unrepeatable matrix

package com.company;

import java.util.*;

public class Main {

    final static int size = 2255;

    public static void main(String[] args) {
        int[][] matrix = algol();
        Set<Integer> hashSet = new HashSet<>(size * size);
        for (int i = 0; i < size; i++) {
            for (int j = 0; j < size; j++) {
                hashSet.add(matrix[i][j]);
            }
        }
        System.out.println("Count in matrix = " + hashSet.size() + ". Expected count = " + size * size); //without unit tests!!!
    }

    public static int[][] algol() {
        Random random = new Random();
        List<Integer> arr = new ArrayList<>(size * size);
        int[][] matrix = new int[size][size];
        int step = Integer.MAX_VALUE / (size * size);
        int last = 0;
        for (int i = 0; i < size * size; i++) {
            int newn = last + random.nextInt(step);
            arr.add(newn);
            last = newn + 1;
        }
        Collections.shuffle(arr);
        int n = 0;
        for (int i = 0; i < size; i++) {
            for (int j = 0; j < size; j++) {
                matrix[i][j] = arr.get(n++);
            }
        }
        return matrix;
    }

}