从课堂上更改文字

我创建了一个简单的Java类。这应该花费时间。使用System.out.println也可以！但是，如何通知某个TextView它也应该更改？

Countdown_Test.java

public class Countdown_test {

    private static long START_TIME_IN_MILLIS;
    private CountDownTimer mCountDownTimer;
    private boolean mTimerRunning;
    private long mTimeLeftInMillis;
    //private TextView timer;

    public Countdown_test(long start_time) {
        START_TIME_IN_MILLIS = start_time;
        mTimeLeftInMillis = START_TIME_IN_MILLIS;
        //timer = findViewById(R.id.timer); dosen't work
    }

    public void startTimer() {
        mCountDownTimer = new CountDownTimer(mTimeLeftInMillis, 1000) {
            @Override
            public void onTick(long millisUntilFinished) {
                mTimeLeftInMillis = millisUntilFinished;
                updateCountDownText();
            }
            @Override
            public void onFinish() {
                mTimerRunning = false;
            }
        }.start();
        mTimerRunning = true;
    }

    public void updateCountDownText() {
        int minutes = (int) (mTimeLeftInMillis / 1000) / 60;
        int seconds = (int) (mTimeLeftInMillis / 1000) % 60;
        String timeLeftFormatted = String.format(Locale.getDefault(), "%02d:%02d", minutes, seconds);
        System.out.println(timeLeftFormatted);
    }
}

MainActivity.java

 Countdown_test ct = new Countdown_test(600000);

        button_start.setOnClickListener(v -> {
          
                ct.startTimer();

        });
        ct.updateCountDownText();