Question

I was just trying to figure out if I could get a NULL pointer exception with the following code. The cause could be this: The check is done at time X. But, I post the runnable to the handler, he will execute at X+5. He should have a strong reference, therefore preventing the Runnable being gc-ed in between.

Am I correct? (that what I call easy reputation, a YES is enough. A no, you have to explain 🙂

public class WeakRunnableUiList
{
    private ArrayList<WeakReference<Runnable>> _items = new ArrayList<WeakReference<Runnable>>();
    private Handler _handler = new Handler(Looper.getMainLooper());

    public void Add(Runnable r)
    {
        _items.add(new WeakReference<Runnable>(r));
    }

    public void Execute()
    {
        ArrayList<WeakReference<Runnable>> remove = new ArrayList<WeakReference<Runnable>>();
        for (WeakReference<Runnable> item : _items)
        {
            if (item.get() == null)
            {
                remove.add(item);
            }
            else
            {
                _handler.post(item.get());
            }
        }
        _items.removeAll(remove);
    }
}

Answer 1

He should have a strong reference, therefore preventing the Runnable
being gc-ed in between

No.
Put this into your code just before _handler.post(...:

...
byte[] b=new byte[1024*32]; // this can occur in an other thread!
if (item.get() == null) {
    Log.e("Item is NULL now!", "Item is NULL now!");
}
_handler.post(item.get());
...

And then in the main program:

for(int i=0;i<100;i++) {
    weakRunnableUiList.Add(new X());
}
weakRunnableUiList.Execute();

Will give you 12-03 21:56:01.521: E/Item is NULL now!(1071): Item is NULL now!

So the runnable can get NULL after your first check!

Do it like this:

Runnable r=item.get();
if (r==null) ...

---

But you can post nulls to handlers: _handler.post(null);, and it will not throw a nullpointer actually.