从另一个类获取TextView进行倒计时

// CountdownTimerActivity.java
public class CountdownTimerActivity extends AppCompatActivity {
    private TextView countdownTextView;
    private CountDownTimer countDownTimer;

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

        countdownTextView = findViewById(R.id.countdownTextView);

        countDownTimer = new CountDownTimer(60000, 1000) {
            @Override
            public void onTick(long millisUntilFinished) {
                countdownTextView.setText("倒计时：" + millisUntilFinished / 1000 + "秒");
            }

            @Override
            public void onFinish() {
                countdownTextView.setText("倒计时结束");
            }
        };
    }

    public void startCountdown() {
        countDownTimer.start();
    }

    public void stopCountdown() {
        countDownTimer.cancel();
    }
}

// MainActivity.java
public class MainActivity extends AppCompatActivity {
    private CountdownTimerActivity countdownTimerActivity;

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

        countdownTimerActivity = new CountdownTimerActivity();

        TextView textView = findViewById(R.id.textView);
        Button startButton = findViewById(R.id.startButton);
        Button stopButton = findViewById(R.id.stopButton);

        startButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                countdownTimerActivity.startCountdown();
            }
        });

        stopButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                countdownTimerActivity.stopCountdown();
            }
        });
    }
}