Android findViewById() not working when using a seperate class

Hi I am new to Android

I would like keep the following code seperatly from my MainActivity file. However, when I try to findViewById() inside the seperate class, I get the error

"cannot be resolved"

Now I know I cannot extend the MainActivity class as it will lead to stack overflow, but could someone tell me how to go about access a textview from this seperate file?

public class Counter {

    private TextView tempTextView;
    //Temporary TextView
    private Button tempBtn;
    public Handler mHandler = new Handler();
    public long startTime;
    public long elapsedTime;
    public final int REFRESH_RATE = 100;
    public String hours,minutes,seconds,milliseconds;
    public long secs,mins,hrs,msecs;
    public boolean stopped = false;

    public Runnable startTimer = new Runnable() {
        public void run() {
            elapsedTime = System.currentTimeMillis() - startTime;
            updateTimer(elapsedTime);
            mHandler.postDelayed(this,REFRESH_RATE);
        }
    };


    private void updateTimer (float time) {
        secs = (long) (time / 1000);
        mins = (long) ((time / 1000) / 60);
        hrs = (long) (((time / 1000) / 60) / 60); /* Convert the seconds to String * and format to ensure it has * a leading zero when required */
        secs = secs % 60;
        seconds = String.valueOf(secs);
        if (secs == 0) {
            seconds = "00";
        }
        if (secs < 10 && secs > 0) {
            seconds = "0" + seconds;
        } /* Convert the minutes to String and format the String */
        mins = mins % 60;
        minutes = String.valueOf(mins);
        if (mins == 0) {
            minutes = "00";
        }
        if (
                mins < 10 && mins > 0
                ) {
            minutes = "0" + minutes;
        } /* Convert the hours to String and format the String */
        hours = String.valueOf(hrs);
        if (hrs == 0) {
            hours = "00";
        }
        if (hrs < 10 && hrs > 0) {
            hours = "0" + hours;
        }
        /* Although we are not using milliseconds on the timer in this example * I included the code in the event that you wanted to include it on your own */
        milliseconds = String.valueOf((long) time);
        if (milliseconds.length() == 2) {
            milliseconds = "0" + milliseconds;
        }
        if (milliseconds.length() <= 1) {
            milliseconds = "00";
        }
        milliseconds = milliseconds.substring(milliseconds.length() - 3, milliseconds.length() - 2);
        /* Setting the timer text to the elapsed time */
       // ((TextView) findViewById(R.id.elapsed_value)).setText(hours + ":" + minutes + ":" + seconds);
        //  ((TextView)findViewById(R.id.timerMs)).setText("." + milliseconds); }

    }
}