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  1. #1
    kavitha_0821 is offline Member
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    Post Exception in thread "main" java.lang.NullPointerException at LinkedList.main(Link

    // LinkedList class
    //
    // CONSTRUCTION: with no initializer
    // Access is via LinkedListIterator class
    //
    // ******************PUBLIC OPERATIONS*********************
    // boolean isEmpty( ) --> Return true if empty; else false
    // void makeEmpty( ) --> Remove all items
    // LinkedListIterator zeroth( )
    // --> Return position to prior to first
    // LinkedListIterator first( )
    // --> Return first position
    // void insert( x, p ) --> Insert x after current iterator position p
    // void remove( x ) --> Remove x
    // LinkedListIterator find( x )
    // --> Return position that views x
    // LinkedListIterator findPrevious( x )
    // --> Return position prior to x
    // ******************ERRORS************************** ******
    // No special errors

    /**
    * Linked list implementation of the list
    * using a header node.
    * Access to the list is via LinkedListIterator.
    * @author Mark Allen Weiss
    * @see LinkedListIterator
    */
    import java.io.*;
    class BeamNode
    {
    int x;
    int y;
    }
    public class LinkedList {
    /**
    * Construct the list
    */
    public LinkedList( ) {
    header = new ListNode( null );
    }

    /**
    * Test if the list is logically empty.
    * @return true if empty, false otherwise.
    */
    public boolean isEmpty( ) {
    return header.next == null;
    }

    /**
    * Make the list logically empty.
    */
    public void makeEmpty( ) {
    header.next = null;
    }

    /**
    * Return an iterator representing the header node.
    */
    public LinkedListIterator zeroth( ) {
    return new LinkedListIterator( header );
    }

    /**
    * Return an iterator representing the first node in the list.
    * This operation is valid for empty lists.
    */
    public LinkedListIterator first( ) {
    return new LinkedListIterator( header.next );
    }

    /**
    * Insert after p.
    * @param x the item to insert.
    * @param p the position prior to the newly inserted item.
    */
    public void insert( Object x, LinkedListIterator p ) {
    if( p != null && p.current != null )
    p.current.next = new ListNode( x, p.current.next );
    }

    /**
    * Return iterator corresponding to the first node containing an item.
    * @param x the item to search for.
    * @return an iterator; iterator is not valid if item is not found.
    */
    public LinkedListIterator find( Object x ) {
    ListNode itr = header.next;

    while( itr != null && !itr.element.equals( x ) )
    itr = itr.next;

    return new LinkedListIterator( itr );
    }

    /**
    * Return iterator prior to the first node containing an item.
    * @param x the item to search for.
    * @return appropriate iterator if the item is found. Otherwise, the
    * iterator corresponding to the last element in the list is returned.
    */
    public LinkedListIterator findPrevious( Object x ) {
    ListNode itr = header;

    while( itr.next != null && !itr.next.element.equals( x ) )
    itr = itr.next;

    return new LinkedListIterator( itr );
    }

    /**
    * Remove the first occurrence of an item.
    * @param x the item to remove.
    */
    public void remove( Object x ) {
    LinkedListIterator p = findPrevious( x );

    if( p.current.next != null )
    p.current.next = p.current.next.next; // Bypass deleted node
    }

    // Simple print method
    public static void printList( LinkedList theList ) {
    if( theList.isEmpty( ) )
    System.out.print( "Empty list" );
    else {
    LinkedListIterator itr = theList.first( );
    for( ; itr.isValid( ); itr.advance( ) )
    System.out.print( itr.retrieve( ) + " " );
    }

    System.out.println( );
    }

    private ListNode header;

    // In this routine, LinkedList and LinkedListIterator are the
    // classes written in Section 17.2.
    public static int listSize( LinkedList theList ) {
    LinkedListIterator itr;
    int size = 0;

    for( itr = theList.first(); itr.isValid(); itr.advance() )
    size++;

    return size;
    }

    public static void main( String [ ] args ) {
    LinkedList theList = new LinkedList( );
    LinkedListIterator theItr;
    BeamNode[] b = new BeamNode[3];
    int i,x=0,y=0;
    String line1 = null,line2 = null;
    theItr = theList.zeroth( );
    printList( theList );

    for( i = 1; i < 4; i++ )
    {
    try
    {
    BufferedReader is = new BufferedReader(new InputStreamReader(System.in));
    System.out.println("Enter the value of x\n");
    line1 = is.readLine();
    b[i].x = Integer.parseInt(line1);
    System.out.println("Enter the value of y\n");
    line2 = is.readLine();
    b[i].y = Integer.parseInt(line2);
    }
    catch (NumberFormatException ex)
    {
    System.err.println("Not a valid number: " + line1);
    }
    catch (IOException e)
    {
    System.err.println("Unexpected IO ERROR: " + e);
    }

    theList.insert( b[i], theItr );
    // printList( theList );
    theItr.advance( );

    }
    System.out.println( "Size was: " + listSize( theList ) );

    /* for( i = 0; i < 10; i += 2 )
    theList.remove( new Integer( i ) );

    for( i = 0; i < 10; i++ )
    if( ( i % 2 == 0 ) == ( theList.find( new Integer( i ) ).isValid( ) ) )
    System.out.println( "Find fails!" );

    System.out.println( "Finished deletions" );*/
    printList( theList );


    }
    }


    // Basic node stored in a linked list
    // Note that this class is not accessible outside
    // of package weiss.nonstandard

    class ListNode {
    // Constructors
    public ListNode( Object theElement ) {
    this( theElement, null );
    }

    public ListNode( Object theElement, ListNode n ) {
    element = theElement;
    next = n;
    }

    public Object element;
    public ListNode next;
    }
    Last edited by kavitha_0821; 07-16-2009 at 11:20 AM.

  2. #2
    Tolls is offline Moderator
    Join Date
    Apr 2009
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    Default

    Please put your formatted code in CODE tags (the '#' button above the message box).
    Unformatted code is nigh on impossible to read.

    Also, you ought to give us the stack trace for the exception, and point out in the code where the exception is being thrown.

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