你可以使用CountdownTimer類來創建一個簡單的計時器，下面是一個示例代碼：

import android.os.CountDownTimer;
import android.widget.TextView;

public class MyCountdownTimer extends CountDownTimer {

    private TextView textView;

    public MyCountdownTimer(long millisInFuture, long countDownInterval, TextView textView) {
        super(millisInFuture, countDownInterval);
        this.textView = textView;
    }

    @Override
    public void onTick(long millisUntilFinished) {
        long secondsUntilFinished = millisUntilFinished / 1000;
        textView.setText("倒計時：" + secondsUntilFinished + "秒");
    }

    @Override
    public void onFinish() {
        textView.setText("倒計時結束");
    }
}

然后在你的Activity中使用這個計時器：

public class MainActivity extends AppCompatActivity {

    TextView textView;
    MyCountdownTimer countdownTimer;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        textView = findViewById(R.id.textView);

        countdownTimer = new MyCountdownTimer(60000, 1000, textView); // 倒計時60秒，每秒更新一次
        countdownTimer.start();
    }
}

這樣，你就可以在TextView中看到一個簡單的計時器，顯示倒計時的秒數，直到計時結束。