Thread: cant get LinkedList to compile

So here is my interface for my LinkedList:

package data_structures;
 
import java.util.Iterator;
import java.util.NoSuchElementException;
 
public interface ListADT<E> extends Iterable<E> {
 
//  Adds the Object obj to the beginning of the list
    public void addFirst(E obj);
 
//  Adds the Object obj to the end of the list
    public void addLast(E o);
 
//  Removes the first Object in the list and returns it.
//  Returns null if the list is empty.
    public E removeFirst();
 
//  Removes the last Object in the list and returns it.
//  Returns null if the list is empty.
    public E removeLast();
 
//  Returns the first Object in the list, but does not remove it.
//  Returns null if the list is empty.
    public E peekFirst();
 
//  Returns the last Object in the list, but does not remove it.
//  Returns null if the list is empty.
    public E peekLast();
 
//  Removes the specific Object obj from the list, if it exists.
//  Returns true if the Object obj was found and removed, otherwise fa$
    public boolean remove(E obj);
 
//  The list is returned to an empty state.
    public void makeEmpty();
 
//  Returns true if the list contains the Object obj, otherwise false
    public boolean contains(E obj);
 
//  Returns true if the list is empty, otherwise false
    public boolean isEmpty();
 
//  Returns true if the list is full, otherwise false
    public boolean isFull();
 
//  Returns the number of Objects currently in the list.
    public int size();
 
//  Returns an Iterator of the values in the list, presented in
//  the same order as the list.
    public Iterator<E> iterator();
}

I tried to compile my LinkedList but get the following error: data_structures/LinkedListDS.java:6: data_structures.LinkedListDS is not abstract and does not override abstract method iterator() in data_structures.ListADT
public class LinkedListDS<E> implements ListADT<E> {

Here is my LinkedList:

package data_structures;
 
import java.util.Iterator;
import java.util.NoSuchElementException;
 
public class LinkedListDS<E> implements ListADT<E> {
        class Node<E> {
        E data;
        Node<E> next;
        public Node(E data) {
                this.data = data;
                next = null;
        }
}
private Node<E> head, tail;
private int currentSize;
public void LinkedListDS() {
        head=tail=null;
        currentSize= 0;
        }
public void addFirst(E obj){
        Node<E> newNode = new Node<E>(obj);
        if(head==null)
        head = tail= null;
        else {
                newNode.next=head;
                head = newNode;
             }
 
currentSize=currentSize++;
}
 
public E removeFirst() {
        if (head==null) return null;
        E tmp=head.data;
        head =head.next;
        if (head==null) tail=null;
        currentSize--;
return tmp;
}
}

Also i made a Stack code but get the following error when compiling: data_structures/Stack.java:7: '(' or '[' expected
list = new LinkedListDS<E>;
^
data_structures/Stack.java:17: '(' or '[' expected
}
^

package data_structures;
 
public class Stack implements Iterable<E> {
 
        private ListADT<E> list;
        public Stack() {
                list = new LinkedListDS<E>;
        }
        public void push(E obj) {
                list.addFirst(obj);
        }
        public E pop() {
                return list.removeFirst();
        }
        public Iterator<E> iterator() {
                return new IteratorHelper
        }
 
class IteratorHelper implements Iterator<E> {
        Node<E> iteratorPointer;
        public IteratorHelper() {
                 iteratorPointer = head;
        }
        public boolean hasNext() {
                return iteratorPointer != null;
        }
        public E next() {
                if (!hasNext())
                        throw NoSuchException();
        E temp = iteratorPointer.data;
        iteratorPointer = iteratorPointer.next;
        return tmp;
        }
        public void remove() {
                throw new UnsupportedOperationException();
        }
 
}
}

Any help would b greatly appreciated. I am new to programming and can't figure out what is wrong with my code.

Thanks for all your help:] I just have one more question. I am nearly done with my LinkedList here it is:

package data_structures;
 
import java.util.Iterator;
import java.util.NoSuchElementException;
 
public class LinkedListDS<E> implements ListADT<E> {
        class Node<E> {
        E data;
        Node<E> next;
        public Node(E data) {
                this.data = data;
                next = null;
        }
}
private Node<E> head, tail;
private int currentSize;
public void LinkedListDS() {
        head=tail=null;
        currentSize= 0;
        }
public void addFirst(E obj){
        Node<E> newNode = new Node<E>(obj);
        if(head==null)
        head = tail= null;
        else {
                newNode.next=head;
                head = newNode;
             }
 
        currentSize=currentSize++;
}
 
public void addLast(E obj){
                // Create a new node.
                Node < E > newNode = new Node < E > (obj);
        if(tail==null)
        tail=head=null;
        else {
                 tail.next = newNode; // Step 1
                // Link the new node to the tail.
                newNode.prev = tail; // Step 2
                // The new node is the new tail.
                tail = newNode; // Step 3
                }
        currentSize=currentSize++;
}
public E removeFirst() {  
        if (head==null) return null;
        E tmp=head.data;
        head =head.next;
        if (head==null) tail=null;
        currentSize--;
return tmp;
}
 
public E removeLast(){
        if (tail==null) return null;
        E tmp=tail.data;
        tail =  tail.prev; 
        if (tail==null) head=null;
        currentSize--;
return tmp;
}
 
public E peekFirst(){
        if(head == null) return null;
        return head.data;
}
 
public E peekLast(){
        if(tail == null) return null;
        return tail.data;
}       
 
public boolean remove(E obj){
if (head == null) return false;
if (head.data.equals(obj)) {
head = head.next;
return true;
currentSize--;
}
 
Node<E> current = head;
while (current.next != null) {
if (current.next.data.equals(obj)) {
current.next = current.next.next;
return true;
currentSize--;   
}
current = current.next;
}       
return false;
 
}
 
public void makeEmpty(){
head = tail = null;   
        currenSize = 0;
}
public boolean contains(E obj){
        Node<E> current = head;
        while (current != null) {   
                if (current.data.equals(obj)) {
                        return true;
                                                }
        current = current.next;
                                }
        return false;
}        
public boolean isEmpty(){
return first == null;
}
 
public boolean isFull(){
return false;
}
 
public int size(){
        return currentSize;
 
public Iterator<E> iterator() {
        return new IteratorHelper();
}
 
class IteratorHelper implements Iterator<E>{
        Node<E> IteratorPointer;
        public IteratorHelper(){
        IteratorPointer=head;
}
 
}
}
}

I am getting the following errors when i compile:
data_structures/LinkedListDS.java:122: illegal start of expression
public Iterator<E> iterator() {
^
data_structures/LinkedListDS.java:122: ';' expected
public Iterator<E> iterator() {
^
2 errors

I have tried to change around the IteratorHelper class to see if that was the problem, but had no luck.

I am having such a hard time getting my Iterator method to work. I know that I must include a class for the iterator and this is what I have so far:

package data_structures;
 
 
public class LinkedListDS<E> implements ListADT<E> {
        class Node<E> {
        E data;
        Node<E> next;
        public Node(E data) {
                this.data = data;
                next = null;
        }
}
private Node<E> head, tail;
private int currentSize;
public void LinkedListDS() {
        head=tail=null;
        currentSize= 0;
        }
public void addFirst(E obj){
        Node<E> newNode = new Node<E>(obj);
        if(head==null)
        head = tail= null;
        else {
                newNode.next=head;
                head = newNode;
             }
 
        currentSize=currentSize++;
}
 
public void addLast(E obj){
        Node<E> newNode = new Node<E>(obj);
        if(head == null){
                head = newNode;
                tail = newNode;
        currentSize++;
        return;
                        }
        tail.next = newNode;
        tail = newNode;
        currentSize++;
 
}
 
public E removeFirst() {
        if (head==null) return null;
        E tmp=head.data;
        if (head==null) return null;
        E tmp=head.data;
        head =head.next;
        if (head==null) tail=null;
        currentSize--; 
return tmp;
}
 
public E removeLast(){
        Node<E> previous = null;
        Node<E> current = head;
        if(current == null) return null;
        while(current.next != null){
                previous = current;
                current = current.next;
                                }
        if(previous == null) return removeFirst();
        previous.next = null;
        tail = previous;
        currentSize--;
        return current.data;
}
 
 
public E peekFirst(){
        if(head == null) return null;
        return head.data;
}
 
public E peekLast(){
        if(tail == null) return null;
        return tail.data;
}
 
public boolean remove(E obj){  
if (head == null) return false;
if (head.data.equals(obj)) {
head = head.next;
return true;
}
 
Node<E> current = head;
while (current.next != null) {
if (current.next.data.equals(obj)) {
current.next = current.next.next;
return true;
}
current = current.next;
}
currentSize--;
return false;
 
}
 
public void makeEmpty(){
head = tail = null;
        currentSize = 0;
}
public boolean contains(E obj){
        Node<E> current = head;
        while (current != null) {
                if (current.data.equals(obj)) {
                        return true;
                                                }
        current = current.next;
                                }
        return false;
}
public boolean isEmpty(){
return head == null; 
}
 
public boolean isFull(){
return false;
}
 
public int size(){
        return currentSize;
}
public Iterator<E> iterator() {
        return new IteratorHelper();
}
class IteratorHelper implements Iterator<E>{
        Node<E> IteratorPointer;
        public IteratorHelper(){
        IteratorPointer=head;
}
}
}

And i get the following errors when I compile:
data_structures/LinkedListDS.java:125: cannot find symbol
symbol : class Iterator
location: class data_structures.LinkedListDS<E>
public Iterator<E> iterator() {
^
data_structures/LinkedListDS.java:128: cannot find symbol
symbol : class Iterator
location: class data_structures.LinkedListDS<E>
class IteratorHelper implements Iterator<E>{
^
data_structures/LinkedListDS.java:128: interface expected here
class IteratorHelper implements Iterator<E>{


Can you please help me write the class for my iterator or point me in the right direction? I am relatively new to programming
I simply need the iterator to return an Iterator of the values in the list, presented in the same order as the list.