public?class?ProgressBars?extends?Activity{

	private?ProgressBar?bar;
	private?myAsyncTask?asyncTask;
	@Override
	protected?void?onCreate(Bundle?savedInstanceState)?{
		//?TODO?Auto-generated?method?stub
		super.onCreate(savedInstanceState);
		setContentView(R.layout.progerssbar);
		bar=(ProgressBar)?findViewById(R.id.pro);
		asyncTask.execute();
	}
	
	@Override
	protected?void?onPause()?{
		//?TODO?Auto-generated?method?stub
		super.onPause();
		if(asyncTask!=null&&asyncTask.getStatus()==asyncTask.getStatus().RUNNING){
			
			asyncTask.cancel(true);
		}
	}
	class?myAsyncTask?extends?AsyncTask<Void,?Integer,?Void>{

		@Override
		protected?Void?doInBackground(Void...?arg0)?{
			//?TODO?Auto-generated?method?stub
			for?(int?i?=?0;?i?<?100;?i++)?{
				if(isCancelled()){
					break;
				}
				publishProgress(i);
				try?{
					Thread.sleep(300);
				}?catch?(InterruptedException?e)?{
					//?TODO?Auto-generated?catch?block
					e.printStackTrace();
				}
			}
			return?null;
		}
		
		@Override
		protected?void?onProgressUpdate(Integer...?values)?{
			//?TODO?Auto-generated?method?stub
			super.onProgressUpdate(values);
			if(isCancelled()){
				return;
			}
			bar.setProgress(values[0]);
		}
	}
}

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.astest/com.example.astest.ProgressBars}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.os.AsyncTask com.example.astest.ProgressBars$myAsyncTask.execute(java.lang.Object[])' on a null object reference

主界面activity中設置了按鈕的點擊事件，也聲明了activity，不知道為什么還會報空指針異常