recursively sum the integers in an array

The solution is simpler than it looks, try this (assuming an array with non-zero length):

public int sumOfArray(int[] a, int n) {
    if (n == 0)
        return a[n];
    else
        return a[n] + sumOfArray(a, n-1);
}

Call it like this:

int[] a = { 1, 2, 3, 4, 5 };
int sum = sumOfArray(a, a.length-1);

The issue is that a[n-1] is an int, whereas sumOfArray expects an array of int.

Hint: you can simplify things by making sumOfArray take the array and the starting (or ending) index.

a[n-1]

is getting the int at n-1, not the array from 0 to n-1.

try using

Arrays.copyOf(a, a.length-1);

instead

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