Implementing List interface for CustomList in Java

Question

I need to create a custom list that adds elements in pairs. I copied a similar implementation and adjusted it as needed, but unfortunately, it doesn't work.

class PairStringList implements List<String> {

    private List list;

    public PairStringList() {
        list = new ArrayList();
    }

    @Override
    public boolean add(String s) {
        list.add(list.size(), s);
        list.add(list.size(), s);
        return true;
    }

//other methods
}

And I have this for testing my code so far.

public static void main(String[] args) {
        List<String> list = new PairStringList();
        list.add("1");
        list.add("2");
        list.add("3");

        for(int i = 0; i < list.size(); i++) {
            String s = list.get(i);
            System.out.println(s);
        }

        list.add(4, "0");

        for(int i = 0; i < list.size(); i++) {
            System.out.println(list.get(i));
        }
    }

The problem is that when I call add method the size of the list increases by 2 as it should, but nothing is added and I get an array full of nulls.

Answer 1

You must have to implement get(int index) method in your PairStringList. Currently your get method is simply returning null. Implementation of get(int index) method should be as shown below:

@Override
public String get(int index) {
    return list.get(index);
}