//記錄使用者首次點選返回鍵的時間
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(this,"再按一次退出程式",Toast.LENGTH_SHORT).show();
            firstTime=System.currentTimeMillis();
        }else{
            finish();
            System.exit(0);
        }
        return true;
    }
    return super.onKeyDown(keyCode, event);
}