How does equals() method work in Java [duplicate]

Answer 1

The default implementation, the one of the class java.lang.Object, simply tests the references are to the same object :

150    public boolean equals(Object obj) {
151        return (this == obj);
152    }

The reference equality operator is described like this in the Java Specification :

At run time, the result of == is true if the operand values are both null or both refer to the same object or array; otherwise, the result is false.

This default behavior isn't usually semantically satisfying. For example you can't test equality of big Integer instances using == :

Integer a = new Integer(1000);
Integer b = new Integer(1000);
System.out.println(a==b); // prints false

That's why the method is overridden :

722     public boolean equals(Object obj) {
723         if (obj instanceof Integer) {
724             return value == ((Integer)obj).intValue();
725         }
726         return false;
727     }

which enables this :

System.out.println(a.equals(b)); // prints true

Classes overriding the default behavior should test for semantic equality, based on the equality of identifying fields (usually all of them).

As you seem to know, you should override the hashCode method accordingly.