private long exitTime = 0;    @Override  public boolean onKeyDown(int keyCode, KeyEvent event) {      if(keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN){                        if((System.currentTimeMillis()-exitTime) > 2000){          Toast.makeText(getApplicationContext(), "再按一次退出程序", Toast.LENGTH_SHORT).show();                                exitTime = System.currentTimeMillis();      }      else{          finish();          System.exit(0);          }                        return true;      }      return super.onKeyDown(keyCode, event);  }