Android 两次按Back键退出应用

    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; //返回true表示执行结束不需继续执行父类按键响应        }          return super.onKeyDown(keyCode, event);      }