Question

I am having a problem with one of my methods in my program. The method is designed to take 2 arraylists and the perform multiplication between the two like a polynomial.

For example, if I was to say list1={3,2,1} and list2={5,6,7}; I am trying to get a return value of 15,28,38,20,7. However, all I can get is an error message that says:

Exception in thread “main” java.lang.IndexOutOfBoundsException: Index: 0, Size: 0.

I have provided the method below:

private static ArrayList<Integer> multiply(ArrayList<Integer> list1,ArrayList<Integer> list2) {

    ArrayList<Integer> array =new ArrayList<Integer>(list1.size()+list2.size());

    for (int i=0;i<array.size();i++)
        array.add(i, 0);

    for (int i = 0; i < list1.size(); i++)

        for (int j = 0; j < list2.size(); j++)

            array.set(i+j, ((list1.get(i) * list2.get(j))+array.get(i+j)));

    return array;

}

Any help with solving this problem is greatly appreciated.

Answer 1

Change your first for loop to:

for (int i = 0 ; i < list1.size() + list2.size() ; i++)
    array.add(0);

As you have it, array.size() is initially 0 so that first for loop is never even entered, so nothing is being added to array. An ArrayList‘s capacity is not the same thing as its size.