轉自：http://www.runoob.com/w3cnote/android-tutorial-activity-start.html
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定義一個變量，表識是否退出

// 定義一個變量，來標識是否退出
private static boolean isExit = false;
Handler mHandler = new Handler() {
    @Override
    public void handleMessage(Message msg) {
        super.handleMessage(msg);
        isExit = false;
    }
};

public boolean onKeyDown(int keyCode, KeyEvent event) {
    if (keyCode == KeyEvent.KEYCODE_BACK) {
        if (!isExit) {
            isExit = true;
            Toast.makeText(getApplicationContext(), "再按一次退出程序",
                    Toast.LENGTH_SHORT).show();
            // 利用handler延遲發(fā)送更改狀態(tài)信息
            mHandler.sendEmptyMessageDelayed(0, 2000);
        } else {
            exit(this);
        }
        return false;
    }
return super.onKeyDown(keyCode, event);}

保存點擊時間

//保存點擊的時間
private long exitTime = 0;
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if (keyCode == KeyEvent.KEYCODE_BACK) {
        if ((System.currentTimeMillis() - exitTime) > 2000) {
            Toast.makeText(getApplicationContext(), "再按一次退出程序",
                    Toast.LENGTH_SHORT).show();
            exitTime = System.currentTimeMillis();
        } else {
                        exit();
                      }
        return false;
    }
        return super.onKeyDown(keyCode, event);
}