"Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 0, Size: 0" with ArrayList?

Question

"Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 0, Size: 0" is the main error I get when I compile this method:

public static ArrayList<ArrayList<Integer>> createSparseArray(int len, double den) {
    int counter = 0;
    ArrayList<Integer> placeHolder = new ArrayList<Integer>();
    for (int j = 0; j < len; j++) {
        double randomNumber = Math.random();
        if (randomNumber < den) {
            counter++;
            placeHolder.add(j);
        }
    }
    ArrayList<ArrayList<Integer>> list = new ArrayList<ArrayList<Integer>>();
    for (int k = 0; k < counter; k++) {
        for (int m = 0; m < 2; m++) {
            list.get(0).set(placeHolder.get(k), (int) (Math.random() * (99999) + 1));
        }
    }
    return list;
}

How can I fix this?

Answer 1

ArrayList<ArrayList<Integer>> list doesnt contain any element in (0)th postion and the compiler throws out of bounds exception on iterating when it doesn't find any element in the specified postion.

when you try executing list.get(0).set(placeHolder.get(k), (int) (Math.random() * (99999) + 1)); statement , your list doesnt contain any element inside it. you need to iterate the inner list to set the values for the list.