Question

I was looking over some mock OCJP questions. I came across a really baffling syntax. Here it is:

class OddStuff {
    public static void main(String[] args) {
        boolean b = false;
        System.out.println((b != b));// False
        System.out.println((b =! b));// True
    }
}

Why does the output change between != and =!?

Answer 1

The question is just playing with you with confusing spacing.

b != b is the usual != (not equals) comparison.

On the other hand:

b =! b is better written as b = !b which is parsed as:

b = (!b)

Thus it’s two operators.

1. First invert b.
2. Then assign it back to b.

The assignment operator returns the assigned value. Therefore, (b =! b) evaluates to true – which is what you print out.