Java Tid-bits

Wrapper classes and the == and != operator

Take the following code example:

Integer a = 130;
Integer b = 130;
if (a == b) {
    System.out.println("a == b");
}
Integer c = 100;
Integer d = 100;
if (c == d) {
    System.out.println("c == d");
}

Integer e = new Integer(100);
Integer f = new Integer(100);
if (e == f) {
    System.out.println("e == f");
}

What is output by the following code fragment? The answer is...

c == d

If the equals-equals operator evaluated to true for c==d why did it not for a==b? And furthermore, if c == d then why does e ==f evaluate to false?

This shows an interesting aspect of auto-boxing and the way Java manages/saves memory. The golden rule is that for the types Boolean, Byte, Character (\u0000 to \u007f), Short, and Integer (-128-127), if a variable is created by auto-boxing an evaluation of the == operator will return true for equal value items.

Your next question may be... "Well why is e == f evaluating to false?" This is because when we explicitly call the Integer constructor, passing it 100 we are bypassing the auto-box feature of java and therefore the rule doesn't apply.

PS: You should never rely on == for wrapped objects. Always use the .equals method!