class A{ 
 
 private boolean mIsStarted; 
 void start(){ 
   boolean changed = false; 
   synchronized(this){ 
  if(!mIsStarted){ 
    mIsStarted = true; 
    changed = false; 
   } 
   if(changed){ 
    callBack.onStart(); 
   } 
  } 
 } 
 
boolean isStarted(){ 
 sychronized(this){ 
  return mIstarted; 
 } 
} 
 void stop(){  
  boolean changed = false;  
  synchronized(this){  
 
  if(mStarted){  
   mStarted = false;  
  changed = true;  
  }  
 }  
 
 if(changed){ 
  callBack.onStop();  
  }  
 }  
} 

 if(!mIsStarted){

 mIsStarted = true; 
 changed = false; 
} 

synchronized(this){ 

 if(mStarted){  
  mStarted = false;  
 changed = true;  
 }  
}