MainActivity.java

public class MainActivity extends AppCompatActivity {
    TextView tv;
    Button startBt, stopBt;
    UpdateTask tvCounter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tv = (TextView)findViewById(R.id.textView);
        startBt = (Button)findViewById(R.id.button);
        stopBt = (Button)findViewById(R.id.button2);
      
        tvCounter = new UpdateTask(tv);
    }

    public void startClick(View view){
        tvCounter = new UpdateTask(tv);
        tvCounter.execute();
    }

    public void stopClick(View view){
        tvCounter.cancel(true);
        CharSequence text = "Hello toast!";
        Toast.makeText(this, text, Toast.LENGTH_SHORT).show();
    }
}

class UpdateTask extends AsyncTask<Void, Integer, Void> {
    TextView timeTv;
    public UpdateTask(TextView timeTv){
        this.timeTv = timeTv;
    }
    protected void onProgressUpdate(Integer... args) {
        int counter = args[0];
        timeTv.setText("" + counter);
    }
    @Override
    protected Void doInBackground(Void... params) {
        int counter = 0;
        while(!isCancelled()) {
            counter++;
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            publishProgress(counter); // This will call onProgressUpdate
        }
        return null;
    }
}