negative number not going into int

Question

I've the below code in java. Here i'm trying to see the random combinations of an array.

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Random;

public class Dummy {
    public static void main(String args[]) throws Exception {
        Random rand = new Random();

        int[] a = { 1, 2, 3, 4, 5 };
        int[] b = { 1, 0, 1, 0, 1 };
        Arrays.sort(a);
        Arrays.sort(b);
        int x = a.length * b.length;
        System.out.println(x);

        ArrayList<Integer> list = new ArrayList<Integer>();
        for (int i = 0; i < 25; i++) {

            System.out.println("random of i is" + a[rand.nextInt(i)]
                    + "and j is " + b[rand.nextInt(i)]);

        }
        System.out.println(list);

    }
}

and here i'm getting the below error.

Exception in thread "main" java.lang.IllegalArgumentException: n must be positive
    at java.util.Random.nextInt(Unknown Source)
    at Dummy.main(Dummy.java:20)

please let me know how to fix it. Based on some other posts based on similar issue, i found that only positive numbers are to be given in Random, i want to know how i can give negative numbers also.

Also i want to know how many combinations i can get by perpetuating a[] and b[].

Thanks

Answer 1

Your error is because of you are passing 0 in rand.nextInt(i).

rand.nextInt() expects positive integer greater that 0.

Answer 2

You are looping the array with maximum of 25 and with 0,

Change the maximum upto the size of the array and then initialize the i greater than zero otherwise you'll get ArrayIndexOutOfBoundsException

Do like this

for (int i = 1; i <= a.length; i++) {

            System.out.println("random of i is" + a[rand.nextInt(i)]
                    + "and j is " + b[rand.nextInt(i)]);
}