BAEL-4714 - Java Concurrent HashSet equivalent to ConcurrentHashMap (#11721)

* add article code and unit tests

* fix PMD rule violation

Co-authored-by: vlad fernoaga <vlad.fernoaga-external@getyourguide.com>

core-java-modules/core-java-concurrency-collections-2/src/test/java/com/baeldung/concurrenthashset/ConcurrentHashSetUnitTest.java

@@ -0,0 +1,100 @@
+package com.baeldung.concurrenthashset;
+
+import static java.util.Arrays.asList;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.CopyOnWriteArraySet;
+
+import org.junit.jupiter.api.Test;
+
+public class ConcurrentHashSetUnitTest {
+
+    @Test
+    public void whenCreateConcurrentHashSetWithStaticMethod_thenSetIsCreated() {
+        // when
+        Set<Integer> threadSafeUniqueNumbers = ConcurrentHashMap.newKeySet();
+        threadSafeUniqueNumbers.add(23);
+        threadSafeUniqueNumbers.add(45);
+
+        //then
+        assertTrue(threadSafeUniqueNumbers.stream()
+            .anyMatch(entry -> entry.equals(23)));
+        assertTrue(threadSafeUniqueNumbers.stream()
+            .anyMatch(entry -> entry.equals(45)));
+    }
+
+    @Test
+    public void whenCreateConcurrentHashSetWithKeySetMethod_thenSetIsSyncedWithMapped() {
+        // when
+        ConcurrentHashMap<Integer, String> numbersMap = new ConcurrentHashMap<>();
+        Set<Integer> numbersSet = numbersMap.keySet();
+
+        numbersMap.put(1, "One");
+        numbersMap.put(2, "Two");
+        numbersMap.put(3, "Three");
+
+        System.out.println("Map before remove: " + numbersMap);
+        System.out.println("Set before remove: " + numbersSet);
+
+        numbersSet.remove(2);
+
+        System.out.println("Set after remove: " + numbersSet);
+        System.out.println("Map after remove: " + numbersMap);
+
+        //then
+        assertNull(numbersMap.get(2));
+    }
+
+    @Test
+    public void whenCreateConcurrentHashSetWithKeySetMethodDefaultValue_thenSetIsSyncedWithMapped() {
+        // when
+        ConcurrentHashMap<Integer, String> numbersMap = new ConcurrentHashMap<>();
+        Set<Integer> numbersSet = numbersMap.keySet("SET-ENTRY");
+
+        numbersMap.put(1, "One");
+        numbersMap.put(2, "Two");
+        numbersMap.put(3, "Three");
+
+        System.out.println("Map before add: " + numbersMap);
+        System.out.println("Set before add: " + numbersSet);
+
+        numbersSet.addAll(asList(4, 5));
+
+        System.out.println("Map after add: " + numbersMap);
+        System.out.println("Set after add: " + numbersSet);
+
+        //then
+        assertEquals("One", numbersMap.get(1));
+        assertEquals("SET-ENTRY", numbersMap.get(4));
+        assertEquals("SET-ENTRY", numbersMap.get(5));
+    }
+
+    @Test
+    public void whenSynchronizedSetIsCreated_thenSetIsCreated() {
+        // when
+        Set<Integer> syncNumbers = Collections.synchronizedSet(new HashSet<>());
+        syncNumbers.add(1);
+
+        //then
+        assertTrue(syncNumbers.stream()
+            .anyMatch(entry -> entry.equals(1)));
+    }
+
+    @Test
+    public void whenCopyOnWriteArraySetIsCreated_thenSetIsCreated() {
+        // when
+        Set<Integer> copyOnArraySet = new CopyOnWriteArraySet<>();
+        copyOnArraySet.add(1);
+
+        //then
+        assertTrue(copyOnArraySet.stream()
+            .anyMatch(entry -> entry.equals(1)));
+    }
+
+}