cached bitmap

package com.example;

import java.util.Random;

import android.app.Activity;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.util.AttributeSet;
import android.util.Log;
import android.view.View;

public class Test1 extends Activity {
    /** Called when the activity is first created. */
    private Handler mHandler;
    private DrawView draw;
    private boolean running;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        draw = new DrawView(this, null);
        mHandler = new Handler(new Handler.Callback() {
            @Override
            public boolean handleMessage(Message msg) {
                Log.v("tag", "add random point in 500 ms");
                draw.addRandomPoint();
                if (running)
                    mHandler.sendMessageDelayed(new Message(), 500);
                return true;
            }
        });
        setContentView(draw);
    }

    @Override
    public void onResume() {
        super.onResume();
        running = true;
        mHandler.sendMessage(new Message());
    }

    @Override
    public void onPause() {
        super.onPause();
        running = false;
    }

    public class DrawView extends View {
        public static final String TAG = "DrawView";
        Paint paint = new Paint();
        Bitmap cache;

        public DrawView(Context context, AttributeSet attrs) {
            super(context, attrs);
            paint.setStyle(Paint.Style.FILL_AND_STROKE);
            paint.setColor(Color.WHITE);
            paint.setAntiAlias(true);

        }

        @Override
        public void onDraw(Canvas canvas) {
            if (cache != null)
                canvas.drawBitmap(cache, 0, 0, paint);
        }

        Random random = new Random();
        private final int[] RAINBOW =  {Color.RED, Color.YELLOW, Color.BLUE, Color.GREEN,
                Color.parseColor("#FF7F00"), //orange
                Color.parseColor("#00ffff"),//azure
                Color.parseColor("#8b00ff") //purple
        };

        public void addRandomPoint() {
            int w = getWidth();
            int h = getHeight();
            if (w == 0 || h == 0)
                return;

            Point point = new Point(
                    random.nextFloat() * getWidth(),
                    random.nextFloat() * getHeight(),
                    random.nextFloat() * 30,
                    RAINBOW[random.nextInt(RAINBOW.length)]);

            if (cache == null) {
                cache = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
            }

            Canvas canvas = new Canvas(cache);
            paint.setColor(point.color);
            paint.setStyle((random.nextFloat()> 0.5) ? Paint.Style.STROKE : Paint.Style.FILL_AND_STROKE);
            paint.setStrokeWidth(point.radius / 5);
            canvas.drawCircle(point.x, point.y, point.radius, paint);
            postInvalidate();
        }
    }

    class Point {
        Point(float x, float y, float radius, int color) {
            this.x = x;
            this.y = y;
            this.radius = radius;
            this.color = color;
        }

        float x, y, radius;
        int color;
    }
}