Circles View, Example Code

import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;

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<Circle> circles = new ArrayList<>();
    Activity activity;

    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();
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        int action = event.getAction();
        if (action == MotionEvent.ACTION_UP) {
            circles.add(new Circle(event.getX(),event.getY(), 10, Color.argb(255, random.nextInt(255), random.nextInt(255), random.nextInt(255))));
        }
        return true;
    }

    private void init() {
        circles.add(new Circle(50, 100, 10, Color.GREEN));
        circles.add(new Circle(300, 150, 10, Color.GRAY));
        circles.add(new Circle(100, 300, 10, Color.MAGENTA));
        circles.add(new Circle(50, 350, 10, Color.RED));
        circles.add(new Circle(300, 350, 10, Color.BLUE));
        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 (Circle circle : circles) {
                    circle.translate(random.nextInt(10) - 5, random.nextInt(10) - 5);
                }
            }
        });

    }


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

    private class Circle {

        public float cx, cy;
        public float radius;
        public Paint paint;

        public Circle(float cx, float cy, float radius, int color) {
            this.cx = cx;
            this.cy = cy;
            this.radius = radius;
            this.paint = new Paint();
            paint.setColor(color);
        }

        public void translate(int dx, int dy) {
            invalidate((int) (cx - radius), (int) (cy - radius), (int) (cx + radius + 1), (int) (cy + radius + 1));
            cx += dx;
            cy += dy;
            invalidate((int) (cx - radius), (int) (cy - radius), (int) (cx + radius + 1), (int) (cy + radius + 1));
        }

        public void draw(Canvas canvas) {
            canvas.drawCircle(cx, cy, radius, paint);
        }
    }

}