[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/main/java/com/baeldung/collections/removeallperformance/HashSetBenchmark.java

@@ -0,0 +1,86 @@
+package com.baeldung.collections.removeallperformance;
+
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+import java.util.concurrent.TimeUnit;
+
+import org.apache.commons.lang3.RandomStringUtils;
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.BenchmarkMode;
+import org.openjdk.jmh.annotations.Level;
+import org.openjdk.jmh.annotations.Mode;
+import org.openjdk.jmh.annotations.OutputTimeUnit;
+import org.openjdk.jmh.annotations.Scope;
+import org.openjdk.jmh.annotations.Setup;
+import org.openjdk.jmh.annotations.State;
+import org.openjdk.jmh.annotations.Warmup;
+import org.openjdk.jmh.runner.Runner;
+import org.openjdk.jmh.runner.options.Options;
+import org.openjdk.jmh.runner.options.OptionsBuilder;
+
+import com.baeldung.collections.containsperformance.Employee;
+
+@BenchmarkMode(Mode.AverageTime)
+@OutputTimeUnit(TimeUnit.NANOSECONDS)
+@Warmup(iterations = 5)
+public class HashSetBenchmark {
+
+    @State(Scope.Thread)
+    public static class MyState {
+        private Set<Employee> employeeSet1 = new HashSet<>();
+        private List<Employee> employeeList1 = new ArrayList<>();
+        private Set<Employee> employeeSet2 = new HashSet<>();
+        private List<Employee> employeeList2 = new ArrayList<>();
+
+        private long set1Size = 60000;
+        private long list1Size = 50000;
+        private long set2Size = 50000;
+        private long list2Size = 60000;
+
+        @Setup(Level.Trial)
+        public void setUp() {
+
+            for (long i = 0; i < set1Size; i++) {
+                employeeSet1.add(new Employee(i, RandomStringUtils.random(7, true, false)));
+            }
+
+            for (long i = 0; i < list1Size; i++) {
+                employeeList1.add(new Employee(i, RandomStringUtils.random(7, true, false)));
+            }
+
+            for (long i = 0; i < set2Size; i++) {
+                employeeSet2.add(new Employee(i, RandomStringUtils.random(7, true, false)));
+            }
+
+            for (long i = 0; i < list2Size; i++) {
+                employeeList2.add(new Employee(i, RandomStringUtils.random(7, true, false)));
+            }
+
+        }
+
+    }
+
+    @Benchmark
+    public boolean given_SizeOfHashsetGreaterThanSizeOfCollection_When_RemoveAllFromHashSet_Then_GoodPerformance(MyState state) {
+        return state.employeeSet1.removeAll(state.employeeList1);
+    }
+
+    @Benchmark
+    public boolean given_SizeOfHashsetSmallerThanSizeOfCollection_When_RemoveAllFromHashSet_Then_BadPerformance(MyState state) {
+        return state.employeeSet2.removeAll(state.employeeList2);
+    }
+
+    public static void main(String[] args) throws Exception {
+        Options options = new OptionsBuilder().include(HashSetBenchmark.class.getSimpleName())
+            .threads(1)
+            .forks(1)
+            .shouldFailOnError(true)
+            .shouldDoGC(true)
+            .jvmArgs("-server")
+            .build();
+        new Runner(options).run();
+    }
+
+}

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

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+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));
+    }
+
+}