How to add a node at specified index in LinkedList implementation

Question

So I am implementing a LinkedList from scratch and one method, insertAt(int index, T elem), is really giving me a headache. The method is supposed to insert a node at the specified index and move the rest of the list accordingly. My implementation seems to just copy and paste existing nodes. Any help on what I am doing wrong? (use cases below)

Let me know if it would help to include the interface notes or the full class:

public class LinkedList<T> implements ListInterface<T> {
private Node<T> first;
private Node<T> last;
private int counter;
public LinkedList() {
}
@Override
public ListInterface<T> insertAt(int index, T elem) {
 if(index > counter) {
 throw new IndexOutOfBoundsException();
 }
 Node<T> node = new Node<T>(null, elem, null);
 if(counter == 0) {
 first = last = node;
 }
 else {
 if(index == 0) {
 node.next = first;
 first.prev = node;
 first = node;
 }
 else if(index == counter) {
 node.prev = last;
 last.next = node;
 last = node;
 }
 else {
 Node<T> current = this.first;
 for(int i = 0; i < index; i++) {
 current = current.next;
 }
 node.next = current;
 node.prev = current.prev;
 current.prev.next = node;
 }
 }
 counter++;
 return this;
}

Node Class:

public class Node<T> {
public T data;
public Node<T> next;
public Node<T> prev;
public Node(Node<T> prev, T data, Node<T> next){
 this.data = data;
 this.next = next;
 this.prev = prev;
}
public Node() {
}

Usage Example #1 (Wrong Answers):

 LinkedList<String> list = new LinkedList <String>();
 list.insertFirst("p");
 list.insertFirst("a");
 list.insertFirst("e");
 list.insertFirst("h");
 list.insertAt(2, "A");

Console:

Before: {h, e, a, p}

After: {h, e, e, a, p}

What Console SHOULD BE:

Before: {h, e, a, p}

After: {h, e, A, a, p}

Answer 1

current.prev = node; you lost this one while 0 < index < counter

i test your code, seems works:

public class LinkedList<T> {
 private Node<T> first;
 private Node<T> last;
 private int counter;
 public LinkedList() {
 }
 public static void main(String args[]) {
 LinkedList<String> list = new LinkedList<String>();
 list.insertFirst("p");
 list.insertFirst("a");
 list.insertFirst("e");
 list.insertFirst("h");
 list.insertAt(2, "A");
 for (Node n = list.first; n != null; n = n.next) {
 System.out.println(n.data);
 }
 }
 private void insertFirst(T s) {
 insertAt(0, s);
 }
 public LinkedList<T> insertAt(int index, T elem) {
 if (index > counter) {
 throw new IndexOutOfBoundsException();
 }
 Node<T> node = new Node<T>(null, elem, null);
 if (counter == 0) {
 first = last = node;
 } else {
 if (index == 0) {
 node.next = first;
 first.prev = node;
 first = node;
 } else if (index == counter) {
 node.prev = last;
 last.next = node;
 last = node;
 } else {
 Node<T> current = this.first;
 for (int i = 0; i < index; i++) {
 current = current.next;
 }
 node.next = current;
 node.prev = current.prev;
 current.prev.next = node;
 current.prev = node;
 }
 }
 counter++;
 return this;
 }
}
class Node<T> {
 public T data;
 public Node<T> next;
 public Node<T> prev;
 public Node(Node<T> prev, T data, Node<T> next) {
 this.data = data;
 this.next = next;
 this.prev = prev;
 }
 public Node() {
 }
}