java - What is the difference between identity and equality in OOP?

What is the difference between identity and equality in OOP (Object Oriented Programming)?

Answer

• identity: a variable holds the
  
  same instance as another variable.




• equality: two distinct objects can
  be used interchangeably. they often
  have the same id.

For example:

Integer a = new Integer(1);
Integer b = a;

a is identical to b.

In Java, identity is tested with ==. For example, if( a == b ).

Integer c =  new Integer(1);
Integer d = new Integer(1);

c is equal but not identical to d.

Of course, two identical variables are always equal.

In Java, equality is defined by the equals method. Keep in mind, if you implement equals you must also implement hashCode.