Android - onPause and onResume -

i have code randomly draws red circles. didn't work until had pause , resume methods in both classes. without pause , resume methods, screen black , not change. why did need onpause , onresume method , why in both classes?

the commented code pause/resume methods.

public class randomcircles extends activity {      mysurfaceview mysurfaceview;      /** called when activity first created. */     @override     public void oncreate(bundle savedinstancestate) {         super.oncreate(savedinstancestate);         mysurfaceview = new mysurfaceview(this);         setcontentview(mysurfaceview);     }    /*   @override     protected void onresume() {         // todo auto-generated method stub         super.onresume();         mysurfaceview.onresumemysurfaceview();     }      @override     protected void onpause() {         // todo auto-generated method stub         super.onpause();         mysurfaceview.onpausemysurfaceview();     }*/      class mysurfaceview extends surfaceview implements runnable{          thread thread = null;         surfaceholder surfaceholder;         volatile boolean running = false;          private paint paint = new paint(paint.anti_alias_flag);         random random;          public mysurfaceview(context context) {             super(context);             // todo auto-generated constructor stub             surfaceholder = getholder();             random = new random();         }          /*public void onresumemysurfaceview(){             running = true;             thread = new thread(this);             thread.start();         }          public void onpausemysurfaceview(){             boolean retry = true;             running = false;             while(retry){                 try {                     thread.join();                     retry = false;                 } catch (interruptedexception e) {                     // todo auto-generated catch block                     e.printstacktrace();                 }             }         }*/          @override         public void run() {             // todo auto-generated method stub             while(running){                 if(surfaceholder.getsurface().isvalid()){                     canvas canvas = surfaceholder.lockcanvas();                     //... actual drawing on canvas                      int x = random.nextint(getwidth());                      if(getwidth() - x < 100)                         x -= 100;                     else if(getwidth() - x > getwidth() - 100)                         x += 100;                      int y = random.nextint(getheight());                      if(getheight() - y < 100)                         y -= 100;                     else if(getheight() - x > getheight() - 100)                         y += 100;                      int radius;                     radius = 100;                     paint paint = new paint();                     paint.setstyle(paint.style.fill);                     paint.setcolor(color.white);                     canvas.drawpaint(paint);                     // use color.parsecolor define html colors                     paint.setcolor(color.parsecolor("#cd5c5c"));                     canvas.drawcircle(x, y, radius, paint);                      surfaceholder.unlockcanvasandpost(canvas);                 }             }         }      } } 

the first 2 onpause() , onresume() methods part of activity life cycle , invoked when activity paused/resumed.

this image illustrates android activity life cycle. can read on here

the reason works additional onresume , onpause methods in activity because invoke surfaceview onpausemysurfaceview() , onresumemysurfaceview() methods respective methods in activity. if didn't that, surfaceview methods never have been called , never stopped/started thread.