How is `return ||

Question

I was reading a book, and saw this code:

public Animal getWinner(List<Animal> animals) {
    return animals == null || animals.size() == 0 ? null : animals.get(0);
}

The latter expression is fine as it returns an Animal or null. The former expression is puzzling though, as it's just a boolean expression.

I made a test class and called getWinner(null). It returned null. As far as I know, animals == null would be true, the expression would short-circuit, and I expect the method to return true instead of null.

How does this line compile, and even work as expected?

Wow, a member for 8 years and _now_ you ask a question. You really lurk in the shadows :-) — oligofren, Oct 27 '18 at 23:18

score 3 · Accepted Answer · edited Oct 31 '18 at 01:19

3

This has to do with operator precedence in Java and which operator - the boolean or the ternary, will be executed first.

The ternary has lower precedence, so the or will be evaluated first, meaning it would look like this with parenthesis:

return (animals == null || animals.size() == 0) ? null : animals.get(0);

So the result of the entire line is either to return null or return animals.get(0).

edited Oct 31 '18 at 01:19

Samuel Liew

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answered Oct 27 '18 at 23:09

oligofren

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2

When in doubt: add parenthesis. The next programmer will thank you :D – oligofren Oct 27 '18 at 23:15

1 Answers1