private boolean isExit;     private Handler handler;     @Override     protected void onCreate(Bundle savedInstanceState) {         super.onCreate(savedInstanceState);         setContentView(R.layout.activity_main);         handler = new Handler(){             @Override             public void handleMessage(Message msg) {                 super.handleMessage(msg);                 isExit = false;             }         };     } 　　public boolean onKeyDown(int keyCode, KeyEvent event) {         if(keyCode==KeyEvent.KEYCODE_BACK){             if(!isExit){                 isExit = true;                 handler.sendEmptyMessageDelayed(0, 1500);                 Toast.makeText(this, "再按一次推出程序", 1000).show();                 return false;             }else{                 finish();             }         }         return super.onKeyDown(keyCode, event);     }

private long exitTimeMillis = System.currentTimeMillis();     @Override     protected void onCreate(Bundle savedInstanceState) {         super.onCreate(savedInstanceState);         setContentView(R.layout.activity_main);     }     @Override     public boolean onKeyDown(int keyCode, KeyEvent event) {         if(keyCode==KeyEvent.KEYCODE_BACK){             long currentTime = System.currentTimeMillis();             if(currentTime-exitTimeMillis==0||currentTime-exitTimeMillis>1500){                 exitTimeMillis = System.currentTimeMillis();                 Toast.makeText(this, "再按一次推出程序", 1000).show();                 return false;             }else{                 finish();             }         }         return super.onKeyDown(keyCode, event);     }