BAEL-9485 The Java Object Class (#18883)

* Implement Car class with Object method overrides

The Car class implements Cloneable and overrides Object methods for equality, hashing, string representation, and cloning.

* Rename Car.java to follow package naming convention

* Add package declaration to CarObjectUnitTests

* Add package declaration to Car.java

* Rename test methods for better readability

* Update Car object creation in unit test

* Remove synchronization tests for Car class

Removed tests for wait(), notify(), and notifyAll() methods due to synchronization issues.

* Refactor CarObjectUnitTests for clarity and conciseness

Refactored JUnit tests for the Car class by removing comments and simplifying assertions.

* Fix formatting in toString() method of Car class

Author: Deepak-Vohra

core-java-modules/core-java-lang-oop-types/src/main/java/com/baeldung/objectclassguide/Car.java

@@ -0,0 +1,78 @@
+package com.baeldung.objectclassguide;
+
+import java.util.Objects;
+
+
+/**
+ * Represents a car object, implementing Cloneable and overriding Object methods.
+ *
+ * The class uses standard implementation for equals(), hashCode(), and toString().
+ * The clone() method performs a shallow copy, which is sufficient since 'make' (String)
+ * and 'year' (int) are immutable or primitive.
+ */
+public class Car implements Cloneable {
+    private String make;
+    private int year;
+
+
+    public Car(String make, int year) {
+        this.make = make;
+        this.year = year;
+    }
+
+
+    // Getters for external access (useful for testing)
+    public String getMake() {
+        return make;
+    }
+
+
+    public int getYear() {
+        return year;
+    }
+
+
+    /**
+     * Standard implementation of equals() for value equality.
+     */
+    @Override
+    public boolean equals(Object obj) {
+        if (this == obj) return true;
+        if (obj == null || getClass() != obj.getClass()) return false;
+        Car car = (Car) obj;
+        // Use Objects.equals for safe String comparison
+        return year == car.year && Objects.equals(make, car.make);
+    }
+
+
+    /**
+     * Standard implementation of hashCode() based on make and year.
+     */
+    @Override
+    public int hashCode() {
+        return Objects.hash(make, year);
+    }
+
+
+    /**
+     * Standard implementation of toString() for debugging and logging.
+     */
+    @Override
+    public String toString() {
+        return "Car{"
+          + "make='" + make + '\''
+          + ", year=" + year
+          + '}';
+    }
+
+
+    /**
+     * Overrides the protected clone() method from Object to perform a shallow copy.
+     * This is the standard pattern when implementing the Cloneable marker interface.
+     */
+    @Override
+    public Object clone() throws CloneNotSupportedException {
+        // Calls Object's native clone() method
+        return super.clone();
+    }
+}

core-java-modules/core-java-lang-oop-types/src/test/java/com/baeldung/objectclassguide/CarObjectUnitTests.java

@@ -0,0 +1,92 @@
+package com.baeldung.objectclassguide;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotSame;
+
+public class CarObjectUnitTests {
+
+    @Test
+    void givenACarObject_whenTestingObjectEqualsItself_thenEqualsReflexive() {
+        Car car1 = new Car("Honda", 2020);
+        assertTrue(car1.equals(car1));
+    }
+
+    @Test
+    void givenTwoCarObjects_whenTestingSymmetric_thenEqualsSymmetric() {
+        Car car1 = new Car("Honda", 2020);
+        Car car2 = new Car("Honda", 2020);
+        assertTrue(car1.equals(car2));
+        assertTrue(car2.equals(car1));
+    }
+
+    @Test
+    void givenThreeCarObjects_whenTestingTransitive_thenEqualsTransitive() {
+        Car car1 = new Car("Honda", 2020);
+        Car car2 = new Car("Honda", 2020);
+        Car car3 = new Car("Honda", 2020);
+        
+        assertTrue(car1.equals(car2));
+        assertTrue(car2.equals(car3));
+        assertTrue(car1.equals(car3));
+    }
+
+    @Test
+    void givenTwoDifferentCarObjects_whenComparingWithEquals_thenEqualsReturnsFalse() {
+        Car car1 = new Car("Honda", 2020);
+        Car car2 = new Car("Toyota", 2020);
+
+        assertFalse(car1.equals(car2));
+    }
+
+    @Test
+    void givenANonNullCarObject_whenTestingAgainstNull_thenEqualsReturnsFalse() {
+        Car car1 = new Car("Honda", 2020);
+        
+        assertFalse(car1.equals(null));
+    }
+
+    @Test
+    void givenTwoEqualCarObjects_whenComparingHashCodes_thenReturnsEqualHashCodes() {
+        Car car1 = new Car("Honda", 2020);
+        Car car2 = new Car("Honda", 2020);
+        
+        assertEquals(car1.hashCode(), car2.hashCode());
+    }
+
+    @Test
+    void givenACarObject_whenTestingHashCodeConsistency_thenReturnsSameHashCodeAcrossMultipleCalls() {
+        Car car = new Car("Honda", 2020);
+        int initialHash = car.hashCode();
+        
+        assertEquals(initialHash, car.hashCode());
+    }
+
+    @Test
+    void givenACarObject_whenTestingToString_thenReturnsExpectedString() {
+        Car car = new Car("Tesla", 2023);
+        String expected = "Car{make='Tesla', year=2023}";
+
+        assertEquals(expected, car.toString());
+    }
+
+    @Test
+    void givenACarObject_whenTestingGetClass_thenReturnsCarClass() {
+        Car car = new Car("Ford", 2015);
+        
+        assertEquals(Car.class, car.getClass());
+    }
+
+    @Test
+    void givenACarObject_whenTestingClone_thenCloneSuccess() throws CloneNotSupportedException {
+        Car original = new Car("Honda", 2020);
+        Car cloned = (Car) original.clone();
+        
+        assertNotSame(original, cloned);
+        assertEquals(original, cloned);
+        assertEquals(original.getMake(), cloned.getMake()); 
+        assertEquals(original.getYear(), cloned.getYear());
+    }
+}