How to Increment a class Integer references value in java from another method

No, objects aren't passed by reference. References are passed by value - there's a big difference. Integer is an immutable type, therefore you can't change the value within the method.

Your n++; statement is effectively

n = Integer.valueOf(n.intValue() + 1);

So, that assigns a different value to the variable n in Increment - but as Java only has pass-by-value, that doesn't affect the value of n in the calling method.

EDIT: To answer your update: that's right. Presumably your "MyIntegerObj" type is mutable, and changes its internal state when you call plusplus(). Oh, and don't bother looking around for how to implement an operator - Java doesn't support user-defined operators.

You could use an AtomicInteger from java.util.concurrent.atomic. It has a 'incrementAndGet' method which is suitable for your showcase.