Change text from a class out

I created a simple Java class. This should spend the time. With System.out.println it works too! But how can I inform a certain TextView that it should also change?

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();

You could write an interface, a TimeLeftListener with a method onTimeLeftChanged(String timeleft). Then your activity extends TimeLeftListener and implements the onTimeLeftChanged()-method.
This is where you update your views.
When you initiate your CountDownTest you pass the activity like this.

Countdown_test ct = new Countdown_test(600000, this);

And create al listener in you CountDownTest-class like this

myTimeLeftListener =(TimeleftListener)activity;

Then in your updateCountDownText()-Method you put

myTimeLeftListener.onTimeLeftChanged(timeleft)

You could also use ViewModel and LiveData.