private long mTime;

  @Override

     public boolean onKeyDown(int keyCode, KeyEvent event) {

         if (keyCode == event.KEYCODE_BACK) {

             if ((System.currentTimeMillis() - mTime) > 2000) {

                 Toast.makeText(this, "再按一次退出APP", Toast.LENGTH_SHORT).show();

                 //获取系统当前时间

                 mTime = System.currentTimeMillis();

             } else {

                 finish();

             }

             return true;

         }

         return super.onKeyDown(keyCode, event);

     }