Different behavior of OR operator in Java

Question

I have a simple code below:

public class Foo {
    public static void main(String[] args) {
        boolean v = true;
        v = v || hello();
        System.out.println(v);
    }

    public static boolean hello() {
        System.out.println("inside hello");
        return true;
    }
}

and it prints:

true

But if I change the expression:

v = v || hello();

to

v |= hello();

It prints:

inside hello
true

Can anyone please explain why this is happening? I assumed that they should have identical behavior but in case of |= operator, the short-circuit is not happening.

Why would you assume they're the same? `v |=` is equivalent to `v = v |`. If you want short-circuiting behavior, you'll have to use the nonexistent `v ||=`. — shmosel, Aug 15 '18 at 04:00
This is happening as per the standard short-circuit behavior. Which says if any condition in || statement becomes true then it will not check further as the result will be true only. so once it's getting v as true it's returning immediately if you set v = false then it will give the same output as second. For the second it needs to execute both statements of |= operator and apply the operator. — Shivang Agarwal, Aug 15 '18 at 04:02
@shmosel - The expression `x |=` is what they use for booleans too. — Gurwinder Singh, Aug 15 '18 at 05:11
How did I imply otherwise? You know `|` is also a boolean operator, right? — shmosel, Aug 15 '18 at 05:20
@shmosel - Sun's answer is what I was looking for. Thanks for your help. — Gurwinder Singh, Aug 15 '18 at 05:23

score 2 · Accepted Answer · answered Aug 15 '18 at 03:58

15.26.2. Compound Assignment Operators will not cause short circuit :

First, the left-hand operand is evaluated to produce a variable. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason; the right-hand operand is not evaluated and no assignment occurs.

Otherwise, the value of the left-hand operand is saved and then the right-hand operand is evaluated.

...

score 0 · Answer 2 · answered Aug 15 '18 at 03:57

In Java || has a short-circuit, the right hand side will not be evaluated if the left hand side is true - because the value on the right doesn't matter in that case. So in your first example, hello() is never called.

The second example is actually using a different operator, the bitwise OR operator | which cannot do the short-circuit because it is for doing bit operations on numbers (but also works on booleans). It will work the same if you expand it out to v = v | hello().

Different behavior of OR operator in Java

2 Answers2