Java 16961 (#13236)

* [JAVA-16961] Moved code for article "Create an Empty Map in Java"

* [JAVA-16961] Alter README.md

* [JAVA-16961] Moved code for article "Sorting a Hashset in Java"

* [JAVA-16961] Added links to README.md files

* [JAVA-16961] Revert link changes

* [JAVA-16961] Replaced junit4 with junit5 annotations

* [JAVA-16961] test build

* [JAVA-16961] Added junit annotations

* [JAVA-16961] Added links

Co-authored-by: panagiotiskakos <panagiotis.kakos@libra-is.com>

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

@@ -0,0 +1,62 @@
+package com.baeldung.map;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.LinkedHashMap;
+import java.util.Map;
+import java.util.NavigableMap;
+import java.util.SortedMap;
+import java.util.TreeMap;
+
+import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.Maps;
+
+public class EmptyMapInitializer {
+
+  public static Map<String, String> articleMap;
+  static {
+    articleMap = new HashMap<>();
+  }
+
+  public static Map<String, String> createEmptyMap() {
+    return Collections.emptyMap();
+  }
+
+  public void createMapUsingConstructors() {
+    Map hashMap = new HashMap();
+    Map linkedHashMap = new LinkedHashMap();
+    Map treeMap = new TreeMap();
+  }
+
+  public Map<String, String> createEmptyMapUsingMapsObject() {
+    Map<String, String> emptyMap = Maps.newHashMap();
+    return emptyMap;
+  }
+
+  public Map createGenericEmptyMapUsingGuavaMapsObject() {
+    Map genericEmptyMap = Maps.<String, Integer>newHashMap();
+    return genericEmptyMap;
+  }
+
+  public static Map<String, String> createMapUsingGuava() {
+    Map<String, String> emptyMapUsingGuava =
+        Maps.newHashMap(ImmutableMap.of());
+    return emptyMapUsingGuava;
+  }
+
+  public static Map<String, String> createImmutableMapUsingGuava() {
+	    Map<String, String> emptyImmutableMapUsingGuava = ImmutableMap.of();
+	    return emptyImmutableMapUsingGuava;
+  }
+  
+  public SortedMap<String, String> createEmptySortedMap() {
+    SortedMap<String, String> sortedMap = Collections.emptySortedMap();
+    return sortedMap;
+  }
+
+  public NavigableMap<String, String> createEmptyNavigableMap() {
+    NavigableMap<String, String> navigableMap = Collections.emptyNavigableMap();
+    return navigableMap;
+  }
+
+}

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

@@ -0,0 +1,37 @@
+package com.baeldung.map;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.Map;
+
+import org.junit.Test;
+
+public class EmptyMapInitializerUnitTest {
+
+  @Test(expected=UnsupportedOperationException.class)
+  public void givenEmptyMap_whenAddingEntries_throwsException() {
+    Map<String, String> map = EmptyMapInitializer.createEmptyMap();
+    map.put("key", "value");
+  }
+
+  @Test
+  public void givenEmptyMap_whenChecked_returnsTrue() {
+    assertTrue(EmptyMapInitializer.articleMap.isEmpty());
+  }
+
+  @Test
+  public void givenEmptyMap_whenCreatedUsingGuava_returnsEmptyOrNot() {
+    Map<String, String> emptyMapUsingGuava =
+        EmptyMapInitializer.createMapUsingGuava();
+    assertTrue(emptyMapUsingGuava.isEmpty());
+    emptyMapUsingGuava.put("key", "value");
+    assertFalse(emptyMapUsingGuava.isEmpty());
+  }
+
+  @Test(expected=UnsupportedOperationException.class)
+  public void givenImmutableEmptyMapUsingGuava_whenAddingEntries_throwsException() {
+    Map<String, String> map = EmptyMapInitializer.createImmutableMapUsingGuava();
+    map.put("key", "value");
+  }
+}