Grouping by collector java (#10986)

* Initial Commit - Ports and Adapters

* Restructured code to implement input and outbou ports and adapters

* Added tests for groupingBy Collector

* Revert "Initial Commit - Ports and Adapters"

This reverts commit 55152dea

* Added tests for groupingBy Collector

* reverting

* corrected typo

* used BDD naming strategy for tests

* Added a test for groupingByConcurrent

* added new module and package

* added core-java-streams-4 module in main pom.xml

* updated pom.xml and changed artifactId

core-java-modules/core-java-streams-4/src/test/java/com/baeldung/streamcollectors/StreamGroupingByCollectorUnitTest.java

@@ -0,0 +1,99 @@
+package com.baeldung.streamcollectors;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.Map;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class StreamGroupingByCollectorUnitTest
+{
+        static class Employee {
+
+                Integer employeeId;
+                String employeeName;
+                Integer departmentId;
+
+                Employee(Integer employeeId, String employeeName, Integer departmentId) {
+                        this.employeeId = employeeId;
+                        this.employeeName = employeeName;
+                        this.departmentId = departmentId;
+                }
+
+                public Integer getEmployeeId() {
+                        return employeeId;
+                }
+
+                public void setEmployeeId(Integer employeeId) {
+                        this.employeeId = employeeId;
+                }
+
+                public String getEmployeeName() {
+                        return employeeName;
+                }
+
+                public void setEmployeeName(String employeeName) {
+                        this.employeeName = employeeName;
+                }
+
+                public Integer getDepartmentId() {
+                        return departmentId;
+                }
+
+                public void setDepartmentId(Integer departmentId) {
+                        this.departmentId = departmentId;
+                }
+        }
+
+        @Test
+        public void
+        givenListOfStrings_whenGroupingEqualStrings_thenUseCollectorsGroupingByToGroupEqualStringsAndCountOfOccurrences()  {
+                List<String> list = new ArrayList<>(
+                    Arrays.asList("Foo", "Bar", "Bar", "Foo", "Bar")
+                );
+
+                Map<String, Long> result = list.stream().collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));
+                Assert.assertEquals(new Long(2), result.get("Foo"));
+                Assert.assertEquals(new Long(3), result.get("Bar"));
+
+        }
+
+        @Test
+        public void
+        givenListOfStrings_whenGroupingEqualLengthStrings_thenUseCollectorsGroupingByConcurrentToGroupEqualLengthStringsAndCountOfOccurrences()  {
+
+                List<String> list = new ArrayList<>(
+                    Arrays.asList("Adam", "Bill", "Jack", "Joe", "Ian")
+                );
+
+                Map<Integer, Long> result = list.stream()
+                    .collect(Collectors.groupingByConcurrent(String::length, Collectors.counting()));
+                Assert.assertEquals(new Long(2), result.get(3));
+                Assert.assertEquals(new Long(3), result.get(4));
+
+        }
+
+
+        @Test
+        public void
+        givenListOfEmployees_whenGroupingDepartmentId_thenUseCollectorsGroupingByDepartmentIdAndCountNumberOfEmployeesWithinEveryDepartment()  {
+
+                List<Employee> list = new ArrayList<>(
+                    Arrays.asList(new Employee(1, "Joe", 1),
+                                  new Employee(2, "Josh", 1),
+                                  new Employee(3, "Jamie", 2),
+                                  new Employee(4, "Jim", 2),
+                                  new Employee(5, "Jack", 2))
+                );
+
+                Map<Integer, Long> result = list.stream() .collect(Collectors.groupingBy(Employee::getDepartmentId, Collectors.counting()));
+                Assert.assertEquals(new Long(2), result.get(1));
+                Assert.assertEquals(new Long(3), result.get(2));
+
+        }
+
+}