Invalid java syntax using ternary operator

Question

I am implementing a stack using queues. My pop() function originally looked like:

public void pop(){
    queue1.isEmpty() ? queue2.poll() : queue1.poll();
}

This did not compile. What is wrong with this definition?

Does `queue2.poll()` and `queue1.poll()` return the _same_ type? — Tim Biegeleisen, Jun 22 '15 at 01:18
What Throwable (Exception or Error) did compilation produce? In case you are implementing an ArrayDeque it has a pop method that would need overriding -- and it is a natural fit for stack implementation. — , Jun 22 '15 at 01:26
Although this use of the ternary operator is illegal in Java, interestingly it is legal in C/C++. (Not to say that it's good or bad.) — Nayuki, Jun 22 '15 at 02:40

score 1 · Accepted Answer · answered Jun 22 '15 at 01:18

You need to assign (or return) the Object you're polling. Something like

public void pop(){
    Object obj = queue1.isEmpty() ? queue2.poll() : queue1.poll();
}

or (what I think you really want) - something like

public Object pop(){
    return queue1.isEmpty() ? queue2.poll() : queue1.poll();
}

score 1 · Answer 2 · answered Jun 22 '15 at 01:25

The conditional operator only works in an expression context. A statement is not an expression. In your case you need to use an if statement:

public void pop(){
    if (queue1.isEmpty()) {
        queue2.poll();
    } else {
        queue1.poll();
    }
}

If you are concerned about performance, then don't be. There is absolutely no performance penalty for using an if statement.

2 Answers2