Access UI thread in Handler

package com.example.androidrunnable;

import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.app.Activity;
import android.widget.TextView;

public class MainActivity extends Activity {
 
 static TextView prompt;
 static int counter;
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        //setContentView(R.layout.activity_main);
        prompt = new TextView(this);
        setContentView(prompt);
        
        prompt.setText("Hello");
        
        Thread myThread = new Thread(myRunnable);
        myThread.start();
    }
    
 static Handler myHandler = new Handler(){

  @Override
  public void handleMessage(Message msg) {
   prompt.setText(String.valueOf(counter));
  }
     
    };

    Runnable myRunnable = new Runnable(){

  @Override
  public void run() {
   counter = 0;
   while(true){
    try {
     Thread.sleep(1000);
     myHandler.sendMessage(myHandler.obtainMessage());
     counter++;
    } catch (InterruptedException e) {
     // TODO Auto-generated catch block
     e.printStackTrace();
    }
   }
  } 
    };
    
}