Android Java rectangle collision detection not working

Question

I had been hard coding a collision detection system which was buggy. Then I came across using rectangles for collision detection. So I put it all in and it does not work, I put a log in and it never logged.

Note to Java programmers who are not Android programers: Android uses the word Rect instead of Rectangle.

Code for Block.java:

public Rect getBounds(){
    return new Rect (this.x, this.y, 10, 20);
}

Code for Sprite.java:

public Rect getBounds(){
    return new Rect (this.x, this.y, 20, 20);
}

Code for MainGame.java:

for(Block block : BLOCKS) {
    block.draw(canvas);
    block.rigidbody();

    Rect spriter = sprite.getBounds();
    Rect blockr = block.getBounds();

    if(spriter.intersect(blockr)){
        showgameover = 1;
        Log.d(TAG, "Game Over");
    }

}

Is anyone able to help?

Answer 1

Have you tried the debugger? The code looks like it should work, if the sprite and a block are indeed colliding.

Here's what you do, create a sprite and a block at the same position. Have a breakpoint on your if(spriter.intersect(blockr)) statement. When it breaks inspect the elements rectangle values to ensure they do intersect.

I imagine that if these are currently intersecting you're just not set up to see the log properly and/or not doing anything with the showgameover variable.

EDIT

According to the documentation you're using the intersect method correctly.

One of the few remaining things to check is to ensure that your for(Block block : BLOCKS) is actually getting run. What I told you to do with the debugger should help answer that. Otherwise you can stick a Log.d() inside the loop and ensure that it's actually being run. But I recommend the debugger method, as that's more useful in the long run.

Answer 2

I am not sure if this is your problem, but I struggled with a subtle intersection bug for a long time. I traced it to Android's implementation of intersects:

public boolean intersects(int left, int top, int right, int bottom) {
     return this.left < right && left < this.right
            && this.top < bottom && top < this.bottom;
 }

Whereas the algorithm I was using assumed that 'touching' rects are intersecting, using the following implementation:

public static boolean intersects(Rect rect, Rect otherRect) {
    return  rect.left <= otherRect.right && otherRect.left <= rect.right
        && rect.top <= otherRect.bottom && otherRect.top <= rect.bottom;
}

Additionally, you should probably be using the intersects method instead of intersect as the latter modifies the rectangle (of the sprite rect in your case) to match the intersection area.