map.containsKey("key");
+
+        assertNull(val);
+        assertTrue(valPresent);
+
+    }
+
+    @Test
+    public void whenPutReturnsPrevValue_thenCorrect() {
+        Map<String, String> map = new HashMap<>();
+        map.put("key1", "val1");
+        String rtnVal = map.put("key1", "val2");
+
+        assertEquals("val1", rtnVal);
+    }
+
+    @Test
+    public void whenCallsEqualsOnCollision_thenCorrect() {
+        HashMap<MyKey, String> map = new HashMap<>();
+        MyKey k1 = new MyKey(1, "firstKey");
+        MyKey k2 = new MyKey(2, "secondKey");
+        MyKey k3 = new MyKey(2, "thirdKey");
+
+        LOG.debug("storing value for k1");
+        map.put(k1, "firstValue");
+
+        LOG.debug("storing value for k2");
+        map.put(k2, "secondValue");
+
+        LOG.debug("storing value for k3");
+        map.put(k3, "thirdValue");
+
+        LOG.debug("retrieving value for k1");
+        String v1 = map.get(k1);
+
+        LOG.debug("retrieving value for k2");
+        String v2 = map.get(k2);
+
+        LOG.debug("retrieving value for k3");
+        String v3 = map.get(k3);
+
+        assertEquals("firstValue", v1);
+        assertEquals("secondValue", v2);
+        assertEquals("thirdValue", v3);
+
+    }
+
+    @Test
+    public void givenLinkedHashMap_whenGetsOrderedKeyset_thenCorrect() {
+        LinkedHashMap<Integer, String> map = new LinkedHashMap<>();
+        map.put(1, null);
+        map.put(2, null);
+        map.put(3, null);
+        map.put(4, null);
+        map.put(5, null);
+        Set<Integer> keys = map.keySet();
+        Integer[] arr = keys.toArray(new Integer[0]);
+        for (int i = 0; i < arr.length; i++) {
+            assertEquals(new Integer(i + 1), arr[i]);
+
+        }
+    }
+
+    @Test
+    public void givenLinkedHashMap_whenAccessOrderWorks_thenCorrect() {
+        LinkedHashMap<Integer, String> map = new LinkedHashMap<>(16, .75f, true);
+        map.put(1, null);
+        map.put(2, null);
+        map.put(3, null);
+        map.put(4, null);
+        map.put(5, null);
+        Set<Integer> keys = map.keySet();
+        assertEquals("[1, 2, 3, 4, 5]", keys.toString());
+        map.get(4);
+        assertEquals("[1, 2, 3, 5, 4]", keys.toString());
+        map.get(1);
+        assertEquals("[2, 3, 5, 4, 1]", keys.toString());
+        map.get(3);
+        assertEquals("[2, 5, 4, 1, 3]", keys.toString());
+    }
+
+    @Test
+    public void givenLinkedHashMap_whenRemovesEldestEntry_thenCorrect() {
+        LinkedHashMap<Integer, String> map = new MyLinkedHashMap<>(16, .75f, true);
+        map.put(1, null);
+        map.put(2, null);
+        map.put(3, null);
+        map.put(4, null);
+        map.put(5, null);
+        Set<Integer> keys = map.keySet();
+        assertEquals("[1, 2, 3, 4, 5]", keys.toString());
+        map.put(6, null);
+        assertEquals("[2, 3, 4, 5, 6]", keys.toString());
+        map.put(7, null);
+        assertEquals("[3, 4, 5, 6, 7]", keys.toString());
+        map.put(8, null);
+        assertEquals("[4, 5, 6, 7, 8]", keys.toString());
+    }
+
+    @Test
+    public void givenTreeMap_whenOrdersEntriesNaturally_thenCorrect() {
+        TreeMap<Integer, String> map = new TreeMap<>();
+        map.put(3, "val");
+        map.put(2, "val");
+        map.put(1, "val");
+        map.put(5, "val");
+        map.put(4, "val");
+        assertEquals("[1, 2, 3, 4, 5]", map.keySet().toString());
+    }
+
+    @Test
+    public void givenTreeMap_whenOrdersEntriesNaturally_thenCorrect2() {
+        TreeMap<String, String> map = new TreeMap<>();
+        map.put("c", "val");
+        map.put("b", "val");
+        map.put("a", "val");
+        map.put("e", "val");
+        map.put("d", "val");
+
+        assertEquals("[a, b, c, d, e]", map.keySet().toString());
+    }
+
+    @Test
+    public void givenTreeMap_whenOrdersEntriesByComparator_thenCorrect() {
+        TreeMap<Integer, String> map = new TreeMap<>(Comparator.reverseOrder());
+        map.put(3, "val");
+        map.put(2, "val");
+        map.put(1, "val");
+        map.put(5, "val");
+        map.put(4, "val");
+
+        assertEquals("[5, 4, 3, 2, 1]", map.keySet().toString());
+    }
+
+    @Test
+    public void givenTreeMap_whenPerformsQueries_thenCorrect() {
+        TreeMap<Integer, String> map = new TreeMap<>();
+        map.put(3, "val");
+        map.put(2, "val");
+        map.put(1, "val");
+        map.put(5, "val");
+        map.put(4, "val");
+
+        Integer highestKey = map.lastKey();
+        Integer lowestKey = map.firstKey();
+        Set<Integer> keysLessThan3 = map.headMap(3).keySet();
+        Set<Integer> keysGreaterThanEqTo3 = map.tailMap(3).keySet();
+
+        assertEquals(new Integer(5), highestKey);
+        assertEquals(new Integer(1), lowestKey);
+        assertEquals("[1, 2]", keysLessThan3.toString());
+        assertEquals("[3, 4, 5]", keysGreaterThanEqTo3.toString());
+    }
+
+}

core-java-modules/core-java-collections-maps/src/test/java/com/baeldung/map/MapUtilUnitTest.java

@@ -0,0 +1,103 @@
+/**
+ * 
+ */
+package com.baeldung.map;
+
+import com.google.common.collect.HashBiMap;
+import org.apache.commons.collections4.BidiMap;
+import org.apache.commons.collections4.bidimap.DualHashBidiMap;
+import org.junit.Test;
+
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.stream.Collectors;
+
+import static org.junit.Assert.assertEquals;
+
+/**
+ * @author swpraman
+ *
+ */
+public class MapUtilUnitTest {
+    
+    
+    @Test
+    public void whenUsingImperativeWayForSingleKey_shouldReturnSingleKey() {
+        Map<String, String> capitalCountryMap = new HashMap<>();
+        capitalCountryMap.put("Tokyo", "Japan");
+        capitalCountryMap.put("New Delhi", "India");
+        assertEquals("New Delhi", MapUtil.getKey(capitalCountryMap, "India"));
+    }
+    
+    @Test
+    public void whenUsingImperativeWayForAllKeys_shouldReturnAllKeys() {
+        Map<String, String> capitalCountryMap = new HashMap<>();
+        capitalCountryMap.put("Tokyo", "Japan");
+        capitalCountryMap.put("Berlin", "Germany");
+        capitalCountryMap.put("Cape Town", "South Africa");
+        capitalCountryMap.put("Pretoria", "South Africa");
+        capitalCountryMap.put("Bloemfontein", "South Africa");
+
+        assertEquals(new HashSet<String>(Arrays.asList(
+                new String[] {"Cape Town", "Pretoria", "Bloemfontein"})), 
+                MapUtil.getKeys(capitalCountryMap, "South Africa"));
+    }
+    
+    @Test
+    public void whenUsingFunctionalWayForSingleKey_shouldReturnSingleKey() {
+        Map<String, String> capitalCountryMap = new HashMap<>();
+        capitalCountryMap.put("Tokyo", "Japan");
+        capitalCountryMap.put("Berlin", "Germany");
+        assertEquals("Berlin", MapUtil.keys(capitalCountryMap, "Germany").findFirst().get());
+    }
+    
+    @Test
+    public void whenUsingFunctionalWayForAllKeys_shouldReturnAllKeys() {
+        Map<String, String> capitalCountryMap = new HashMap<>();
+        capitalCountryMap.put("Tokyo", "Japan");
+        capitalCountryMap.put("Berlin", "Germany");
+        capitalCountryMap.put("Cape Town", "South Africa");
+        capitalCountryMap.put("Pretoria", "South Africa");
+        capitalCountryMap.put("Bloemfontein", "South Africa");
+        assertEquals(new HashSet<String>(Arrays.asList(
+                new String[] {"Cape Town", "Pretoria", "Bloemfontein"})), 
+                MapUtil.keys(capitalCountryMap, "South Africa").collect(Collectors.toSet()));
+    }
+    
+    @Test
+    public void whenUsingBidiMap_shouldReturnKey() {
+        BidiMap<String, String> capitalCountryMap = new DualHashBidiMap<String, String>();
+        capitalCountryMap.put("Berlin", "Germany");
+        capitalCountryMap.put("Cape Town", "South Africa");
+        assertEquals("Berlin", capitalCountryMap.getKey("Germany"));
+    }
+    
+    @Test
+    public void whenUsingBidiMapAddDuplicateValue_shouldRemoveOldEntry() {
+        BidiMap<String, String> capitalCountryMap = new DualHashBidiMap<String, String>();
+        capitalCountryMap.put("Berlin", "Germany");
+        capitalCountryMap.put("Cape Town", "South Africa");
+        capitalCountryMap.put("Pretoria", "South Africa");
+        assertEquals("Pretoria", capitalCountryMap.getKey("South Africa"));
+    }
+    
+    @Test
+    public void whenUsingBiMap_shouldReturnKey() {
+        HashBiMap<String, String> capitalCountryMap = HashBiMap.create();
+        capitalCountryMap.put("Berlin", "Germany");
+        capitalCountryMap.put("Cape Town", "South Africa");
+        assertEquals("Berlin", capitalCountryMap.inverse().get("Germany"));
+    }
+    
+    @Test(expected=IllegalArgumentException.class)
+    public void whenUsingBiMapAddDuplicateValue_shouldThrowException() {
+        HashBiMap<String, String> capitalCountryMap = HashBiMap.create();
+        capitalCountryMap.put("Berlin", "Germany");
+        capitalCountryMap.put("Cape Town", "South Africa");
+        capitalCountryMap.put("Pretoria", "South Africa");
+        assertEquals("Berlin", capitalCountryMap.inverse().get("Germany"));
+    }
+
+}

core-java-modules/core-java-collections-maps/src/test/java/com/baeldung/map/MultiValuedMapUnitTest.java

@@ -0,0 +1,227 @@
+package com.baeldung.map;
+
+import org.apache.commons.collections4.MultiMapUtils;
+import org.apache.commons.collections4.MultiSet;
+import org.apache.commons.collections4.MultiValuedMap;
+import org.apache.commons.collections4.multimap.ArrayListValuedHashMap;
+import org.apache.commons.collections4.multimap.HashSetValuedHashMap;
+import org.junit.Test;
+
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.Set;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.Assert.assertTrue;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+
+public class MultiValuedMapUnitTest {
+
+    @Test
+    public void givenMultiValuesMap_whenPuttingMultipleValuesUsingPutMethod_thenReturningAllValues() {
+        MultiValuedMap<String, String> map = new ArrayListValuedHashMap<>();
+        
+        map.put("fruits", "apple");
+        map.put("fruits", "orange");
+        
+        assertThat((Collection<String>) map.get("fruits")).containsExactly("apple", "orange");
+
+    }
+ 
+    @Test
+    public void givenMultiValuesMap_whenPuttingMultipleValuesUsingPutAllMethod_thenReturningAllValues() {
+        MultiValuedMap<String, String> map = new ArrayListValuedHashMap<>();
+       
+        map.putAll("vehicles", Arrays.asList("car", "bike"));
+        
+        assertThat((Collection<String>) map.get("vehicles")).containsExactly("car", "bike");
+
+    }
+
+    @Test
+    public void givenMultiValuesMap_whenGettingValueUsingGetMethod_thenReturningValue() {
+        MultiValuedMap<String, String> map = new ArrayListValuedHashMap<>();
+      
+        map.put("fruits", "apple");
+        
+        assertThat((Collection<String>) map.get("fruits")).containsExactly("apple");
+    }
+
+    @Test
+    public void givenMultiValuesMap_whenUsingEntriesMethod_thenReturningMappings() {
+        MultiValuedMap<String, String> map = new ArrayListValuedHashMap<>();
+        map.put("fruits", "apple");
+        map.put("fruits", "orange");
+
+        Collection<Entry<String, String>> entries = (Collection<Entry<String, String>>) map.entries();
+        
+        for(Map.Entry<String,String> entry : entries) {
+            assertThat(entry.getKey()).contains("fruits");
+            assertTrue(entry.getValue().equals("apple") || entry.getValue().equals("orange") );
+        }
+    }
+
+    @Test
+    public void givenMultiValuesMap_whenUsingKeysMethod_thenReturningAllKeys() {
+        MultiValuedMap<String, String> map = new ArrayListValuedHashMap<>();
+        map.put("fruits", "apple");
+        map.put("fruits", "orange");
+        map.put("vehicles", "car");
+        map.put("vehicles", "bike");
+        
+        MultiSet<String> keys = map.keys();
+        
+        assertThat((keys)).contains("fruits", "vehicles");
+
+    }
+
+    @Test
+    public void givenMultiValuesMap_whenUsingKeySetMethod_thenReturningAllKeys() {
+        MultiValuedMap<String, String> map = new ArrayListValuedHashMap<>();
+        map.put("fruits", "apple");
+        map.put("fruits", "orange");
+        map.put("vehicles", "car");
+        map.put("vehicles", "bike");
+        
+        Set<String> keys = map.keySet();
+        
+        assertThat(keys).contains("fruits", "vehicles");
+
+    }
+
+    @Test
+    public void givenMultiValuesMap_whenUsingValuesMethod_thenReturningAllValues() {
+        MultiValuedMap<String, String> map = new ArrayListValuedHashMap<>();
+
+        map.put("fruits", "apple");
+        map.put("fruits", "orange");
+        map.put("vehicles", "car");
+        map.put("vehicles", "bike");
+        
+        assertThat(((Collection<String>) map.values())).contains("apple", "orange", "car", "bike");
+    }
+
+    @Test
+    public void givenMultiValuesMap_whenUsingRemoveMethod_thenReturningUpdatedMap() {
+        MultiValuedMap<String, String> map = new ArrayListValuedHashMap<>();
+
+        map.put("fruits", "apple");
+        map.put("fruits", "orange");
+        map.put("vehicles", "car");
+        map.put("vehicles", "bike");
+        assertThat(((Collection<String>) map.values())).contains("apple", "orange", "car", "bike");
+        
+        map.remove("fruits");
+        
+        assertThat(((Collection<String>) map.values())).contains("car", "bike");
+
+    }
+
+    @Test
+    public void givenMultiValuesMap_whenUsingRemoveMappingMethod_thenReturningUpdatedMapAfterMappingRemoved() {
+        MultiValuedMap<String, String> map = new ArrayListValuedHashMap<>();
+
+        map.put("fruits", "apple");
+        map.put("fruits", "orange");
+        map.put("vehicles", "car");
+        map.put("vehicles", "bike");
+        assertThat(((Collection<String>) map.values())).contains("apple", "orange", "car", "bike");
+        
+        map.removeMapping("fruits", "apple");
+        
+        assertThat(((Collection<String>) map.values())).contains("orange", "car", "bike");
+    }
+
+    @Test
+    public void givenMultiValuesMap_whenUsingClearMethod_thenReturningEmptyMap() {
+        MultiValuedMap<String, String> map = new ArrayListValuedHashMap<>();
+        map.put("fruits", "apple");
+        map.put("fruits", "orange");
+        map.put("vehicles", "car");
+        map.put("vehicles", "bike");
+        assertThat(((Collection<String>) map.values())).contains("apple", "orange", "car", "bike");
+        
+        map.clear();
+  
+        assertTrue(map.isEmpty());
+    }
+
+    @Test
+    public void givenMultiValuesMap_whenUsingContainsKeyMethod_thenReturningTrue() {
+        MultiValuedMap<String, String> map = new ArrayListValuedHashMap<>();
+        map.put("fruits", "apple");
+        map.put("fruits", "orange");
+        map.put("vehicles", "car");
+        map.put("vehicles", "bike");
+        
+        assertTrue(map.containsKey("fruits"));
+    }
+
+    @Test
+    public void givenMultiValuesMap_whenUsingContainsValueMethod_thenReturningTrue() {
+        MultiValuedMap<String, String> map = new ArrayListValuedHashMap<>();
+        map.put("fruits", "apple");
+        map.put("fruits", "orange");
+        map.put("vehicles", "car");
+        map.put("vehicles", "bike");
+        
+        assertTrue(map.containsValue("orange"));
+    }
+
+    @Test
+    public void givenMultiValuesMap_whenUsingIsEmptyMethod_thenReturningFalse() {
+        MultiValuedMap<String, String> map = new ArrayListValuedHashMap<>();
+        map.put("fruits", "apple");
+        map.put("fruits", "orange");
+        map.put("vehicles", "car");
+        map.put("vehicles", "bike");
+        
+        assertFalse(map.isEmpty());
+    }
+
+    @Test
+    public void givenMultiValuesMap_whenUsingSizeMethod_thenReturningElementCount() {
+        MultiValuedMap<String, String> map = new ArrayListValuedHashMap<>();
+        map.put("fruits", "apple");
+        map.put("fruits", "orange");
+        map.put("vehicles", "car");
+        map.put("vehicles", "bike");
+        
+        assertEquals(4, map.size());
+    }
+
+    @Test
+    public void givenArrayListValuedHashMap_whenPuttingDoubleValues_thenReturningAllValues() {
+        MultiValuedMap<String, String> map = new ArrayListValuedHashMap<>();
+        map.put("fruits", "apple");
+        map.put("fruits", "orange");
+        map.put("fruits", "orange");
+        
+        assertThat((Collection<String>) map.get("fruits")).containsExactly("apple", "orange", "orange");
+    }
+
+    @Test
+    public void givenHashSetValuedHashMap_whenPuttingTwiceTheSame_thenReturningOneValue() {
+        MultiValuedMap<String, String> map = new HashSetValuedHashMap<>();
+        map.put("fruits", "apple");
+        map.put("fruits", "apple");
+  
+        assertThat((Collection<String>) map.get("fruits")).containsExactly("apple");
+    }
+
+    @Test(expected = UnsupportedOperationException.class)
+    public void givenUnmodifiableMultiValuedMap_whenInserting_thenThrowingException() {
+        MultiValuedMap<String, String> map = new ArrayListValuedHashMap<>();
+        map.put("fruits", "apple");
+        map.put("fruits", "orange");
+        MultiValuedMap<String, String> immutableMap = MultiMapUtils.unmodifiableMultiValuedMap(map);
+        
+        immutableMap.put("fruits", "banana");
+
+    }
+    
+
+}

core-java-modules/core-java-collections-set/README.md

@@ -11,3 +11,4 @@ This module contains articles about the Java Set collection
 - [Guide to EnumSet](https://www.baeldung.com/java-enumset)
 - [Set Operations in Java](https://www.baeldung.com/java-set-operations)
 - [Copying Sets in Java](https://www.baeldung.com/java-copy-sets)
+- [Immutable Set in Java](https://www.baeldung.com/java-immutable-set)

core-java-modules/core-java-collections-set/pom.xml

@@ -34,7 +34,23 @@
         </dependency>
     </dependencies>
 
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <version>${maven-compiler-plugin.version}</version>
+                <configuration>
+                    <source>${maven.compiler.source}</source>
+                    <target>${maven.compiler.target}</target>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+
     <properties>
+        <maven.compiler.source>11</maven.compiler.source>
+        <maven.compiler.target>11</maven.compiler.target>
         <commons-collections4.version>4.3</commons-collections4.version>
         <gson.version>2.8.5</gson.version>
     </properties>

core-java-modules/core-java-collections-set/src/main/java/com/baeldung/set/UnmodifiableSet.java

@@ -1,4 +1,4 @@
-package com.baeldung.java9.set;
+package com.baeldung.set;
 
 import com.google.common.collect.ImmutableSet;
 

core-java-modules/core-java-collections-set/src/test/java/com/baeldung/set/SetExamplesUnitTest.java

@@ -1,9 +1,10 @@
-package com.baeldung.java9;
+package com.baeldung.set;
+
+import org.junit.Test;
 
 import java.util.Collections;
 import java.util.HashSet;
 import java.util.Set;
-import org.junit.Test;
 
 import static org.junit.Assert.assertEquals;
 

core-java-modules/core-java-collections/README.md

@@ -3,7 +3,6 @@
 This module contains articles about Java collections
 
 ### Relevant Articles: 
-- [Collect a Java Stream to an Immutable Collection](https://www.baeldung.com/java-stream-immutable-collection)
 - [Introduction to the Java ArrayDeque](https://www.baeldung.com/java-array-deque)
 - [An Introduction to Java.util.Hashtable Class](https://www.baeldung.com/java-hash-table)
 - [Thread Safe LIFO Data Structure Implementations](https://www.baeldung.com/java-lifo-thread-safe)
@@ -13,4 +12,4 @@ This module contains articles about Java collections
 - [Defining a Char Stack in Java](https://www.baeldung.com/java-char-stack)
 - [Guide to the Java Queue Interface](https://www.baeldung.com/java-queue)
 - [An Introduction to Synchronized Java Collections](https://www.baeldung.com/java-synchronized-collections)
-- [[More -->]](/core-java-modules/core-java-collections-2)
+- [[More -->]](/core-java-modules/core-java-collections-2)

core-java-modules/core-java-concurrency-advanced-3/README.md

@@ -10,4 +10,5 @@ This module contains articles about advanced topics about multithreading with co
 - [Guide to RejectedExecutionHandler](https://www.baeldung.com/java-rejectedexecutionhandler)
 - [Guide to Work Stealing in Java](https://www.baeldung.com/java-work-stealing)
 - [Asynchronous Programming in Java](https://www.baeldung.com/java-asynchronous-programming)
+- [Java Thread Deadlock and Livelock](https://www.baeldung.com/java-deadlock-livelock)
 - [[<-- previous]](/core-java-modules/core-java-concurrency-advanced-2)

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/concurrent/volatilekeyword/TaskRunner.java

@@ -0,0 +1,25 @@
+package com.baeldung.concurrent.volatilekeyword;
+
+public class TaskRunner {
+
+    private static int number;
+    private volatile static boolean ready;
+
+    private static class Reader extends Thread {
+
+        @Override
+        public void run() {
+            while (!ready) {
+                Thread.yield();
+            }
+
+            System.out.println(number);
+        }
+    }
+
+    public static void main(String[] args) {
+        new Reader().start();
+        number = 42;
+        ready = true;
+    }
+}

core-java-modules/core-java-concurrency-basic-2/README.md

@@ -8,4 +8,5 @@ This module contains articles about basic Java concurrency
 - [Difference Between Wait and Sleep in Java](https://www.baeldung.com/java-wait-and-sleep)
 - [Guide to the Synchronized Keyword in Java](https://www.baeldung.com/java-synchronized)
 - [Life Cycle of a Thread in Java](https://www.baeldung.com/java-thread-lifecycle)
-- [[<-- Prev]](/core-java-modules/core-java-concurrency-basic)
+- [Guide to AtomicMarkableReference](https://www.baeldung.com/java-atomicmarkablereference)
+- [[<-- Prev]](/core-java-modules/core-java-concurrency-basic)

core-java-modules/core-java-concurrency-collections-2/README.md

@@ -0,0 +1,3 @@
+### Relevant Articles:
+
+- [Introduction to Lock Striping](https://www.baeldung.com/java-lock-stripping)

core-java-modules/core-java-concurrency-testing/README.md

@@ -0,0 +1,7 @@
+=========
+
+## Core Java Concurrency Testing Examples
+
+### Relevant Articles: 
+- [Testing Multi-Threaded Code in Java](https://www.baeldung.com/java-testing-multithreaded)
+

core-java-modules/core-java-concurrency-testing/pom.xml

@@ -0,0 +1,93 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>core-java-concurrency-testing</artifactId>
+    <version>0.1.0-SNAPSHOT</version>
+    <name>core-java-concurrency-testing</name>
+    <packaging>jar</packaging>
+
+    <parent>
+        <groupId>com.baeldung</groupId>
+        <artifactId>parent-java</artifactId>
+        <version>0.0.1-SNAPSHOT</version>
+        <relativePath>../../parent-java</relativePath>
+    </parent>
+	
+    <dependencies>
+        <dependency>
+            <groupId>junit</groupId>
+            <artifactId>junit</artifactId>
+            <version>4.13</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>com.googlecode.thread-weaver</groupId>
+            <artifactId>threadweaver</artifactId>
+            <version>0.2</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>com.google.code.tempus-fugit</groupId>
+            <artifactId>tempus-fugit</artifactId>
+            <version>1.1</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>com.googlecode.multithreadedtc</groupId>
+            <artifactId>multithreadedtc</artifactId>
+            <version>1.01</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.openjdk.jcstress</groupId>
+            <artifactId>jcstress-core</artifactId>
+            <version>0.5</version>
+        </dependency>
+    </dependencies>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <version>3.1</version>
+                <configuration>
+                    <compilerVersion>${javac.target}</compilerVersion>
+                    <source>${javac.target}</source>
+                    <target>${javac.target}</target>
+                </configuration>
+            </plugin>
+
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-shade-plugin</artifactId>
+                <version>2.2</version>
+                <executions>
+                    <execution>
+                        <id>main</id>
+                        <phase>package</phase>
+                        <goals>
+                            <goal>shade</goal>
+                        </goals>
+                        <configuration>
+                            <finalName>jcstress</finalName>
+                            <transformers>
+                                <transformer
+                                    implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
+                                    <mainClass>org.openjdk.jcstress.Main</mainClass>
+                                </transformer>
+                                <transformer
+                                    implementation="org.apache.maven.plugins.shade.resource.AppendingTransformer">
+                                    <resource>META-INF/TestList</resource>
+                                </transformer>
+                            </transformers>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
+        </plugins>
+    </build>
+
+</project>

core-java-modules/core-java-concurrency-testing/src/main/java/com/baeldung/concurrent/MyCounter.java

@@ -0,0 +1,22 @@
+package com.baeldung.concurrent;
+
+public class MyCounter {
+
+    private int count;
+
+    public void increment() {
+        int temp = count;
+        count = temp + 1;
+    }
+
+    public synchronized void incrementWithWait() throws InterruptedException {
+        int temp = count;
+        wait(100);
+        count = temp + 1;
+    }
+
+    public int getCount() {
+        return count;
+    }
+
+}

core-java-modules/core-java-concurrency-testing/src/main/resources/logback.xml

@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<configuration>
+    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
+        <encoder>
+            <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n
+            </pattern>
+        </encoder>
+    </appender>
+
+    <logger name="org.springframework" level="WARN" />
+    <logger name="org.springframework.transaction" level="WARN" />
+
+    <!-- in order to debug some marshalling issues, this needs to be TRACE -->
+    <logger name="org.springframework.web.servlet.mvc" level="WARN" />
+
+    <root level="INFO">
+        <appender-ref ref="STDOUT" />
+    </root>
+</configuration>

core-java-modules/core-java-concurrency-testing/src/test/java/com/baeldung/concurrent/MyCounterJCStressUnitTest.java

@@ -0,0 +1,36 @@
+package com.baeldung.concurrent;
+
+import static org.openjdk.jcstress.annotations.Expect.ACCEPTABLE;
+import static org.openjdk.jcstress.annotations.Expect.ACCEPTABLE_INTERESTING;
+
+import org.openjdk.jcstress.annotations.Actor;
+import org.openjdk.jcstress.annotations.Arbiter;
+import org.openjdk.jcstress.annotations.JCStressTest;
+import org.openjdk.jcstress.annotations.Outcome;
+import org.openjdk.jcstress.annotations.State;
+import org.openjdk.jcstress.infra.results.I_Result;
+
+@JCStressTest
+@Outcome(id = "1", expect = ACCEPTABLE_INTERESTING, desc = "One update lost.")
+@Outcome(id = "2", expect = ACCEPTABLE, desc = "Both updates.")
+@State
+public class MyCounterJCStressUnitTest {
+
+    private MyCounter counter;
+
+    @Actor
+    public void actor1() {
+        counter.increment();
+    }
+
+    @Actor
+    public void actor2() {
+        counter.increment();
+    }
+
+    @Arbiter
+    public void arbiter(I_Result r) {
+        r.r1 = counter.getCount();
+    }
+
+}

core-java-modules/core-java-concurrency-testing/src/test/java/com/baeldung/concurrent/MyCounterMultithreadedTCUnitTest.java

@@ -0,0 +1,35 @@
+package com.baeldung.concurrent;
+
+import org.junit.Test;
+
+import edu.umd.cs.mtc.MultithreadedTestCase;
+import edu.umd.cs.mtc.TestFramework;
+
+public class MyCounterMultithreadedTCUnitTest extends MultithreadedTestCase {
+
+    private MyCounter counter;
+
+    @Override
+    public void initialize() {
+        counter = new MyCounter();
+    }
+
+    public void thread1() throws InterruptedException {
+        counter.increment();
+    }
+
+    public void thread2() throws InterruptedException {
+        counter.increment();
+    }
+
+    @SuppressWarnings("deprecation")
+    @Override
+    public void finish() {
+        assertEquals(2, counter.getCount());
+    }
+
+    @Test
+    public void testCounter() throws Throwable {
+        TestFramework.runManyTimes(new MyCounterMultithreadedTCUnitTest(), 1000);
+    }
+}

core-java-modules/core-java-concurrency-testing/src/test/java/com/baeldung/concurrent/MyCounterSimpleUnitTest.java

@@ -0,0 +1,57 @@
+package com.baeldung.concurrent;
+
+import static org.junit.Assert.assertEquals;
+
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+import org.junit.Test;
+
+public class MyCounterSimpleUnitTest {
+
+    @Test
+    public void testCounter() {
+        MyCounter counter = new MyCounter();
+        for (int i = 0; i < 500; i++)
+            counter.increment();
+        assertEquals(500, counter.getCount());
+    }
+
+    // @Test
+    public void testCounterWithConcurrency() throws InterruptedException {
+        int numberOfThreads = 100;
+        ExecutorService service = Executors.newFixedThreadPool(10);
+        CountDownLatch latch = new CountDownLatch(numberOfThreads);
+        MyCounter counter = new MyCounter();
+        for (int i = 0; i < numberOfThreads; i++) {
+            service.execute(() -> {
+                counter.increment();
+                latch.countDown();
+            });
+        }
+        latch.await();
+        assertEquals(numberOfThreads, counter.getCount());
+    }
+
+    // @Test
+    public void testSummationWithConcurrencyAndWait() throws InterruptedException {
+        int numberOfThreads = 2;
+        ExecutorService service = Executors.newFixedThreadPool(10);
+        CountDownLatch latch = new CountDownLatch(numberOfThreads);
+        MyCounter counter = new MyCounter();
+        for (int i = 0; i < numberOfThreads; i++) {
+            service.submit(() -> {
+                try {
+                    counter.incrementWithWait();
+                } catch (InterruptedException e) {
+                    e.printStackTrace();
+                }
+                latch.countDown();
+            });
+        }
+        latch.await();
+        assertEquals(numberOfThreads, counter.getCount());
+    }
+
+}

core-java-modules/core-java-concurrency-testing/src/test/java/com/baeldung/concurrent/MyCounterTempusFugitUnitTest.java

@@ -0,0 +1,35 @@
+package com.baeldung.concurrent;
+
+import static org.junit.Assert.assertEquals;
+
+import org.junit.AfterClass;
+import org.junit.Rule;
+import org.junit.Test;
+
+import com.google.code.tempusfugit.concurrency.ConcurrentRule;
+import com.google.code.tempusfugit.concurrency.RepeatingRule;
+import com.google.code.tempusfugit.concurrency.annotations.Concurrent;
+import com.google.code.tempusfugit.concurrency.annotations.Repeating;
+
+public class MyCounterTempusFugitUnitTest {
+
+    @Rule
+    public ConcurrentRule concurrently = new ConcurrentRule();
+    @Rule
+    public RepeatingRule rule = new RepeatingRule();
+
+    private static MyCounter counter = new MyCounter();
+
+    @Test
+    @Concurrent(count = 2)
+    @Repeating(repetition = 10)
+    public void runsMultipleTimes() {
+        counter.increment();
+    }
+
+    @AfterClass
+    public static void annotatedTestRunsMultipleTimes() throws InterruptedException {
+        assertEquals(counter.getCount(), 20);
+    }
+
+}

core-java-modules/core-java-concurrency-testing/src/test/java/com/baeldung/concurrent/MyCounterThreadWeaverUnitTest.java

@@ -0,0 +1,42 @@
+package com.baeldung.concurrent;
+
+import static org.junit.Assert.assertEquals;
+
+import org.junit.Test;
+
+import com.google.testing.threadtester.AnnotatedTestRunner;
+import com.google.testing.threadtester.ThreadedAfter;
+import com.google.testing.threadtester.ThreadedBefore;
+import com.google.testing.threadtester.ThreadedMain;
+import com.google.testing.threadtester.ThreadedSecondary;
+
+public class MyCounterThreadWeaverUnitTest {
+
+    private MyCounter counter;
+
+    @ThreadedBefore
+    public void before() {
+        counter = new MyCounter();
+    }
+
+    @ThreadedMain
+    public void mainThread() {
+        counter.increment();
+    }
+
+    @ThreadedSecondary
+    public void secondThread() {
+        counter.increment();
+    }
+
+    @ThreadedAfter
+    public void after() {
+        assertEquals(2, counter.getCount());
+    }
+
+    @Test
+    public void testCounter() {
+        new AnnotatedTestRunner().runTests(this.getClass(), MyCounter.class);
+    }
+
+}

core-java-modules/core-java-concurrency-testing/src/test/resources/.gitignore

@@ -0,0 +1,13 @@
+*.class
+
+#folders#
+/target
+/neoDb*
+/data
+/src/main/webapp/WEB-INF/classes
+*/META-INF/*
+
+# Packaged files #
+*.jar
+*.war
+*.ear

core-java-modules/core-java-date-operations-2/README.md

@@ -7,4 +7,5 @@ This module contains articles about date operations in Java.
 - [Checking If Two Java Dates Are on the Same Day](https://www.baeldung.com/java-check-two-dates-on-same-day)
 - [Converting Java Date to OffsetDateTime](https://www.baeldung.com/java-convert-date-to-offsetdatetime)
 - [How to Set the JVM Time Zone](https://www.baeldung.com/java-jvm-time-zone)
+- [How to determine day of week by passing specific date in Java?](https://www.baeldung.com/java-get-day-of-week)
 - [[<-- Prev]](/core-java-modules/core-java-date-operations-1)

core-java-modules/core-java-io-2/README.md

@@ -12,4 +12,5 @@ This module contains articles about core Java input and output (IO)
 - [Java – Append Data to a File](https://www.baeldung.com/java-append-to-file)
 - [How to Copy a File with Java](https://www.baeldung.com/java-copy-file)
 - [Create a Directory in Java](https://www.baeldung.com/java-create-directory) 
+- [Java IO vs NIO](https://www.baeldung.com/java-io-vs-nio)
 - [[<-- Prev]](/core-java-modules/core-java-io)

core-java-modules/core-java-lambdas/src/test/java/com/baeldung/java8/Java8SortUnitTest.java

@@ -1,18 +1,17 @@
 package com.baeldung.java8;
 
-import static org.hamcrest.Matchers.equalTo;
+import com.baeldung.java8.entity.Human;
+import com.google.common.collect.Lists;
+import com.google.common.primitives.Ints;
+import org.junit.Assert;
+import org.junit.Test;
 
 import java.util.Collections;
 import java.util.Comparator;
 import java.util.List;
 import java.util.stream.Collectors;
 
-import org.junit.Assert;
-import org.junit.Test;
-
-import com.baeldung.java8.entity.Human;
-import com.google.common.collect.Lists;
-import com.google.common.primitives.Ints;
+import static org.hamcrest.Matchers.equalTo;
 
 public class Java8SortUnitTest {
 
@@ -113,11 +112,11 @@ public class Java8SortUnitTest {
         humans.sort(Comparator.comparing(Human::getName));
         Assert.assertThat(humans.get(0), equalTo(new Human("Jack", 12)));
     }
-    
+
     @Test
     public final void givenStreamNaturalOrdering_whenSortingEntitiesByName_thenCorrectlySorted() {
         final List<String> letters = Lists.newArrayList("B", "A", "C");
-        
+
         final List<String> sortedLetters = letters.stream().sorted().collect(Collectors.toList());
         Assert.assertThat(sortedLetters.get(0), equalTo("A"));
     }
@@ -126,7 +125,7 @@ public class Java8SortUnitTest {
     public final void givenStreamCustomOrdering_whenSortingEntitiesByName_thenCorrectlySorted() {
         final List<Human> humans = Lists.newArrayList(new Human("Sarah", 10), new Human("Jack", 12));
         final Comparator<Human> nameComparator = (h1, h2) -> h1.getName().compareTo(h2.getName());
-        
+
         final List<Human> sortedHumans = humans.stream().sorted(nameComparator).collect(Collectors.toList());
         Assert.assertThat(sortedHumans.get(0), equalTo(new Human("Jack", 12)));
     }
@@ -164,4 +163,48 @@ public class Java8SortUnitTest {
         Assert.assertThat(reverseSortedHumans.get(0), equalTo(new Human("Sarah", 10)));
     }
 
+    @Test(expected = NullPointerException.class)
+    public final void givenANullElement_whenSortingEntitiesByName_thenThrowsNPE() {
+        final List<Human> humans = Lists.newArrayList(null, new Human("Jack", 12));
+
+        humans.sort((h1, h2) -> h1.getName().compareTo(h2.getName()));
+    }
+
+    @Test
+    public final void givenANullElement_whenSortingEntitiesByNameManually_thenMovesTheNullToLast() {
+        final List<Human> humans = Lists.newArrayList(null, new Human("Jack", 12), null);
+
+        humans.sort((h1, h2) -> {
+            if (h1 == null) return h2 == null ? 0 : 1;
+            else if (h2 == null) return -1;
+
+            return h1.getName().compareTo(h2.getName());
+        });
+
+        Assert.assertNotNull(humans.get(0));
+        Assert.assertNull(humans.get(1));
+        Assert.assertNull(humans.get(2));
+    }
+
+    @Test
+    public final void givenANullElement_whenSortingEntitiesByName_thenMovesTheNullToLast() {
+        final List<Human> humans = Lists.newArrayList(null, new Human("Jack", 12), null);
+
+        humans.sort(Comparator.nullsLast(Comparator.comparing(Human::getName)));
+
+        Assert.assertNotNull(humans.get(0));
+        Assert.assertNull(humans.get(1));
+        Assert.assertNull(humans.get(2));
+    }
+
+    @Test
+    public final void givenANullElement_whenSortingEntitiesByName_thenMovesTheNullToStart() {
+        final List<Human> humans = Lists.newArrayList(null, new Human("Jack", 12), null);
+
+        humans.sort(Comparator.nullsFirst(Comparator.comparing(Human::getName)));
+
+        Assert.assertNull(humans.get(0));
+        Assert.assertNull(humans.get(1));
+        Assert.assertNotNull(humans.get(2));
+    }
 }

core-java-modules/core-java-lang-2/README.md

@@ -10,4 +10,5 @@ This module contains articles about core features in the Java language
 - [How to Return Multiple Values From a Java Method](https://www.baeldung.com/java-method-return-multiple-values)
 - [Guide to the Java finally Keyword](https://www.baeldung.com/java-finally-keyword)
 - [The Java Headless Mode](https://www.baeldung.com/java-headless-mode)
+- [Comparing Long Values in Java](https://www.baeldung.com/java-compare-long-values)
 - [[<-- Prev]](/core-java-modules/core-java-lang)

core-java-modules/core-java-lang-2/src/test/java/com/baeldung/comparelong/CompareLongUnitTest.java

@@ -0,0 +1,84 @@
+package com.baeldung.comparelong;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatCode;
+
+import org.junit.Test;
+
+import java.util.Objects;
+
+public class CompareLongUnitTest {
+
+    @Test
+    public void givenLongValuesLessThan128_whenUsingReferenceComparater_thenSuccess() {
+
+        Long l1 = 127L;
+        Long l2 = 127L;
+
+        assertThat(l1 == l2).isTrue();
+    }
+
+    @Test
+    public void givenLongValuesGreaterOrEqualsThan128_whenUsingReferenceComparater_thenFails() {
+
+        Long l1 = 128L;
+        Long l2 = 128L;
+
+        assertThat(l1 == l2).isFalse();
+    }
+
+    @Test
+    public void givenLongValuesGreaterOrEqualsThan128_whenUsingEquals_thenSuccess() {
+
+        Long l1 = 128L;
+        Long l2 = 128L;
+
+        assertThat(l1.equals(l2)).isTrue();
+    }
+
+    @Test
+    public void givenLongValuesLessThan128_whenUsingObjectsEquals_thenSuccess() {
+
+        Long l1 = 127L;
+        Long l2 = 127L;
+
+        assertThat(Objects.equals(l1, l2)).isTrue();
+    }
+
+    @Test
+    public void givenLongValuesGreaterOrEqualsThan128_whenUsingObjectsEquals_thenSuccess() {
+
+        Long l1 = 128L;
+        Long l2 = 128L;
+
+        assertThat(Objects.equals(l1, l2)).isTrue();
+    }
+
+    @Test
+    public void givenNullReference_whenUsingObjectsEquals_thenNoException() {
+
+        Long l1 = null;
+        Long l2 = 128L;
+
+        assertThatCode(() -> Objects.equals(l1, l2)).doesNotThrowAnyException();
+    }
+
+    @Test
+    public void givenLongValuesGreaterOrEqualsThan128_whenUsingComparisonOperator_andLongValue_thenSuccess() {
+
+        Long l1 = 128L;
+        Long l2 = 128L;
+
+        assertThat(l1.longValue() == l2.longValue()).isTrue();
+    }
+
+    @Test
+    public void givenLongValuesGreaterOrEqualsThan128_whenUsingCasting_thenSuccess() {
+
+        Long l1 = 128L;
+        Long l2 = 128L;
+
+        assertThat((long) l1 == (long) l2).isTrue();
+    }
+
+}

core-java-modules/core-java-perf/src/main/java/com/baeldung/jmx/JMXTutorialMainlauncher.java

@@ -3,7 +3,12 @@ package com.baeldung.jmx;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-import javax.management.*;
+import javax.management.InstanceAlreadyExistsException;
+import javax.management.MBeanRegistrationException;
+import javax.management.MBeanServer;
+import javax.management.MalformedObjectNameException;
+import javax.management.NotCompliantMBeanException;
+import javax.management.ObjectName;
 import java.lang.management.ManagementFactory;
 
 public class JMXTutorialMainlauncher {
@@ -11,24 +16,21 @@ public class JMXTutorialMainlauncher {
     private static final Logger LOG = LoggerFactory.getLogger(JMXTutorialMainlauncher.class);
 
     public static void main(String[] args) {
-        // TODO Auto-generated method stub
 
         LOG.debug("This is basic JMX tutorial");
-        ObjectName objectName = null;
+
         try {
-            objectName = new ObjectName("com.baeldung.tutorial:type=basic,name=game");
-        } catch (MalformedObjectNameException e) {
-            e.printStackTrace();
-        }
-        MBeanServer server = ManagementFactory.getPlatformMBeanServer();
-        Game gameObj = new Game();
-        try {
-            server.registerMBean(gameObj, objectName);
-        } catch (InstanceAlreadyExistsException | MBeanRegistrationException | NotCompliantMBeanException e) {
+            ObjectName objectName = new ObjectName("com.baeldung.tutorial:type=basic,name=game");
+            MBeanServer server = ManagementFactory.getPlatformMBeanServer();
+            server.registerMBean(new Game(), objectName);
+        } catch (MalformedObjectNameException | InstanceAlreadyExistsException |
+                MBeanRegistrationException | NotCompliantMBeanException e) {
             e.printStackTrace();
         }
+
         LOG.debug("Registration for Game mbean with the platform server is successfull");
         LOG.debug("Please open jconsole to access Game mbean");
+
         while (true) {
             // to ensure application does not terminate
         }

core-java-modules/core-java-regex/README.md

@@ -9,3 +9,4 @@
 - [Pre-compile Regex Patterns Into Pattern Objects](https://www.baeldung.com/java-regex-pre-compile)
 - [Difference Between Java Matcher find() and matches()](https://www.baeldung.com/java-matcher-find-vs-matches)
 - [How to Use Regular Expressions to Replace Tokens in Strings](https://www.baeldung.com/java-regex-token-replacement)
+- [Regular Expressions \s and \s+ in Java](https://www.baeldung.com/java-regex-s-splus)

core-java-modules/core-java-security-2/README.md

@@ -1,3 +1,12 @@
+## Core Java Security 
+
+This module contains articles about core Java Security
+
 ### Relevant Articles:
 
 - [Guide To The Java Authentication And Authorization Service (JAAS)](https://www.baeldung.com/java-authentication-authorization-service)
+- [MD5 Hashing in Java](http://www.baeldung.com/java-md5)
+- [Hashing a Password in Java](https://www.baeldung.com/java-password-hashing)
+- [SHA-256 and SHA3-256 Hashing in Java](https://www.baeldung.com/sha-256-hashing-java)
+- [Checksums in Java](https://www.baeldung.com/java-checksums)
+- More articles: [[<-- prev]](/core-java-modules/core-java-security)

core-java-modules/core-java-security-2/pom.xml

@@ -16,4 +16,45 @@
         <relativePath>../../parent-java</relativePath>
     </parent>
 
+    <dependencies>
+
+        <dependency>
+            <groupId>commons-codec</groupId>
+            <artifactId>commons-codec</artifactId>
+            <version>${commons-codec.version}</version>
+        </dependency>
+        
+        <dependency>
+            <groupId>org.bouncycastle</groupId>
+            <artifactId>bcprov-jdk15on</artifactId>
+            <version>${bouncycastle.version}</version>
+        </dependency>
+        
+         <!-- test scoped -->
+        <dependency>
+            <groupId>org.assertj</groupId>
+            <artifactId>assertj-core</artifactId>
+            <version>${assertj-core.version}</version>
+            <scope>test</scope>
+        </dependency>
+
+        <!-- https://mvnrepository.com/artifact/javax.xml.bind/jaxb-api -->
+        <dependency>
+            <groupId>javax.xml.bind</groupId>
+            <artifactId>jaxb-api</artifactId>
+            <version>2.3.1</version>
+        </dependency>
+
+
+    </dependencies>
+    
+    <properties>
+        <!-- util -->
+        <bouncycastle.version>1.60</bouncycastle.version>
+        <commons-codec.version>1.11</commons-codec.version>
+
+        <!-- testing -->
+        <assertj-core.version>3.10.0</assertj-core.version>
+    </properties>
+
 </project>

core-java-modules/core-java-security-2/src/main/java/com/baeldung/checksums/ChecksumUtils.java

@@ -0,0 +1,23 @@
+package com.baeldung.checksums;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.zip.CRC32;
+import java.util.zip.CheckedInputStream;
+import java.util.zip.Checksum;
+
+public class ChecksumUtils {
+
+    public static long getChecksumCRC32(byte[] bytes) {
+        Checksum crc32 = new CRC32();
+        crc32.update(bytes, 0, bytes.length);
+        return crc32.getValue();
+    }
+
+    public static long getChecksumCRC32(InputStream stream, int bufferSize) throws IOException {
+        CheckedInputStream checkedInputStream = new CheckedInputStream(stream, new CRC32());
+        byte[] buffer = new byte[bufferSize];
+        while (checkedInputStream.read(buffer, 0, buffer.length) >= 0) {}
+        return checkedInputStream.getChecksum().getValue();
+    }
+}

core-java-modules/core-java-security-2/src/test/java/com/baeldung/checksums/ChecksumUtilsUnitTest.java

@@ -0,0 +1,51 @@
+package com.baeldung.checksums;
+
+
+import org.junit.Before;
+import org.junit.Test;
+
+import java.io.ByteArrayInputStream;
+import java.io.IOException;
+import java.io.InputStream;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class ChecksumUtilsUnitTest {
+
+    byte[] arr;
+
+    @Before
+    void setUp() {
+        arr =  new byte[]{0,10,21,20,35,40,120,56,72,22};
+    }
+
+    @Test
+    void givenByteArray_whenChecksumCreated_checkCorrect() {
+
+        long checksum = ChecksumUtils.getChecksumCRC32(arr);
+
+        assertEquals(3915397664L, checksum);
+    }
+
+    @Test
+    void givenTwoDifferentStrings_whenChecksumCreated_checkCollision() {
+
+        String plumless = "plumless";
+        String buckeroo = "buckeroo";
+
+        long plumlessChecksum = ChecksumUtils.getChecksumCRC32(plumless.getBytes());
+        long buckerooChecksum = ChecksumUtils.getChecksumCRC32(buckeroo.getBytes());
+
+        assertEquals(plumlessChecksum, buckerooChecksum);
+    }
+
+    @Test
+    void givenInputString_whenChecksumCreated_checkCorrect() throws IOException {
+
+        InputStream inputStream = new ByteArrayInputStream(arr);
+        long checksum = ChecksumUtils.getChecksumCRC32(inputStream, 10);
+
+        assertEquals(3915397664L, checksum);
+
+    }
+}

core-java-modules/core-java-security/README.md

@@ -3,17 +3,16 @@
 This module contains articles about core Java Security
 
 ### Relevant Articles: 
-- [MD5 Hashing in Java](http://www.baeldung.com/java-md5)
+
 - [Guide to the Cipher Class](http://www.baeldung.com/java-cipher-class)
 - [Introduction to SSL in Java](http://www.baeldung.com/java-ssl)
 - [Java KeyStore API](http://www.baeldung.com/java-keystore)
 - [Encrypting and Decrypting Files in Java](http://www.baeldung.com/java-cipher-input-output-stream)
-- [Hashing a Password in Java](https://www.baeldung.com/java-password-hashing)
 - [SSL Handshake Failures](https://www.baeldung.com/java-ssl-handshake-failures)
-- [SHA-256 and SHA3-256 Hashing in Java](https://www.baeldung.com/sha-256-hashing-java)
 - [Enabling TLS v1.2 in Java 7](https://www.baeldung.com/java-7-tls-v12)
 - [The Java SecureRandom Class](https://www.baeldung.com/java-secure-random)
 - [An Introduction to Java SASL](https://www.baeldung.com/java-sasl)
 - [A Guide to Java GSS API](https://www.baeldung.com/java-gss)
 - [Intro to the Java SecurityManager](https://www.baeldung.com/java-security-manager)
+- More articles: [[next -->]](/core-java-modules/core-java-security-2)
 

core-java-modules/core-java-security/pom.xml

@@ -24,24 +24,9 @@
             <version>${assertj-core.version}</version>
             <scope>test</scope>
         </dependency>
-
-        <dependency>
-            <groupId>commons-codec</groupId>
-            <artifactId>commons-codec</artifactId>
-            <version>${commons-codec.version}</version>
-        </dependency>
-        <dependency>
-            <groupId>org.bouncycastle</groupId>
-            <artifactId>bcprov-jdk15on</artifactId>
-            <version>${bouncycastle.version}</version>
-        </dependency>
     </dependencies>
 
     <properties>
-        <!-- util -->
-        <bouncycastle.version>1.60</bouncycastle.version>
-        <commons-codec.version>1.11</commons-codec.version>
-
         <!-- testing -->
         <assertj-core.version>3.10.0</assertj-core.version>
     </properties>

core-java-modules/core-java-streams-3/README.md

@@ -9,4 +9,5 @@ This module contains articles about the Stream API in Java.
 - [Guide to Java 8’s Collectors](https://www.baeldung.com/java-8-collectors)
 - [Primitive Type Streams in Java 8](https://www.baeldung.com/java-8-primitive-streams)
 - [Debugging Java 8 Streams with IntelliJ](https://www.baeldung.com/intellij-debugging-java-streams)
+- [Add BigDecimals using the Stream API](https://www.baeldung.com/java-stream-add-bigdecimals)
 - More articles: [[<-- prev>]](/../core-java-streams-2)

core-java-modules/core-java-streams-3/src/test/java/com/baeldung/streams/bigdecimals/AddNumbersUnitTest.java

@@ -0,0 +1,42 @@
+package com.baeldung.streams.bigdecimals;
+
+import static org.junit.Assert.assertEquals;
+
+import java.math.BigDecimal;
+import java.util.Arrays;
+import java.util.List;
+import java.util.stream.IntStream;
+import java.util.stream.Stream;
+
+import org.junit.Test;
+
+public class AddNumbersUnitTest {
+
+    @Test
+    public void givenIntStream_whenSum_thenResultIsCorrect() {
+        IntStream intNumbers = IntStream.range(0, 3);
+        assertEquals(3, intNumbers.sum());
+    }
+
+    @Test
+    public void givenCollectionOfDouble_whenUsingMapToDoubleToSum_thenResultIsCorrect() {
+        List<Double> doubleNumbers = Arrays.asList(23.48, 52.26, 13.5);
+        double result = doubleNumbers.stream()
+            .mapToDouble(Double::doubleValue)
+            .sum();
+        assertEquals(89.24, result, .1);
+    }
+
+    public void givenStreamOfIntegers_whenUsingReduceToSum_thenResultIsCorrect() {
+        Stream<Integer> intNumbers = Stream.of(0, 1, 2);
+        int result = intNumbers.reduce(0, Integer::sum);
+        assertEquals(106, result);
+    }
+
+    public void givenStreamOfBigDecimals_whenUsingReduceToSum_thenResultIsCorrect() {
+        Stream<BigDecimal> bigDecimalNumber = Stream.of(BigDecimal.ZERO, BigDecimal.ONE, BigDecimal.TEN);
+        BigDecimal result = bigDecimalNumber.reduce(BigDecimal.ZERO, BigDecimal::add);
+        assertEquals(11, result);
+    }
+
+}

core-java-modules/core-java-string-operations-2/README.md

@@ -11,4 +11,5 @@ This module contains articles about string operations.
 - [Case-Insensitive String Matching in Java](https://www.baeldung.com/java-case-insensitive-string-matching)
 - [L-Trim and R-Trim in Java](https://www.baeldung.com/l-trim-and-r-trim-in-java)
 - [L-Trim and R-Trim Alternatives in Java](https://www.baeldung.com/java-trim-alternatives)
+- [Java Convert PDF to Base64](https://www.baeldung.com/java-convert-pdf-to-base64)
 - More articles: [[<-- prev]](../core-java-string-operations)

core-java-modules/core-java-string-operations/src/main/java/com/baeldung/newline/AddingNewLineToString.java

@@ -38,6 +38,11 @@ public class AddingNewLineToString {
 		System.out.println("6. Using System.getProperty(\"line.separator\")");
 		rhyme = line1 + System.getProperty("line.separator") + line2;
 		System.out.println(rhyme);
+                
+                //7. Using %n
+                System.out.println("7. Using %n");
+                rhyme = "Humpty Dumpty sat on a wall.%nHumpty Dumpty had a great fall.";
+                System.out.println(rhyme);
 		
 		System.out.println("***HTML to rendered in a browser***");
 		//1. Line break for HTML using <br>

core-java-modules/core-java-strings/README.md

@@ -12,3 +12,4 @@ This module contains articles about strings in Java.
 - [Java String Interview Questions and Answers](https://www.baeldung.com/java-string-interview-questions)
 - [Java Multi-line String](https://www.baeldung.com/java-multiline-string)
 - [Guide to Java String Pool](https://www.baeldung.com/java-string-pool)
+- [Fixing “constant string too long” Build Error](https://www.baeldung.com/java-constant-string-too-long-error)

core-java-modules/pom.xml

@@ -42,7 +42,10 @@
         <module>core-java-collections-list</module>
         <module>core-java-collections-list-2</module>
         <module>core-java-collections-list-3</module>
-        <module>core-java-collections-set</module>
+        <module>core-java-collections-maps</module>
+        <module>core-java-collections-maps-2</module>
+        <module>core-java-collections-maps-3</module>
+        <!-- <module>core-java-collections-set</module> --> <!-- We haven't upgraded to java 11. Fixing in BAEL-10841 -->
 
         <module>core-java-concurrency-2</module>
         <module>core-java-concurrency-advanced</module>