Question

I don’t know if this is a simple problem or not, but I just can’t see what the problem is. I’ve gotten three reports now from my app in Google Play of an IndexOutOfBoundsException at points.get(++i).

public GameThread(SurfaceHolder sHolder, Context context, Handler handler)
{
    points = new ArrayList<Float>();
    running = true;
    mSurfaceHolder = sHolder;
}
protected void doDraw(Canvas canvas)
{
    Paint p = new Paint();
    p.setStyle(Paint.Style.FILL);
    p.setColor(Color.WHITE);

    for (int i = 0; i < points.size(); i++)
    {
        float x = points.get(i);
        float y = points.get(++i);

        canvas.drawPoint(x, y, p);
    }
}

public boolean onTouchEvent(MotionEvent event)
{   
    float x = event.getX();
    float y = event.getY();

    points.add(x);
    points.add(y);

    return true;
}

The index it fails at is trying to access the location equal to its size (Actually one error says
java.lang.IndexOutOfBoundsException: Invalid index 2205, size is 2206 which makes NO sense at all to me), and its size varies.

The only way I can see this happening is if for some reason only one object is added to points, and I don’t know why that would happen. onTouchEvent isn’t running in its own thread, is it?

Answer 1

The problem is at this line:

float y = points.get(++i);

When i = points.size()-1, you preincrement, and then you invoke points.get(++i);. At this point, i = points.size() => Out of bounds. Just remove the ++ to fix it.

EDIT
Alternatively, you can try this approach:

List<Point> points = ...

protected void doDraw(Canvas canvas) {
   Paint p = new Paint();
   p.setStyle(Paint.Style.FILL);
   p.setColor(Color.WHITE);

   for (Point p : points) {
      canvas.drawPoint(p.x, p.y, p);
   }
}

public boolean onTouchEvent(MotionEvent event) {   
   float x = event.getX();
   float y = event.getY();

   points.add(new Point(x, y));

   return true;
}