android实现按两次返回键退出程序

//重写返回键，按两次退出程序

private long mTime;
 public boolean onKeyUp(int keyCode, KeyEvent event) {
  if (keyCode == KeyEvent.KEYCODE_BACK) {
   if ((System.currentTimeMillis() - mTime) > 2000) {
    Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();
    mTime = System.currentTimeMillis();
   } else {
    finish();
   }
   return true;
  }
  return super.onKeyUp(keyCode, event);
 }