Show Max And Min Values In Array List

I'm trying to find out the min and max values of randomly generated values in a array list. Its a checkout system that randomly add a customer to a queue and does the same to remove a customer depending on a randomly generated number. I've managed to figure out the mean of the queue but can only display the total values added to the list rather than display when the queue was at it biggest value.

import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.ListIterator;
import java.util.Queue;
import java.util.Random;

public class Checkout {
private Queue<String> tillQueue;
private int rndNumber;
private int currentLen;
private ArrayList <Integer>lengthList;
private Random r;
public Checkout() .....