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Thread: cant get LinkedList to compile

  1. #1
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    Default 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.


  2. #2
    Super Moderator Norm's Avatar
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    Default Re: cant get LinkedList to compile

    LinkedListDS is not abstract and does not override abstract method iterator()
    The compiler can't find a definition for the iterator() method in the LinkedListDS class.
    If you implement an interface you must define all of its methods.

    '(' or '[' expected
    Where is the ^ located in the error messages. The posting lost its position. Look at the location in the statement above the ^ for the problem. The compiler is expecting to see a '(' or a '[' there.
    Last edited by Norm; September 29th, 2012 at 04:44 PM.
    If you don't understand my answer, don't ignore it, ask a question.

  3. #3
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    Default Re: cant get LinkedList to compile

    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.

  4. #4
    Super Moderator Norm's Avatar
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    Default Re: cant get LinkedList to compile

    Check that all the {s have a matching } and that no methods are defined inside of other methods.
    If you don't understand my answer, don't ignore it, ask a question.

  5. #5
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    Default Re: cant get LinkedList to compile

    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.

  6. #6
    Super Moderator Norm's Avatar
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    Default Re: cant get LinkedList to compile

    cannot find symbol
    symbol : class Iterator
    You need an import statement so the compiler can find the definition of the Iterator interface.

    Start by making a skeleton class that implements the interface and has all the methods.
    Then work on them one at a time.

    You need to fix the formatting of the posted code. There should not be a column of }s. Each } should be in its own column underneath the start of the statement with the matching {
    Too many statements start in the first column. Unformated code is hard to read and understand.
    If you don't understand my answer, don't ignore it, ask a question.

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