如何在Java中访问我的CountDownTimer来取消它？

CountDownTimer timer = new CountDownTimer(30000, 1000) {
    public void onTick(long millisUntilFinished) {
        // 每秒执行的操作
    }

    public void onFinish() {
        // 倒计时结束时执行的操作
    }
};

timer.start();

timer.cancel();

import android.os.CountDownTimer;

public class MainActivity extends AppCompatActivity {
    private CountDownTimer timer;

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

        timer = new CountDownTimer(30000, 1000) {
            public void onTick(long millisUntilFinished) {
                // 每秒执行的操作
            }

            public void onFinish() {
                // 倒计时结束时执行的操作
            }
        };

        timer.start();
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        timer.cancel();
    }
}