Question

When I tested this code, it gave me java.lang.ArrayIndexOutOfBoundsException: 2
The code isn’t finished yet but this is what I am trying to do:
A program that asks the user for two numbers. Print all numbers from the smaller
number to the larger number (inclusive) in a random order. Using this order, find the
largest sum of 2 numbers adjacent to one another.
Could some please point out to me what needs to be fixed as well as how to fix it? Thanks.

 boolean isNum = false;
    int in1 = 0;
    int in2 = 0;
    int size = 0;
    int largestsum = 0;
    while (!isNum) {
        try {
            System.out.print("Enter a number: ");
            in1 = Integer.parseInt(in.readLine());
            boolean isSame = false;
            while (!isSame) {
                System.out.print("Enter a number: ");
                try {
                    in2 = Integer.parseInt(in.readLine());
                    if (in1 != in2) {
                        isSame = true;
                    } else {
                        System.out.println("Same number.");
                    }
                } catch (NumberFormatException nfe) {
                    System.out.println("Not a number.");
                }
            }
            isNum = true;
        } catch (NumberFormatException nfe) { //catches error
            System.out.println("Not a number."); //informs user of the error
        }
    }
    if (in1 > in2) {
        size = in1 - in2 + 1;
        int[] num = new int[size];
        for (int i = 0; i < size; i++) {
            num[i] = in2 + i;
        }
        int[] order = new int[size];
        boolean[] used = new boolean[size];
        for (int i = 0; i < size; i++) {
            int r = (int) ((size) * (Math.random()));
            while (used[r]) {
                r = (int) ((size) * Math.random());
            }
            order[i] = r; //fill array
            used[r] = true; //r is now used
        }
        for (int i = 0; i < size; i++) {
            if (num[order[i]] + num[order[i + 1]] > largestsum) {
                largestsum = num[order[i]] + num[order[i + 1]];
            }
        }

Answer 1

The last for loop will overflow when i=size-1

In your question, you want to get the sum between 2 adjacent numbers. Your logic in the last for loop says that you calculate the sum by adding number i to i+1. The last item in your array is at position size-1, so when you then try to add this to the next adjacent number, (size-1)+1 overflows the array.

To fix the problem, your last for loop should only continue until i<size-1, like so…

    for (int i = 0; i < size-1; i++) {
        if (num[order[i]] + num[order[i + 1]] > largestsum) {
            largestsum = num[order[i]] + num[order[i + 1]];
        }
    }

With this fixed logic, the largest item in the for loop is size-2, and when you add 1 to it (for the next adjacent number), it will choose item (size-2)+1, which is the last item in the array, rather than overflowing the array.