How to check, remove and replace duplicate values inside an Array List?

I would like to generate 5 random numbers and if the number already exists, remove it and replace it with a new one. Repeating the process until all the numbers generated are unique. However, I am stuck as I keep getting duplicate values inside my array.

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

public class Question2_NEW
{
    public static void main (String[] args)
    {
        System.out.println("Generating 5 random numbers from 0 to 5...");

        Random rand = new Random();
        int generate = rand.nextInt((5) + 1);
        ArrayList<Integer> randomNumbers = new ArrayList<Integer>();
        
        for (int i = 0; i < 5; i++)
        {
            generate = rand.nextInt((5) + 1);
            randomNumbers.add(generate);

            while (randomNumbers.contains(generate)) 
            {
                randomNumbers.remove(randomNumbers.get(i));
                generate = rand.nextInt((5) + 1);
                randomNumbers.add(generate);
                break;
            }
        }

        for (Integer i : randomNumbers) 
        {
            System.out.println(i);
        }
    }
}

This happens because, inside the while loop, you are not checking that the replacement number isn't also already in the ArrayList. It is simply accepted without question.

A better solution is to check whether the number is valid before you add it:

    for (int i = 0; i < 5; i++)
    {
        generate = rand.nextInt((5) + 1);
        while (randomNumbers.contains(generate)) 
        {
            generate = rand.nextInt((5) + 1);
        }
        randomNumbers.add(generate);
    }

Note that there are more efficient algorithms for this task. For one, membership checking is faster with a HashSet. For another, if you just want to generate a random permutation of the numbers 1 through N, a Fisher-Yates shuffle is the undisputed best solution. For something as tiny as 5 numbers, it probably doesn't matter though.

Write and call this method in your code.

private boolean arrayListIncludes(Integer i, ArrayList<Integer> randomNumbers) {
    for(int x = 0; x<randomNumbers.size(); x++) {
         if(randomNumbers.get(i).intValue() == i.intValue()) {
               return true;
         }
    }
    return false;
}

If you want to use List I would the following:

    public static void main(String[] args) {
        System.out.println("Generating 5 random numbers from 0 to 5...");

        Random rand = new Random();
        int generate;
        List<Integer> randomNumbers = new ArrayList<Integer>();

        for (int i = 0; i <= 5; i++) {
            do {
                generate = rand.nextInt((5) + 1);
            } while (randomNumbers.contains(generate));

            randomNumbers.add(generate);
        }

        for (Integer i : randomNumbers) {
            System.out.println(i);
        }
    }

If you can also use sets:

    public static void main(String[] args) {
        System.out.println("Generating 5 random numbers from 0 to 5...");

        Random rand = new Random();
        Set<Integer> randomNumbers = new HashSet<Integer>();

        while (randomNumbers.size() <= 5) {
            randomNumbers.add(rand.nextInt((5) + 1));
        }

        for (Integer i : randomNumbers) {
            System.out.println(i);
        }
    }

Edit: fixed issue with length of Set/ArrayList