I need to generate and display a random number in SystemClock.sleep() with no range

I'm a student at a community college. The following is the Main Activity in which I was tasked to generate a random number instead of the 5 milliseconds to run the sleep()method. I then have to display that random number. I figured out how to accomplish it for the first button, but I keep running into snags for the next two(converting to int, converting to a string for resultsTextView.setText();. I have tried several times to code a method, but then I'm running into other errors because onCreate is void.

package com.example.dan.hour5application;

import android.content.Context;
import android.content.Intent;
import android.content.res.Resources;
import android.os.AsyncTask;
import android.os.SystemClock;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import java.util.Random;

public class MainActivity extends AppCompatActivity {

    Button uiThreadButton;
    Button postButton;
    Button asyncTaskButton;
    TextView resultsTextView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Resources res = getResources();
        float fontSize = res.getDimension(R.dimen.font_size);

        resultsTextView = (TextView) findViewById(R.id.textView);
        uiThreadButton = (Button) findViewById(R.id.uiThreadButton);
        uiThreadButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {   
        /////Achieved a random number here////
                Random random = new Random();
                int x = random.nextInt(10);
                SystemClock.sleep(x);
                String aString = Integer.toString(x);
                resultsTextView.setText(aString);
            }
        });

        postButton = (Button) findViewById(R.id.post);
        postButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                new Thread(new Runnable() {
                    public void run() {
                        resultsTextView.post(new Runnable() {
                            @Override
                            public void run() {
                                resultsTextView.setText("Updated");
                            }
                        });
                    }
                }).start();
            }
        });

        asyncTaskButton = (Button) findViewById(R.id.asynctask);

        class DelayTask extends AsyncTask<Integer, Integer, Integer> {

            @Override
            protected void onPreExecute() {
                resultsTextView.setText("Starting AsyncTask");
            }

            @Override
            protected void onPostExecute(Integer result) {
                if (result == 0) {
                    resultsTextView.setText("Updated via AsyncTask");
                }
            }

            @Override
            protected Integer doInBackground(Integer... params) {
                SystemClock.sleep(5000);
                return 0;
            }
        }
    }
}

I have looked at several other examples on Stack, but they aren't quite what I'm doing. I'm only starting my 4th month in Android Development, and I'm still trying to learn Java. The assignment is long gone, and my instructor didn't provide me with help when I asked. Thanks for your help!