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Thread: java.lang.NullPointerException - Help

  1. #1
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    Default java.lang.NullPointerException - Help

    Im trying to run this code i have written but cannot see whats wrong with it. i get the java.lang.NullPointerException error when running it.

    Basically what im trying to do is to move a background x position by using a new method i have created in Dice class (moveBackground).

    The code i am running is

    di.moveBackground(); where di is the object name

    can any one help as i havent got a clue as im new to java

    Class Called Dice only part of code is pasted here
    private Square background;
     
     public Square getBackground()
       {
          return background;
       }
     
      public void moveBackground()    /** this is what method i am trying to run */
         {
            background.setXPos(75);
          }


    Square class
     public void setXPos(int x)
       {
          this.xPos = x;
          this.update();
       }

    Thanks in advance

  2. #2
    Administrator copeg's Avatar
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    Default Re: java.lang.NullPointerException - Help

    Is the background variable instantiated prior to calling moveBackground, for example in the constructor or using a setter?

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    Java kindergarten chronoz13's Avatar
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    Default Re: java.lang.NullPointerException - Help

    yah i think the object background is not properly instantiated... i think thats the object that the compiler is reffering to..


    try to make a constructor that will instantiate the object background as your new object for the square class
     
     public <Class Name> () {
     
             background  = new Square();
     }
    Last edited by chronoz13; November 28th, 2009 at 08:22 PM.

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    Default Re: java.lang.NullPointerException - Help

    not sure what you mean all my code is below


    To get a dice with 1 spot i run the code:
    Square s = new Square();
    Circle c1 = new Circle();
    Circle c2 = new Circle();
    Circle c3 = new Circle();
    Circle c4 = new Circle();
    Circle c5 = new Circle();
    Circle c6 = new Circle();
    Circle c7 = new Circle();
    Dice di = new Dice(s, c1, c2, c3, c4, c5, c6, c7);

    then the idea would be to run:
    di.moveBackground();

    to move the square created above to the right by 75
    Last edited by mds1256; December 3rd, 2009 at 06:13 PM.

  5. #5
    Administrator copeg's Avatar
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    Default Re: java.lang.NullPointerException - Help

    You never set or create the background variable. Look at the constructor of the class Dice, there should be something like
    background = bg;
    or
    background = new Square();

  6. The Following 2 Users Say Thank You to copeg For This Useful Post:

    JavaPF (November 30th, 2009), mds1256 (November 30th, 2009)

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    Default Re: java.lang.NullPointerException - Help

    Quote Originally Posted by copeg View Post
    You never set or create the background variable. Look at the constructor of the class Dice, there should be something like
    background = bg;
    or
    background = new Square();
    brilliant. thanks very much for this

    Really appreciate it

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