为什么我的arraylist的一次迭代工作，而另一次没有？

Question

( TLDR: Why does the while-loop repeat infinitely, where the for-loop always works? )

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I am writing an Android app, and have a queue of Runnable s that need to be run, then removed from this queue:

List<Runnable> queue = new ArrayList<Runnable>();

I wanted to do this the most optimal way, and thought of the following algorithm:

synchronized(queue)
{
    while(queue.size() > 0) 
    {
        queue.remove(0).run();
    }
}

This caused errors, so I had to add this check: if (queue.size() > 0) before the loop (is that really necessary?). Now, however, this seems to be stuck in an infinite loop. I had to change my loop to this, which works well:

synchronized(queue)
{
    for (int i = 0; i < queue.size(); i++)
    {
        queue.get(i).run();
    }
    queue.clear();
}

My question is - what is the low-level difference between these loops? Why does the while loop fail?

Answer 1

Actually I can not mention the reason why the while loop snippet freezes. A noticeable difference is that in the first scenario the condition in the while loop depends on what happens in the body. What i mean is this line

queue.remove(0).run()

is supposed to decrease the size of the queue and if it does not the loop won't end. In the second scenario the loop condition does not depend on what happens in the loop body. The queue is not changed in the for loop body and when the loop starts it is predefined how many iterations there will be. So my conclusion is that something with this line:

queue.remove(0).run()

messes the things up. Can you please try this:

synchronized(queue)
{
    for (int i = 0; i < queue.size(); i++)
    {
        queue.remove(0).run();
    }
}

and see what happens. The possible reasons for this problem may be: - The remove method may not work properly - Threads started within the loop may cause the problem.

Answer 2

I just tried this (not in Android though) and it worked fine:

public class Main
{           
    public void run()
    {       
        final Runnable runMe = new Runnable()
        {
            @Override
            public void run()
            {
                System.out.println("Hi");               
            }           
        };

        List<Runnable> queue = new ArrayList<Runnable>() {
            private static final long   serialVersionUID    = 1L;
            {
                add(runMe); add(runMe); add(runMe); add(runMe);
            }
        };

        synchronized(queue)
        {
            while(queue.size() > 0)
            {
                queue.remove(0).run();
            }
        }
    }   

    public static void main(String[] args)
    {       
        new Main().run();
    }
}