baeldung-articles : BAEL-7649 (#16452)

Unit Test for hashCode() in Java.

core-java-modules/core-java-lang-oop-methods/src/test/java/com/baeldung/testhashcode/HahCodeUnitTest.java

@@ -0,0 +1,42 @@
+package com.baeldung.testhashcode;
+
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+
+import static org.junit.Assert.assertEquals;
+
+public class HahCodeUnitTest {
+    @Test
+    public void givenObject_whenTestingHashCodeConsistency_thenConsistentHashCodeReturned() {
+        MyClass obj = new MyClass("value");
+        int hashCode1 = obj.hashCode();
+        int hashCode2 = obj.hashCode();
+        assertEquals(hashCode1, hashCode2);
+    }
+
+    @Test
+    public void givenTwoEqualObjects_whenTestingHashCodeEquality_thenEqualHashCodesReturned() {
+        MyClass obj1 = new MyClass("value");
+        MyClass obj2 = new MyClass("value");
+        assertEquals(obj1.hashCode(), obj2.hashCode());
+    }
+
+    @Test
+    public void givenMultipleObjects_whenTestingHashCodeDistribution_thenEvenDistributionOfHashCodes() {
+        List<MyClass> objects = new ArrayList<>();
+        for (int i = 0; i < 1000; i++) {
+            objects.add(new MyClass("value" + i));
+        }
+
+        Set<Integer> hashCodes = new HashSet<>();
+        for (MyClass obj : objects) {
+            hashCodes.add(obj.hashCode());
+        }
+
+        assertEquals(objects.size(), hashCodes.size(), 10);
+    }
+}

core-java-modules/core-java-lang-oop-methods/src/test/java/com/baeldung/testhashcode/MyClass.java

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+package com.baeldung.testhashcode;
+
+public class MyClass {
+    private String value;
+
+    public MyClass(String value) {
+        this.value = value;
+    }
+
+    @Override
+    public int hashCode() {
+        return value != null ? value.hashCode() : 0;
+    }
+
+}