Android控制双击返回按钮退出程序

两次按钮返回按钮，只在第二次按下时相应事件，并通知用户再次按下即可退出。

private static long back_pressed;

@Override
public void onBackPressed()
{
        if (back_pressed + 2000 > System.currentTimeMillis()) super.onBackPressed();
        else Toast.makeText(getBaseContext(), "Press once again to exit!", Toast.LENGTH_SHORT).show();
        back_pressed = System.currentTimeMillis();
}