重写   onKeyDown()方法。

 

@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();             //释放内存             System.exit(0);         }         return true;     }     return super.onKeyDown(keyCode, event); }