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Object Equals

layout: default title: Object.equals()

Every class has an equals() method, because every class descends from java.lang.Object.

package java.lang;
/**
 * Class {@code Object} is the root of the class hierarchy.
 * Every class has {@code Object} as a superclass. All objects,
 * including arrays, implement the methods of this class.
 *
 * @author  unascribed
 * @see     java.lang.Class
 * @since   JDK1.0
 */
public class Object {
//...
    public boolean equals(Object obj) { /* ... */ }
//...
}
  • This means you can invoke equals() on any object reference, and pass it any other object.

When you use equals you are asking an object to compare itself to the reference passed to it.

The implementation of Object.equals does a simple comparison of the references themselves:

public boolean equals(Object obj) {
    return (this == obj);
}

Every class can override Object's implementation of equals.

Drill

EqualsAndHashcode/com.example.equalsandhashcode.drills.TestObject * Given the following code, what will each of the println statements print? 

Object o1 = new Object();
Object o2 = new Object();
Object o3 = o1;

System.out.println(o1 == o2);      // true or false?
System.out.println(o1.equals(o2)); // true or false?
System.out.println(o1 == o3);      // true or false?
System.out.println(o1.equals(o3)); // true or false?
Uncomment the statements in TestObject and make sure you understand the results.

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