What is Cookie Testing and can someone shares test cases for Website Cookie Testing?

Question

Can someone help me to build Failure Test case for these Functions..

public int sumFromTo(int[] arr, int from, int to) { // 1 Case
        if (arr == null || arr.length == 0 || to > arr.length - 1) {
            throw new IllegalArgumentException();
        }
        int sum = 0;
        for (int i = from; i <= to; i++) {
            sum += arr[i];
        }
        return sum;
    }

    // Second Function
    public String replaceCharacter(String word, char oldchar, char newchar) {
        if (word == null) {
            throw new IllegalArgumentException(); // 2 cases
        }
        int i = 0;
        String s = new String();
        do {
            if (word.toLowerCase().charAt(i) == oldchar) {
                s += newchar;
            } else {
                s += word.charAt(i);
            }
            i++;
        } while (i < word.length());
        return s;
    }

    // Third Function
    public boolean isPrime(int num) { // 3 cases
        for (int i = 2; i < 10; i++) {
            if (num % i == 0 && i != num) {
                return false;
            }
        }
        return true;
    }