[BAEL-4495] Performance of removeAll() in a HashSet (#10011)

* [BAEL-4495] Performance of removeAll() in a HashSet

* [BAEL-4495] Add unit test for hashset removeAll()

* [BAEL-4495] Update test methods to camelCase

core-java-modules/core-java-collections-3/src/test/java/com/baeldung/collections/hashset/HashSetUnitTest.java

@@ -0,0 +1,33 @@
+package com.baeldung.collections.hashset;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.HashSet;
+import java.util.Set;
+
+import org.junit.jupiter.api.Test;
+
+class HashSetUnitTest {
+
+    @Test
+    void whenRemoveAllFromHashset_thenRemovesAllElementsFromHashsetThatArePresentInCollection() {
+        Set<Integer> set = new HashSet<>();
+        Collection<Integer> collection = new ArrayList<>();
+        set.add(1);
+        set.add(2);
+        set.add(3);
+        set.add(4);
+        collection.add(1);
+        collection.add(3);
+
+        set.removeAll(collection);
+
+        assertEquals(2, set.size());
+        Integer[] actualElements = new Integer[set.size()];
+        Integer[] expectedElements = new Integer[] { 2, 4 };
+        assertArrayEquals(expectedElements, set.toArray(actualElements));
+    }
+
+}