Single Linkedlist

public class LinkedList<DataType> {

    private Link<DataType> firstLink;

    private Link lastLink() {
        Link l = firstLink.getNext();
        if (l == null) {
            return firstLink;
        }
        while (l.getNext() != null) {
            l = l.getNext();
        }
        return l;
    }

    public int size() {
        int counter = 2;
        Link l = firstLink.getNext();
        if (l == null) {
            return 1;
        }
        while (l.getNext() != null) {
            l = l.getNext();
            counter++;
        }
        return counter;
    }

    public DataType get(int index) {
        if (index == 0) {
            return firstLink.getData();
        }

        int counter = 1;
        Link<DataType> l = firstLink.getNext();

        while (true) {
            if (counter == index) {
                return l.getData();
            }
            l = l.getNext();
            counter++;
        }
    }

    private Link getLink(int index) {
        if (index == 0) {
            return firstLink;
        }

        int counter = 1;
        Link<DataType> l = firstLink.getNext();

        while (true) {
            if (counter == index) {
                return l;
            }
            l = l.getNext();
            counter++;
        }
    }

    public void add(DataType data) {
        if (firstLink == null) {
            firstLink = new Link<>(data);
        } else {
            lastLink().link(new Link(data));
        }
    }

    public void pop() {
        if (size() == 1) {
            firstLink = null;
        } else if (size() != 0) {
            getLink(size() - 2).removeLink();
        }
    }

    public void pop(int index) {
        if (index == 0) {
            firstLink = null;
        } else {
            getLink(index - 1).link(getLink(index).getNext());
        }
    }

    public void insert(DataType data, int index) {
        Link l = new Link(data);
        l.link(getLink(index));
        if (index == 0) {
            firstLink = l;
        } else {
            getLink(index - 1).link(l);
        }
    }

    public void replace(DataType data, int index) {
        Link l = new Link(data);
        l.link(getLink(index).getNext());
        if (index == 0) {
            firstLink = l;
        } else {
            getLink(index - 1).link(l);
        }
    }

    private class Link<DataType> {

        private Link next;
        private DataType data;

        public Link(DataType data) {
            this.data = data;
        }

        public void link(Link link) {
            next = link;
        }

        public DataType getData() {
            return data;
        }

        public Link getNext() {
            return next;
        }

        public void removeLink() {
            next = null;
        }

    }
}