Ball Motion with super-basic collision detection.

package viewmotionexample.ballanimation;

import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Point;
import android.graphics.Rect;
import android.graphics.RectF;
import android.graphics.drawable.Drawable;
import android.util.AttributeSet;
import android.view.Display;
import android.view.MotionEvent;
import android.view.VelocityTracker;
import android.view.View;
import android.view.WindowManager;

import java.util.ArrayList;
import java.util.Random;


/**
 * Created by David Olsen, 2015
 * Public Domain. Use for anything.
 */

public class CirclesView extends View {
    Random random = new Random();
    ArrayList<BallDrawing> circles = new ArrayList<>();
    Activity activity;
    Paint backgroundPaint = new Paint();

    public CirclesView(Context context) {
        super(context);
        this.activity = (Activity) context;
        init();
    }

    public CirclesView(Context context, AttributeSet attrs) {
        super(context, attrs);
        this.activity = (Activity) context;
        init();
    }

    public CirclesView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        this.activity = (Activity) context;

        init();
    }

    VelocityTracker velocityTracker;

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        switch (event.getAction()) {
            case MotionEvent.ACTION_UP:
                BallDrawing bd = new BallDrawing(event.getX(), event.getY());
                velocityTracker.computeCurrentVelocity(10);
                bd.vx = velocityTracker.getXVelocity();
                bd.vy = velocityTracker.getYVelocity();
                circles.add(bd);
                break;
            case MotionEvent.ACTION_MOVE:
                velocityTracker.addMovement(event);
                break;
            case MotionEvent.ACTION_DOWN:
                velocityTracker.clear();
                break;
        }
        return true;
    }

    Drawable drawable;
    Rect drawableBounds = null;

    int maxX, maxY;
    int minX = 0, minY = 0;

    private void init() {
        WindowManager wm = (WindowManager) getContext().getSystemService(Context.WINDOW_SERVICE);
        Display display = wm.getDefaultDisplay();
        Point size = new Point();
        display.getSize(size);
        maxX = size.x;
        maxY = size.y;

        velocityTracker = VelocityTracker.obtain();

        backgroundPaint.setColor(Color.BLUE);
        drawable = getResources().getDrawable(R.drawable.ball);
        if (drawable != null) {
            drawable.setBounds(0, 0, 40, 40);
            drawableBounds = drawable.getBounds();
        }
        Thread t = new Thread(new Runnable() {
            @Override
            public void run() {

                while (true) {
                    try {
                        Thread.sleep(100);
                        tick();

                    } catch (InterruptedException e) {
                        return;
                    }
                }
            }
        });
        t.start();
    }

    public void tick() {
        activity.runOnUiThread(new Runnable() {
            @Override
            public void run() {
                for (BallDrawing circle : circles) {
                    circle.tickMotion();
                }
                for (int j = 0; j < circles.size(); j++) {
                    BallDrawing jball = circles.get(j);
                    RectF jballbounds = jball.getBounds();
                    for (int k = 0; k < circles.size(); k++) {
                        if (j != k) {
                            BallDrawing kball = circles.get(k);
                            RectF kballBounds = kball.getBounds();
                            if (kballBounds.intersect(jballbounds)) {
                                kball.hit();
                                //jball.hit();
                            }
                        }
                    }
                }
            }
        });

    }


    @Override
    protected void onDraw(Canvas canvas) {
        for (BallDrawing c : circles) {
            c.draw(canvas);
        }
    }

    private class BallDrawing {
        public float cx, cy;
        public float vx, vy;

        public BallDrawing(float cx, float cy) {
            this.cx = cx;
            this.cy = cy;
        }

        public RectF getBounds() {
            return new RectF(cx,cy,cx+drawableBounds.width(),cy+drawableBounds.height());
        }

        public void applyMotion() {
            cx += vx;
            cy += vy;
            if (cx < minX) {
                vx =-vx;
                cx = minX - (cx - minX);
            }
            else if (cx > maxX) {
                vx =-vx;
                cx = maxX - (cx - maxX);
            }
            if (cy < minY) {
                vy =-vy;
                cy = minY - (cy - minY);
            }
            else if (cy > maxY) {
                vy =-vy;
                cy = maxY - (cy - maxY);
            }
        }

        public void hit() {
            vx *= -1;
            vy *= -1;
        }

        public void tickMotion() {
            invalidate((int) cx + drawableBounds.left, (int) cy + drawableBounds.top, (int) cx + 1 + drawableBounds.right, (int) cy + 1 + drawableBounds.bottom);
            applyMotion();
            invalidate((int) cx + drawableBounds.left, (int) cy + drawableBounds.top, (int) cx + 1 + drawableBounds.right, (int) cy + 1 + drawableBounds.bottom);
        }


        public void draw(Canvas canvas) {
            canvas.save();
            canvas.translate(cx, cy);
            drawable.draw(canvas);
            canvas.restore();
        }

    }

}