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Thread: LinkedList -- insert()

  1. #1
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    Default LinkedList -- insert()

    For this program I am dealing with the linkedlist interface. I am designing a method "insert" that will add a new member to a specified location provided by the user (public insert(T newMem, int index) ). Now, I'm not sure how to find the desired index within the list and test whether that index is available to insert the new member. If the list is empty, the new member will be added to the list (should it be added to the head? that just seems confusing to me, because if that member is at index 0 instead of index 7 ... would it be possible to insert the newMember into the desired index of the list? ). Within the final else clause I want to try whether the index is available and if it is empty(), insert the newMember...if it is !empty(), move the member in that index to the tail. Any tips? Is this a good approach?

    public void insert(T newMember, int index) {
     
            if(empty()){
     
            }
            else if(index < 0) { 
                System.out.println("Index Out Of Bound");
            }      
            else{
                try{
                    member.setData(newMember);
                }catch(Exception e){
     
                }
            }
        }


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    Default Re: LinkedList -- insert()

    how to find the desired index within the list
    By following the "next" chain.
    I'd think if the list is empty, insertions wouldn't be possible.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: LinkedList -- insert()

    The hasNext() method supposed to return whether the next element or node is empty and return that node or return the node value.
    I'm having a little trouble with the next() method. I want to test if the next node is null and set that node as the current and add a new node element to the list. When I use the next method in addLast(), error that it cannot find symbol. Why would that be?

    public boolean hasNext() {
            if(current.next() == null) {           
                return false;
            }
            return(current.getNode() != null);
        }
     
        public ListNode<T> next() throws PointerOutOfBoundException {
            if(hasNext()){
                current = current.getNode();
                return current;
            }
            else throw new PointerOutOfBoundException ();
        }

    public void addLast(T newMember) {
             if (empty ()){
                addFirst (newMember);
                head = tail;
        }
            else
            {
                tail = new ListNode<T>();
                tail.next() = tail;
     
            }
             size++;
        }

    ---------EDIT----------
    cannot find symbol
    symbol: method next()
    location: variable newNode of type ListNode<T>
    where T is a type-variable:
    T extends Object declared in class MySinglyLinkedListD
    Last edited by javaStooge; July 12th, 2014 at 10:01 AM. Reason: Error Message

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    Default Re: LinkedList -- insert()

    error that it cannot find symbol.
    Please copy the full text of the error message and paste it here. It has important info about the error.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: LinkedList -- insert()

    Posted here to make sure it notified you.

    cannot find symbol
    symbol: method next()
    location: variable newNode of type ListNode<T>
    where T is a type-variable:
    T extends Object declared in class MySinglyLinkedListD

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    Default Re: LinkedList -- insert()

    You left off the source line where the error happens. The error message needs to have a source line with it so you can see where the problem is.

    cannot find symbol
    symbol: method next()
    The compiler can not find a definition for the next() method in scope where it is used. Where is next() defined? Is it in scope where it is used? Is it in the class of the reference variable used with it?
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: LinkedList -- insert()

    That is all the error message contained.

    next() is defined within the same class where I am trying to use it---that's what I don't understand. I'm having the same issue when I try to use my insert method. And the tail is of the same type as both methods...

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    Default Re: LinkedList -- insert()

    next() is defined within the same class where I am trying to use it-
    If it were, the compiler would find it.
    Please post the code.
    If you don't understand my answer, don't ignore it, ask a question.

  9. #9
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    Default Re: LinkedList -- insert()

    public class MySinglyLinkedListD<T> implements SinglyLinkedList<T> {
     
        protected ListNode<T> head, tail, current;
        String name;
        int size = 0;
     
        //Simple constructor
        public MySinglyLinkedListD() {        
        }
        //Constructor for a list with title
        public MySinglyLinkedListD (String name) {
            this.name = name;
    }
        private int size() {
                return size;
        }
        //Method to move cursor to beginning of list
        public void reset () {
            current = head;
        }
       //Prints contents for entire list
        public void traverse () {
            if (!empty ())
               {
                   reset ();
                   System.out.println (current);
                   while (hasNext ())
                   {
                        current = current.getNode ( );
                        System.out.println (current);
                    }
               }
        }
     
        public void addFirst(T t) {
            ListNode<T> newNode = new ListNode<T> (t);
     
            newNode.setNode (head);
            head = newNode;
     
            if(!empty())
                tail = head;
        }
     
        public void addLast(T newMember) {
             if (empty ()){
                addFirst (newMember);
                head = tail;
        }
            else
            {
                tail = new ListNode<T>();
                tail.next() = tail;
     
            }
             size++;
        }
     
        public void insert(T newMember, int index) throws IndexOutOfBoundsException{
            ListNode<T> newNode = head;
     
            if(index == size()){
                addLast(newMember);
                System.out.println("Tail entry at index " + size++);
            }
            else if(index < 0) throw new IndexOutOfBoundsException();
     
            else{
                for(int i = 1; i < index; i++) {
                    current = current.nextNode;
                    newNode = new ListNode<T>();
                    newNode = current;
                    newNode.next() = newMember;
                    size++;
                }
            }
        }
        ///Method to search from any node in list
        public ListNode search(T key) {
     
            ListNode  found = null;
            if (!empty())
            {
                reset();
                do
                {
                    if (current.getElement ().equals (key))
                    {
                        found = current;
                        break;
                    }
                    else
                    {
                        if (hasNext ( ))
                            current = current.getNode ( );
                        else
                            break;
                    }
     
                } while (true);
            }
            return found;
        }
        ///Method to search for specific string/char in list
        public ListNode searchSpecific(T member) {
            ListNode found = null;
            if(!empty()){
                reset();
                do{
     
                }while(true);
            }
     
            return found;
        }
     
        public boolean delete(T key) {
            //Create object to search node
            ListNode  found = search (key);
            //Test whether node exists
            if (found == null)
            {
                System.out.println ("\nkey = " + key + " not found");
                return false;
            }
            else
            {
                 if (found == head)
                 {
                     head = found.getNode ();
                     reset ();
                 }
                else
                 {
                     current = found;
                     try
                     {
                        getPrevious ( );
                     }
                     catch (PointerOutOfBoundException ex)
                     {
                         ;
                     }
                     current.setNode (found.getNode ());
                 }
     
                 System.out.println (key + " deleted");
                 return true;
            }
        }
     
        public boolean empty(){
            return(head == null);
        }
     
        public boolean hasNext() {
            if(current == null) {           
                return false;
            }
            return(current.getNode() != null);
        }
     
        public ListNode<T> next() throws PointerOutOfBoundException {
            if(hasNext()){
                current = current.getNode();
                return current;
            }
            else throw new PointerOutOfBoundException ();
        }
    }

  10. #10
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    Default Re: LinkedList -- insert()

    The code won't compile because of many missing classes.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: LinkedList -- insert()

    I'm not trying to compile it yet...I'm not ready for that. I have my other classes...I just provided the class with the issue. I thought the issue was contained within this class alone.

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    Default Re: LinkedList -- insert()

    The compiler is looking for the next() method in the ListNode class, not in the class that was posted.
    cannot find symbol
    symbol: method next()
    location: variable newNode of type ListNode<T>
    I'm not trying to compile
    The error you posted is a compiler error.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: LinkedList -- insert()

    Is it possible create a single Node class (Node<T>) that can account for a list for integers and a list for strings? Or is it better to create a separate class to do this?

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    Default Re: LinkedList -- insert()

    That is very well possible, every good LinkedList implementation does it that way.
    Why do you think it would not be possible? At what point does it matter to the Node-class what type its content is?

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    Default Re: LinkedList -- insert()

    a single Node class (Node<T>) that can account for
    That's what the generics definition: <T> means. It says that a specific instance of the Node class can be defined to contain any class.
    Node<String> for Strings
    Node<Integer> for Integers
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: LinkedList -- insert()

    Thank you guys. I thought so, but I just wanted to make sure -- I saw an example where they created separate classes for the two but they weren't using generics.

    public class NodeList<T> {
    	public T element;
    	public String elementString;
    	public int elementInt;
    	public NodeList next;
     
    	public NodeList(String s, NodeList next) {
    		this.next = next;
    		this.elementString = s;
    	}
     
    	public NodeList(int n, NodeList next){
    		this.next = next;
    		this.elementInt = n;
    	}
     
    	public NodeList() {
    	}
     
    	public void setNext(NodeList next) {
    		this.next = next;
    	}
     
    	public NodeList getNext() {
    		return next;
    	}
     
    	public void setElement(T element) {
    		this.element = element;
    	}
     
    }

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    Default Re: LinkedList -- insert()

    Would you mind explaining to me exactly what is going on here. What is newNode.next = tail accomplishing? In my class NodeList, next is defined as NodeList<T>...which is the format I've adopted from another source. Not sure exactly what it mean. Thanks.

    public void addFirst(T newMember) {
    		if(empty()) {
    			NodeList<T> newNode = new NodeList<T>();
    			newNode.element = newMember;
    			newNode.next = tail;
    		} else {
    			tail = new NodeList<T>();
    			tail = tail.next;
    			tail.element = newMember;
    		}
    	}

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    Default Re: LinkedList -- insert()

    It's unclear what new NodeList<T() creates. It appears it creates a new Node that will be added as an element to an existing list, but the name, 'NodeList', suggests that a new List is being created. This demonstrates just one of many reasons why programmers should document their code AND give their variables and methods helpful names.

    Looking back at the code you posted in Post #9, it appears you've attempted to make that code your own by making subtle changes and adding new features, posting that result in Post #17, but your changes aren't correct. Explaining to you what is going on in Post #17 is a futile exercise, because it's confusing at best and probably wrong.

    I recommend you comment the original addFirst() method and study that until you completely understand it. If you have questions, post the commented version here, and we'll help you understand it. Once you've done that, comment in the changes you'd like to make to that method and attempt to code them, again coming back with commented code when you need help.

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    Default Re: LinkedList -- insert()

    One of the best ways to understand how linked lists work is by using a paper and pencil to make a drawing of a list and how the links between nodes work. Work through the changes that need to be made to the nodes and the node's pointers for the methods you are working on. For each method make a list of the steps that need to be done. Use those steps to write the code. Put the steps for each method into the code as comments before trying to write the code.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: LinkedList -- insert()

    Ok, lets start with the addToFront method:
    1. I understand the if statement...its saying if a node is empty, add the new node with its member, assign it as head, and link it with old head.
    2. If the list is not empty - as I understand it - I create a temp node that will be linked with the head node (head = temp.next. After doing this, assign head pointer to tempNode (tempNode = head).
    Hopefully this will be enough information that it won't be as confusing.
    Variables defined within class
    public class MySinglyLinkedList<T> implements SinglyLinkedList<T> {
    	private Node<T> head, tail, newNode, temp;
    	int size = 0;
    }
    private class Node<T> {
     
    		public T element;  //contents within node
    		Node<T> next, previous;    //node pointer
     
    public Node(T element) {
    			this.element = element;
    		}
    }

    public void addToFront(T newMember) {
    		newNode = new Node<T>(newMember);	//Node object
    		//Determine whether list exists, or head is empty.
    		if (empty()) {
    			newNode = head;		   //move node pointer to empty node(tail)
    			tail = head;
                             size++;				//increase list size by 1
    		}  //if node !empty(), create a node to   
    		else {
    			Node temp = new Node<T>();	//a temp node to link with current head node
    			head = temp.next;		
    			newNode = head;
    		}
    	}

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    Default Re: LinkedList -- insert()

    if a node(?? list) is empty, add the new node with its member, assign it as head, and link it with old head
    In an empty list, head would be null.

    create a temp node that will be linked with the head node (head = temp.next)
    What would the value of temp.next be for a newly created node? Where should head point when a new node is added at the beginning of the list? Where should the new node's next value point to?

    Some of the variables used in the code should be local to the method and not class variables. For example: newNode
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: LinkedList -- insert()

    temp.next will have the address of the current head node...but it will contain the newest member in the list. Head should always remain the first link in the list -- that is why I used newNode = head; The new node's value will point to the previous first node(now second).

    Yes, I see what you mean now about newNode being local to method. Why are we unable to modify the visibility of the methods from LinkedList interface? just curious as to why.

    private boolean empty() returns head == null
    public void addToFront(T newMember) {
    		Node<T> firstNode = new Node<T>(newMember);	//Node object
    		//Determine whether list exists, or head is empty.
    		if (empty()) {
    			firstNode.next = head;   //new node linked with head
    			firstNode = head;		   //move node pointer to empty node(head)
    			size++;				//increase list size by 1
    		}  //if node !empty(), create a node to   
    		else {
    			Node<T> temp = new Node<T>();	
    			head = temp.next;		//
    			head = firstNode;
    		}
    	}

  23. #23
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    Default Re: LinkedList -- insert()

    Have you compiled and tested the method yet? Do testing one method at a time. Design, code, compile, execute and check that the method worked correctly before doing the next method.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: LinkedList -- insert()

    I've checked my code against several different source and I don't see any serious difference. there is nothing unique about my code and yet it still returns a nullpointer error. Can you guys please explain to me what is wrong?

    MySinglyLinkedList<String> myStringList = new MySinglyLinkedList<String>();
    		myStringList.addToFront("Orion");
    		System.out.println(myStringList.getNext());

    public void addToFront(T newMember) {
    		Node<T> firstNode = new Node<T>(newMember);
     
    		if (empty()) {
    			firstNode.setLink(head);
    			firstNode = head;
    			current = head;
    			System.out.println(newMember);
    		} 
    		else {
    			Node<T> newNode = new Node<T>();
    			newNode.link = head;
    			head = newNode;
    		}
    	}

    public Node<T> getNext() throws PointerOutOfBoundsException {
     
    		if (hasNext()) {
    			current = current.getLink();
    			return current;
    		} else
    			throw new PointerOutOfBoundsException();
    	}
     
    	public boolean hasNext() {
    		if (current == null) {
    			return false;
    		}
    		return (current.getLink() != null);
    	}

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    Default Re: LinkedList -- insert()

    a nullpointer error
    Please copy the full text of the error message and paste it here. It has important info about the error.

    explain to me what is wrong?
    There is a variable with a null value.

    Also can you make a small, complete program that compiles, executes and shows the error?

    Some how when you copied the code here all the comments have been lost.???
    If you don't understand my answer, don't ignore it, ask a question.

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