Random Unique Number

import java.util.ArrayList;
import java.util.Random;

public class uniqueRNG {
    private ArrayList<Integer> numbers = new ArrayList<Integer>(); //new arraylist, is a list with variable size
    private Random random = new Random();   //Built in java Random class
    private int addnum = -1;    //addnum is the current drawn variable
    private int numberamount = 39;

    public Integer drawNumber() {
        while (true){ //keep drawing numbers if not already in the list.
            addnum = this.random.nextInt(numberamount) ; //draw number from 0 to 39. Can be anything you change numberamount to.

            if (numbers.size() >= numberamount){ // if size is bigger than the max amount of numbers allowed, return -1.
                return -1;
            }

            if (!containsNumber(addnum)){ //if not contain number
                numbers.add(addnum); //add number to list to track number
                return addnum; //return number drawn
            }
        }
    }

    //checks in this function whether the number supplied is in list or not. Works as a boolean function for the if statement in drawNumber();
    public boolean containsNumber(int number) {
        if (numbers.contains(number)){
            return true;
        } else {
            return false;
        }
    }
}