LinkedList 구현 in Java

interface LinkedList {
    ListNode add(ListNode head, ListNode nodeToAdd, int position);
    ListNode remove(ListNode head, int positionToRemove);
    void printList(ListNode head);
    boolean contains(ListNode head, ListNode nodeToCheck);
}

public class ListNode implements LinkedList{
    private int data;
    private ListNode next;

    public ListNode(int data) {
        this.data = data;
        next = null;
    }

    public ListNode() {

    }

    @Override
    public ListNode add(ListNode head, ListNode nodeToAdd, int position) {
        // 초기 상태
        if (position == 0 && head == null) {
            head = nodeToAdd;
            return head;
        }
        // 맨 앞에 add
        else if (position == 0){
            nodeToAdd.next = head;
            head = nodeToAdd;

            return head;
        }

        ListNode node = head;
        // find pos
        for (int i = 0; i < position - 1; i++) {
            node = node.next;
        }
        // 해당 pos에 add
        nodeToAdd.next = node.next;
        node.next = nodeToAdd;
        return head;
    }
    @Override
    public ListNode remove(ListNode head, int positionToRemove) {
        if (positionToRemove == 0) {
            head = head.next;
            return head;
        }
        else {
            ListNode node = head;
            for(int i=0; i< positionToRemove; i++) {
                node = node.next;
            }
            node.next = node.next.next;
            return head;
        }
    }

    @Override
    public void printList(ListNode head) {
        ListNode node = head;
        while (node != null) {
            System.out.print(node.data);
            node = node.next;
        }
        System.out.println();
    }

    @Override
    public boolean contains(ListNode head, ListNode nodeToCheck) {
        ListNode node = head;
        while(node != null) {
            if (node.data == nodeToCheck.data) {
                return true;
            }
            node = node.next;
        }
        return false;
    }
    public static void main(String[] args)  {
        LinkedList nums = new ListNode();

        ListNode head = null;
        for(int i=0; i<5; i++) {
            head = nums.add(head, new ListNode(i), i);
        }
        nums.printList(head);
        head = nums.remove(head, 0);
        head = nums.remove(head, 0);
        nums.printList(head);
        System.out.println(nums.contains(head, new ListNode(3)));
        System.out.println(nums.contains(head, new ListNode(1)));
    }


}

• ListNode add(ListNode head, ListNode nodeToAdd, int position)
  
• ListNode remove(ListNode head, int positionToRemove);