What is the difference between != and =! in Java?

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 =!?

Note that if you assign `true` to b, then both return false. — Mikaël Mayer, Jan 10 '14 at 07:15
Same programmer writing Java and C++ . Take a look http://stackoverflow.com/questions/21029174/whats-this-operator :P — Bahadir Cambel, Feb 01 '14 at 13:40

Mysticial · Accepted Answer · 2012-01-11T22:54:45.167

116

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.

First invert b.
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.

edited Jan 11 '12 at 22:54

answered Jan 11 '12 at 20:05

Mysticial

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5

correct and more correctly it's (b = !b) vs. (b != b) – Umair A. Jan 11 '12 at 20:06
2

I didn't know assignments could be done in that manner. – prometheuspk Jan 11 '12 at 20:10
1

@Prometheus87 Yes, you can put assignments inside statements. – Mysticial Jan 11 '12 at 20:11
Great, i learnt something new. Thanks! – prometheuspk Jan 11 '12 at 20:13
3

Also key is the fact that assignments evaluate to the assigned value, so `System.out.println(b = !b)` is printing the value of `b` after `!b` is assigned to it. – Paul Bellora Jan 11 '12 at 20:31
@PaulBellora Yes, adding that to my answer. – Mysticial Jan 11 '12 at 20:32
Perfect explanation..! – Nirav Dangi Nov 02 '14 at 18:38

score 10 · Answer 2 · answered Jan 11 '12 at 20:07

10

b != b means ! (b == b): the opposite of b == b.

b =! b is actually b = !b, an assignment. It's toggling b's value. An assignment evaluates to the value of the expression, so this will evaluate to !b (along with having changed the value of b).

answered Jan 11 '12 at 20:07

Claudiu

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Nathan S. · Answer 3 · 2014-01-11T17:38:57.840

9

b=!b is an assignment. It assigns b to !b and the expression evaluates to the resulting value, which is true.

edited Jan 11 '14 at 17:38

answered Jan 11 '12 at 20:06

Nathan S.

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Luis Tellez · Answer 4 · 2012-01-11T20:09:22.973

3

b =! b

you are doing an assignment, you are saying that B should have the value of !B.

b != b

You are asking if B is different than b

edited Jan 11 '12 at 20:09

answered Jan 11 '12 at 20:06

Luis Tellez

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What is the difference between != and =! in Java?

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