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Thread: Requesting help.

  1. #1
    Junior Member narf0708's Avatar
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    Default Requesting help.

    I made this code from the book "Head First Java 2nd Edition"(pages 95 and 96). It compiles fine, however, when one runs the program it gives the following error "exception in thread main java.lang.NullPointerException" This error occurs on line 21, most likely due to a variable that was set to Null. I cannot figure out what that variable is, and/or how to fix it. Any suggestions?

    class Dog{
    	int size;
    	String name;
    	void bark(int numOfBarks){
    		while(numOfBarks>0){	
    			if (size>60){
    				System.out.println("Woof! Woof!");
    			}else if(size>14){
    				System.out.println("Ruff! Ruff! Ruff!");
    			}else{
    				System.out.println("YIP!! YIP!! YIP!! YIP!!");
    			}//end if/else
    			numOfBarks=numOfBarks-1;
    		}//end loop
    	}//end bark
    }//end class Dog
     
    class DogTest{
    	public static void main(String[] args){
    		Dog[] Dog=new Dog[3];
    		Dog[0].size=63;  //error occurs here
    		Dog[1].size=8;
    		Dog[2].size=35;
     
    		System.out.println("Bark! Bark, all of ye! Bark, all my dogs!!");
    		Dog[0].bark(1);
    		Dog[1].bark(4);
    		Dog[2].bark(2);
    	}//end main
    }//end class DogTest


  2. #2
    Administrator copeg's Avatar
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    Default Re: Requesting help.

    Adding some println's in there to debug and backtrack can help you find the null variable.
    http://www.javaprogrammingforums.com...t-println.html
    If you do this, you will find that Dog[0] is null. Creating an object array is just that...you need to instantiate the objects within the array before you can use them.

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