Comment by tredre3 | Hacker News Reader

Comment by tredre3 | Hacker News Reader

Parent Full thread

That behavior is inherited from C. The pre/post increment behavior is actually the same in every language that uses them. The priority of operation is also usually the same as well.

The reason the question is tricky is because those operators change the value of a as the full expression is progressively executed.

It's not immediately clear to me what the answer in Java would be.

Just take a++ + ++a for example:

If the value if `a` is hoisted by the jvm then it could be 5++ + ++5, so 5 + 6.

But if it's executed left to right and `a` is looked up every time, then it becomes 5++ + ++6, so 5 + 7.

The value of the variable is not hoisted by the Java compiler. (It's not that JVM, that only executes the byte code, what y doesn't have that kind of ambiguities.)

The semantics of Java is not undefined on multiple assignments to the same variable in an expression, so it can't hoist something if it would change the outcome.

Now, I don't actually know what the outcome is, because I don't remember whether `a += e` reads the value of `a` before or after evaluating `e`. The code is still confusing and unreadable to humans, so you shouldn't write it, but the compiler behavior is not undefined.

And if your variable is accessed from multiple threads, it may be undefined which intermediate values night be seen.

    $ cat a.java 
    class a {
        public static void main (String[] args) {
            int a = 4;
            int b = a++ + ++a;
            System.out.println(b);
            System.out.println(a);
        }
    }

    $ javac a.java 
    $ java a
    10
    6

Reply on news.ycombinator.com