Question

I am a beginner Android developer and I am trying to make a game where you move a bat (catcher) along the bottom of the screen and you have to catch falling blocks.

I am struggling currently with the collision detection. The collision detection itself is working correctly but when I then try to remove the block that is falling the game crashes.

The problem code is the line under the “remove block” comment in the checkCollisions() method but I have posted the whole GameView below for clarity.

Here is the code:

package com.mattdrewery.supercatch;

import android.view.View;
import android.view.MotionEvent;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Rect;
import java.util.Random;
import java.util.ArrayList;

public class GameView extends View 
{
private Catcher catcher;
private ArrayList<NormalBlock> blocks;
private Random rand;
private int screenWidth;
private int screenHeight;
private boolean gameOver;
private int updateCount;

public GameView(Context context)
{
    super(context);
    setFocusable(true);

    updateCount = 1;

    // Set gameOver to false
    gameOver = false;

    // Create the ArrayList of normal blocks
    blocks = new ArrayList<NormalBlock>();

    // Create a random number generator
    rand = new Random();

    // Create the catcher
    catcher = new Catcher(context, R.drawable.catcher, 230, 250);
}

@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh)
{
    screenWidth = this.getWidth();
    screenHeight = this.getHeight();
}

@Override
protected void onDraw(Canvas canvas)
{   
    updateCatcher();
    checkCollisions();
    updateBlocks();

    // Draw the catcher to the canvas
    canvas.drawBitmap(catcher.getImage(), catcher.getPosX(), catcher.getPosY(), null);

    // For each normal block in the block array
    for (NormalBlock block : blocks)
    {
        // Draw block to the canvas
        canvas.drawBitmap(block.getImage(), block.getPosX(), block.getPosY(), null);
    }

    // Redraw the screen
    invalidate();
}

@Override
public boolean onTouchEvent(MotionEvent event)
{
    // Get the action from the touch screen
    int eventAction = event.getAction();

    int X = (int) event.getX();

    // If the user presses on the screen....
    if (eventAction == MotionEvent.ACTION_DOWN)
    {
        if (X < screenWidth / 2)
        {
            catcher.setMovingLeft(true);
            catcher.setMovingRight(false);
        }
        else
        {
            catcher.setMovingLeft(false);
            catcher.setMovingRight(true);
        }
    }

    if (eventAction == MotionEvent.ACTION_MOVE)
    {
        if (X < screenWidth / 2)
        {
            catcher.setMovingLeft(true);
            catcher.setMovingRight(false);
        }
        else
        {
            catcher.setMovingLeft(false);
            catcher.setMovingRight(true);
        }
    }

    if (eventAction == MotionEvent.ACTION_UP)
    {
        catcher.setMovingLeft(false);
        catcher.setMovingRight(false);
    }

    return true;
}

private void updateCatcher()
{
    // Check whether the catcher is moving and update position accordingly
    if (catcher.isMovingLeft())
    {
        catcher.moveLeft();
        if (catcher.getPosX() < 0)
        {
            catcher.setPosX(0);
        }
    }
    else if (catcher.isMovingRight())
    {
        catcher.moveRight();
        if (catcher.getPosX() > (screenWidth - catcher.getImage().getWidth()))
        {
            catcher.setPosX(screenWidth - catcher.getImage().getWidth());
        }
    }
}

private void updateBlocks()
{
    updateCount++;

    if (updateCount >= 100)
    {
        updateCount = 1;
        blocks.add(new NormalBlock(getContext(), R.drawable.nblock,
                rand.nextInt(screenWidth - 20), 0));
    }

    // For each normal block in the block array
    for (NormalBlock block : blocks)
    {
        // Check if block has hit the bottom of the screen
        if (block.getPosY() + block.getImage().getHeight() > screenHeight)
        {
            gameOver = true;
        }
        else
        {
            // Drop the block down
            block.dropDown();
        }
    }
}

private void checkCollisions()
{
    // Get Rectangle for the catcher
    Rect catcherRect = new Rect(catcher.posX, catcher.posY,
            catcher.posX + catcher.image.getWidth(),
            catcher.posY + catcher.image.getHeight());

    // For each block in the array
    for (NormalBlock block : blocks)
    {
        // Get Rectangle for the current block
        Rect blockRect = new Rect(block.posX, block.posY,
                block.posX + block.image.getWidth(),
                block.posY + block.image.getHeight());

        if (Rect.intersects(catcherRect, blockRect))
        {
            // Remove block
            blocks.remove(block);
        }
    }

}

public int getScreenHeight()
{
    return screenHeight;
}

public int getScreenWidth()
{
    return screenWidth;
}

}

Thanks in advance,

Answer 1

Try replacing for (NormalBlock block : blocks) with this:

int count = blocks.size();
for (int i=0;i<count;i++){
Block block = blocks.get(i);


I’m guessing you were getting a ConcurrentModificationException there.