java - Android Studio debugger highlights the wrong lines

Question

I can't see anything functionally wrong in terms of what gets executed, but I've just wasted an hour proving my code and the compiler are doing what they're supposed to.

Consider this code:

public int getAnswer(int a) {

    int retval = 18;

    int x = 25;
    int y = 50;

    while (a-- > 0) {
        if (a == 3) {
            retval = a;
            return retval;
        }
    }

    x = 10;
    y = 20;

    return 0;
}

Called with

int theAnswer = getAnswer(6);

Breakpoint both of the lines containing return and before you run the code, try and predict the result, and therefore which line will be hit.

As you might imagine from the tone of my question, the wrong line is highlighted, but the right result is returned. If you step through the code, it dances around quite a bit.

Is there anything that can be configured differently such that the correct lines are displayed? (I'm using the latest Android Studio, which I appreciate is potentially unstable, but I'm sure I've seen similar behaviour on Eclipse a while back but I never spent the time then tracking it down; perhaps it's a java thing).

Answer 1

This is a problem with dx, which is the part of the build that turns your Java .class files into .dx files for packaging into Android. According to this:

https://code.google.com/p/android/issues/detail?id=34193

if a function has multiple return paths, dx merges the return instruction into a single return instruction, so during debugging, the debugger can't tell which line a return belongs to and things jump around. This corresponds to what I see when I try to reproduce your problem: each time through the loop it does the if (a == 3) check, jumps to the return 0 at the end, and then jumps back into the loop. You're seeing that last return 0 get merged with the return retval in the middle of the loop.

I doubt this will be fixed any time soon, so you may just have to learn to live with it. Sorry, I know it's kinda crazy.