Android 实现再按一次后退键退出应用程序

private boolean isexit = false; 
private boolean hastask = false;
Timer texit = new Timer();

TimerTask task = new TimerTask() {
		public void run() {
		isexit = false;
		hastask = true;
		}
	};

@Override
	public boolean onKeyDown(int keyCode, KeyEvent event) {
		if(keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN){
			//DialogAPI.showExit(this);
			if(isexit == false){
				isexit = true;
				Toast.makeText(getApplicationContext(), "再按一次退出程序", Toast.LENGTH_SHORT).show();
				if(!hastask) {
					texit.schedule(task, 2000);
				}
			}else{
				finish();
				System.exit(0);
			}
			return false;
			}
		
		return super.onKeyDown(keyCode, event);
	}