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Thread: LinkedList Objects

  1. #1
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    Default LinkedList Objects

    Hi all, I'm creating a LinkedList...I've removed most unnecessary code for simplicity. Here's what I expect to happen. I create a list opject, populate it with an item, get the size (which is 1) the I create a list2 object, populate it with an item and get the size of list again...which I would still expect to be 1, but it is 2...why is that happening? Here is my full code:
    import java.io.File;
    import java.io.FileNotFoundException;
    import java.util.Scanner; //Needed for scanner class
     
    /**
     * class TestLinkedList -
     */
    public class TestLinkedList {
    	public static void main(String[] args) throws FileNotFoundException{
    		//Populate list with elements
    		IntLinkedList list = new IntLinkedList();
    //		list = getListFrom("project2.txt");
    		//Display original list
    list.addAfter(4);
    		System.out.println( "The original list size is: " + list.size() + 
    		" and contains the elements:");
    //		displayList( list );	
    		System.out.println();
    		//Sort list
    //		list = sortList( list ); 
    		//Display the sorted list
    IntLinkedList list2 = new IntLinkedList();
    list2.addAfter(5);
    		System.out.println( "The final list size is: " + list.size() + 
    		" and contains the sorted elements:");
    //		displayList( list );			
            System.exit(0);	//End Program
    	}//end method main
    }
     
    /**
     * class IntLinkedList -
     */
    class IntLinkedList{
        private static int     size; //the number of elements in the list
        private static IntNode head;
        private static IntNode tail;
        private static IntNode cursor;
        private static IntNode precursor;
        //
        //IntLinkedList
        //
        public IntLinkedList(){
        }//end IntLinkedList
        //
        //compute the number of nodes
        //
        public static int size(){
        	size = 0;
        	for ( cursor = head; cursor != null; cursor = cursor.getLink() ){
        		  size++;
        	}
        	return size;
        }//end size
        //
        //addAfter
        //
    	public static void addAfter( int value ){
            if ( cursor == null ) {  // either no current element or current element is first  
                // add element to the head of the list
                head = new IntNode( value, head );
                cursor = head;
             } else {               // cursor points to the middle of the list
            	cursor.setLink( new IntNode( value, cursor.getLink()) );
                //cursor = precursor.getLink();
             }
             if ( size() == 0 )     // the list was empty and tail needs to be initialized
                tail = head;
    	}
    }
     
    /**
     * class IntNode -
     */
    class IntNode{
    	//
    	//Variables
    	//
    	private int data; //variable to store an integer value.
    	private IntNode link; //variable to store an IntNode object.
    	//
    	//Non-default constructor to initialize the data fields
    	//
    	public IntNode( int initData, IntNode initLink ){
    		data = initData;
    		link  = initLink;
    	}
    	//
    	//Accessor and mutator methods for data and link fields
    	//
    	public int getData(){
    		return data;
    	}
    	public IntNode getLink(){
    		return link;
    	}
    	public void setData(int newData){
    		data = newData;
    	}
    	public void setLink(IntNode newLink){
    		link = newLink;
    	}
    }//end class IntNode

    Thanks for any help,
    Mason
    Last edited by copeg; October 13th, 2010 at 09:16 AM. Reason: Code Tags


  2. #2
    Administrator copeg's Avatar
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    Default Re: LinkedList Objects

    All of your LinkedList variables are not instance variables, but rather class variables. eg static - when you set their value you end up changing the value for the class and not the instance. See Understanding Instance and Class Members (The Java™ Tutorials > Learning the Java Language > Classes and Objects)

  3. #3
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    Default Re: LinkedList Objects

    OK, so if I declare the instance variable as:
    IntLinkedList list;

    How do I, for example, get the size as the project constraint for the size method is:
    public int size(){
    }//end size

    Thanks again.

  4. #4
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    Default Re: LinkedList Objects

    OK, so if I declare the instance variable as:
    IntLinkedList list;
    The variables of type IntLinkedList are not the problem, the problem is with the variables within that class and how they are declared (eg static).

    How do I, for example, get the size as the project constraint for the size method is:
    public int size(){
    }//end size
    You already have this method so I'm not sure what you are asking. Right now, size is static (and so are the other variables) so the method size() - in layman's terms - gets the size of ALL instances of said class. If you want it to retrieve only the size of the particular instance, see me post above

  5. #5
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    Default Re: LinkedList Objects

    OK, I see that when I use the word static, the method becomes part of the class and is not independent objects. But when I remove static, I get a null pointer exception...why did that happen? Here is the complete updated code, Ignore most of it, just look at list population portion.
    import java.io.File;
    import java.io.FileNotFoundException;
    import java.util.Scanner; //Needed for scanner class
     
    /**
     * class TestLinkedList -
     */
    public class TestLinkedList {
        //
        public static void main(String[] args) throws FileNotFoundException{
          //
          IntLinkedList list = getListFrom("project2.txt");
          System.out.println( "The original list is:" );
          System.out.println();
          //displayList( list );
    //      list = sortList( list );
          System.out.println( list.size() );
     
          IntLinkedList list2 = null;
          list2.addAfter(2);
          System.out.println( list2.size() );
     
          System.out.println();
          System.out.println( "The final list is:" );
          System.out.println();
          //displayList( list );
        }//end main
        //
    	//
    	// getListFrom() method populates the linked list with elements
    	//	
    	private static IntLinkedList getListFrom(String path) throws FileNotFoundException{
    		//Get file
    		File file = new File(path);
    		Scanner inputFile = new Scanner(file);
    		IntLinkedList linkedList = null;
    		//Add elements
    		while (inputFile.hasNext()){
    			linkedList.addAfter(Integer.parseInt(inputFile.nextLine()));
    		}
    		inputFile.close();
    		return linkedList;
    	}//end method getListFrom
    	//
    	// displayList() method prints the elements of the linked list
    	//
    //	private void displayList( IntLinkedList list ){
    //		IntLinkedList.printList();
    //		System.out.println();
    //		System.out.println(IntLinkedList.size());
    //	}//end displayList
        //
    //    private static IntLinkedList mergeSortedLists(
    //      IntLinkedList first,
    //      IntLinkedList second
    //    ){
    //    }//end mergeSortedLists
        //
    //    private static IntLinkedList sortList( IntLinkedList list ){
    //    }//end sortList
        //
      }//end class TestIntLInkedList
     
     
     
     
     
     
     
    /**
     * class IntLinkedList -
     */
    class IntLinkedList{
        private int     size; //the number of elements in the list
        private IntNode head;
        private IntNode tail;
        private IntNode cursor;
        private IntNode precursor;
        //
        //IntLinkedList
        //
        public IntLinkedList(){
        	head=null;
        }//end IntLinkedList
        //
        //compute the number of nodes
        //
        public int size(){
        	size = 0;
        	for ( cursor = head; cursor != null; cursor = cursor.getLink() ){
        		  size++;
        	}
        	return size;
        }//end size
        //
        //addAfter
        //
    	public void addAfter( int value ){
            if ( cursor == null ) {  // either no current element or current element is first  
                // add element to the head of the list
                head = new IntNode( value, head );
                cursor = head;
             } else {               // cursor points to the middle of the list
            	cursor.setLink( new IntNode( value, cursor.getLink()) );
                //cursor = precursor.getLink();
             }
             if ( size() == 0 )     // the list was empty and tail needs to be initialized
                tail = head;
    	}
    	//
        //Prints list elements
    	//
        public void printList() {
        	IntNode curr = head;
            while (curr != null) {
                System.out.print(curr.getData() + " ");
                curr = curr.getLink();
            }
        }
    }
     
     
     
     
     
     
     
    /**
     * class IntNode -
     */
    class IntNode{
    	//
    	//Variables
    	//
    	private int data; //variable to store an integer value.
    	private IntNode link; //variable to store an IntNode object.
    	//
    	//Non-default constructor to initialize the data fields
    	//
    	public IntNode( int initData, IntNode initLink ){
    		data = initData;
    		link  = initLink;
    	}
    	//
    	//Accessor and mutator methods for data and link fields
    	//
    	public int getData(){
    		return data;
    	}
    	public IntNode getLink(){
    		return link;
    	}
    	public void setData(int newData){
    		data = newData;
    	}
    	public void setLink(IntNode newLink){
    		link = newLink;
    	}
    }//end class IntNode
    Last edited by copeg; October 13th, 2010 at 12:56 PM. Reason: Code Tags

  6. #6
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    Default Re: LinkedList Objects

    I have edited your post, but for future reference please surround your code with the
    [highlight=java] [/highlight]
    tags, without which your code is completely illegible.

    Where is the exception thrown? Exceptions give line values and are essential in debugging. Please post the full exception message.

  7. #7
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    Default Re: LinkedList Objects

    Sorry about the TAGS, don't know how to do that. When run, Eclipse:
    Exception in thread "main" java.lang.NullPointerException
    at TestLinkedList.getListFrom(TestLinkedList.java:39)
    at TestLinkedList.main(TestLinkedList.java:12)

    When everything is static it works???

  8. #8
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    Default Re: LinkedList Objects

    The linkedList in the references method is never instantiated.
    IntLinkedList linkedList = null;
    linkedList.add(1);//throws null pointer exception

    You must instantiate the object before using its instance variables/methods.
    IntLinkedList linkedList = new IntLinkedList();

  9. #9
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    Thumbs down Re: LinkedList Objects

    Thank you so much! I'm on the right track now...man...Thank you! Ignore the code below, just wanted to put a tag around it. Again...thank you.

    import java.io.*;
    import java.util.*;
    public class TestLinkedList {
        //
        public static void main(String[] args) {
          //
          IntLinkedList list = getListFrom( "A:/Project3.txt" );
          System.out.println( "The original list is:" );
          System.out.println();
          displayList( list );
          list = sortList( list );
          System.out.println();
          System.out.println( "The final list is:" );
          System.out.println();
          displayList( list );
          //
        }//end main
        //
        private static IntLinkedList getListFrom( String path ) {
        }//end getLinkedList
        //
        private static void displayList( IntLinkedList list ){
        }//end displayList
        //
        private static IntLinkedList mergeSortedLists(
          IntLinkedList first,
          IntLinkedList second
        ){
        }//end mergeSortedLists
        //
        private static IntLinkedList sortList( IntLinkedList list ){
        }//end sortList
        //
      }//end class TestIntLInkedList

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    Default Re: LinkedList Objects

    I used the # tag for code...what do you use for java to get the color?

  11. #11
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    Default Re: LinkedList Objects

    Quote Originally Posted by thedolphin13 View Post
    I used the # tag for code...what do you use for java to get the color?
    PHP Code:
    [highlight=java]place code here[/highlight

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    Default Re: LinkedList Objects

    Quote Originally Posted by thedolphin13 View Post
    I used the # tag for code...what do you use for java to get the color?
    To use java tags:
    [highlight=java] and [/highlight]
    NOTE TO NEW PEOPLE LOOKING FOR HELP ON FORUM:

    When asking for help, please follow these guidelines to receive better and more prompt help:
    1. Put your code in Java Tags. To do this, put [highlight=java] before your code and [/highlight] after your code.
    2. Give full details of errors and provide us with as much information about the situation as possible.
    3. Give us an example of what the output should look like when done correctly.

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