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Thread: NullPointerException help with array

  1. #1
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    Default NullPointerException help with array

    Hi i'm new to Java, when running the following test class on deb1 i get a NullPointerException. Can someone explain how to fix my array?

    Deb1:

    public class Deb1 
    {
        private int[] array; // create integer array
     
        public int[] allocate() 
        {
    	array[0] = 0;
    	array[1] = 1;
    	array[2] = 2;
    	return array;
        }
     
        /**
         * Gets a value from the array of integers.
         */
        public int get(int index) 
        {
    	return array[index];
        }
     
        /**
         * Sets a value in the array of integers; returns true if succesful,
         * false otherwise.
         */
        public boolean set(int index, int value) 
        {
    	if (index < array.length && index >= 0) {
    	    array[index] = value;
    	    return true;
    	}
    	else {
    	    return false;
    	}
        }
    }

    TestDeb1:
    public class TestDeb1 {
        public static void main(String[] args) {
    	Deb1 testObject;
    	testObject = new Deb1();
    	testObject.allocate();
    	if (testObject.get(1) != 1) {
    	    System.out.println("Error in get");
    	}
    	if (!testObject.set(2, 3)) {
    	    System.out.println("Error in set");
    	}
    	if (testObject.get(2) != 2) {
    	    System.out.println("Error in set 2");
    	}
        }
    }


  2. #2
    Super Moderator Norm's Avatar
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    Default Re: NullPointerException help with array

    i get a NullPointerException.
    please copy the full text of the error message and paste it here.
    If you don't understand my answer, don't ignore it, ask a question.

  3. #3
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    Default Re: NullPointerException help with array

    Exception in thread "main" java.lang.NullPointerException
    at Deb1.allocate(Deb1.java:20)
    at TestDeb1.main(TestDeb1.java:12)

    This may not be accurate as i removed some of the comments when posting the code. Just asked a friend on fb and he said i haven't instantiated it.

  4. #4
    Super Moderator Norm's Avatar
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    Default Re: NullPointerException help with array

    xception in thread "main" java.lang.NullPointerException
    at Deb1.allocate(Deb1.java:20)
    at TestDeb1.main(TestDeb1.java:12)
    There is a variable with a null value on line 20. Look at line 20 in the your source and see what variable is null. Then backtrack in the code to see why that variable does not have a valid value.
    If you can not tell which variable it is, add a println just before line 20 and print out the values of all the variables on that line.
    If you don't understand my answer, don't ignore it, ask a question.

  5. #5
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    Default Re: NullPointerException help with array

    Fixed the original error by adding = new int[20] to the end of private int[] array. This solved the error because i hadn't created the array itself or set a value for the size,

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