Flatten nested arrays

FlattenArraysTetst.java

public static void main() {
    Object[] array1 = new Object[]{3}; // [3]
    Object[] array2 = new Object[]{1, 2, array1}; // [ 1, 2, [3] ]
    Object[] nestedArrays = new Object[]{array2, 4, null}; // [ [ 1, 2, [3] ] 4 ]

    ArrayList<Integer> flatOutput = new ArrayList<>();

    try {
        flatten(nestedArrays, flatOutput);
        System.out.println("flatOutput = " + flatOutput);
    } catch (NullPointerException e) {
        System.err.println("something goes wrong");
        e.printStackTrace();
    }
}

public void flatten(Object[] nestedArrays, ArrayList<Integer> flatOutput) throws NullPointerException {

        if (nestedArrays == null) {
            throw new NullPointerException("Input array should not be null");
        }

        if (flatOutput == null) {
            throw new NullPointerException("Output array should not be null");
        }

        for (Object element : nestedArrays) {
            // of course int is not derived from Object, int this case JVM uses autoboxing to store int as Integer
            if (element instanceof Integer) {
                // is number
                flatOutput.add((int) element);
            } else {
                // is array
                flatten((Object[]) element, flatOutput);
            }
        }

    }