//--------------使用onKeyUp()干掉他--------------

  //记录用户首次点击返回键的时间
  private long firstTime=0;

  @Override
  public boolean onKeyUp(int keyCode, KeyEvent event) {
    switch (keyCode){
      case KeyEvent.KEYCODE_BACK:
        long secondTime=System.currentTimeMillis();
        if(secondTime-firstTime>2000){
          Toast.makeText(MainActivity.this,"再按一次退出程序--->onKeyUp",Toast.LENGTH_SHORT).show();
          firstTime=secondTime;
          return true;
        }else{
          System.exit(0);
        }
        break;
    }
    return super.onKeyUp(keyCode, event);
  }

 //--------------使用onKeyDown()干掉他--------------

  //记录用户首次点击返回键的时间
  private long firstTime=0;

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

 @Override
  public boolean onKeyDown(int keyCode, KeyEvent event) {
    if(keyCode==KeyEvent.KEYCODE_BACK){
      exitByDoubleClick();
    }
    return false;
  }

  private void exitByDoubleClick() {
    Timer tExit=null;
    if(!isExit){
      isExit=true;
      Toast.makeText(MainActivity.this,"再按一次退出程序--->Timer",Toast.LENGTH_SHORT).show();
      tExit=new Timer();
      tExit.schedule(new TimerTask() {
        @Override
        public void run() {
          isExit=false;//取消退出
        }
      },2000);// 如果2秒钟内没有按下返回键，则启动定时器取消掉刚才执行的任务
    }else{
      finish();
      System.exit(0);
    }
  }