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Thread: What is wrong with this code, please desperate

  1. #1
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    Default What is wrong with this code, please desperate

    public class Site
    {
       protected class PageNode
       {
          protected PageNode down;
          protected PageNode across;
          protected PageNode up;
          protected Page page;
       }
     
       private PageNode currentPage;
       private PageNode homePage;
     
       public Site()
          {
             this.homePage=new PageNode();
     
             this.currentPage=this.homePage;
     
             PageNode shops=addPage("Shops",this.homePage);
             addPage("News",this.homePage);
             PageNode products=addPage("Products",this.homePage);
     
          // addPage("Paisley",shops);
          // addPage("Hamilton",shops);
     
       //    PageNode kitchen=addPage("Kitchen",products);
       //    addPage("Bedroom",products);
     
       //    addPage("Kettles",kitchen);
       //    addPage("Cookers",kitchen);
       //    addPage("Toasters",kitchen);
          }
     
                public PageNode addPage(String name,PageNode homePage)
                {
                   PageNode newNode=new PageNode();
                      newNode.page=new Page(name);
                      if(this.homePage.down==null)
                      {
                         this.homePage.down=newNode;
                      }
                         else
                         {
                         this.currentPage=this.homePage.down;
                         while(this.currentPage.across!=null)
                         this.currentPage=currentPage.across;
                         this.currentPage.across=newNode;
     
                         this.currentPage.up=this.homePage;
                         }
                      return currentPage;
                }
     
       public void displayCurrentPage()                
     
       {
       PageNode currentPage=this.homePage.down;
       System.out.println("current page name: ");
       System.out.println(this.homePage.page.getname());
       System.out.println("has links to ");
     
        if(currentPage.across==null)
         System.out.println("list is empty");
        else
          while (currentPage!=null)
          {
             System.out.println(currentPage.page.getname());
             currentPage=currentPage.across;
          }
       }
     
     
       public void selectLink()
       {
          // add code for step2
       }
     
    public void moveUp()
      {
          // add code for step3
       }
     
      public void displaySiteMap()
       {
          // add code for step4
       }
     
    }

    and the error is

    ----jGRASP exec: java SiteTest

    1: display current page
    2: select link
    3: move up
    4: display site map
    0: quit
    select option: 1
    current page name:
    Exception in thread "main" java.lang.NullPointerException
    at Site.displayCurrentPage(Site.java:60)
    at SiteTest.main(SiteTest.java:17)

    ----jGRASP wedge2: exit code for process is 1.
    ----jGRASP: operation complete

    Thanks in advance guys


  2. #2
    Forum old-timer
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    Default Re: What is wrong with this code, please desperate

    The 'page' in 'homePage' is null on line 60. The PageNode class can be created without any of its members being initialised, so they are null by default. If you want to access them, you should check they're not null before calling any methods on them.

    The 'Best Practice' solution to this kind of problem is to make sure the members can't be null by providing default initialisers for them, and/or a constructor that forces initialisation of them.

    If you don't know in advance what values to give the members, you could try using the Null Object pattern, where you initialise with a special instance of the object that is safe to use but does nothing.

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