java.lang.NullPointerException Array For Loop

Question

I'm having a problem with a "java.lang.NullPointerException". The objective of this code is generate random numbers with an array with a specific size (in this case, 10 numbers, as given in the driver, but it doesn't matter), calculate the average, find the maximum, minimum and a specific number (as given in the driver, 2 and 3, but it doesn't matter). I think I could write all of this code with some research, but after everything done and compiled, I tried to run the code and for my surprise, I found the java.lang.NullPointerException. I had no idea of what it was, but with some research I learned and (I think) I know what's the problem. I want to access an array using a "for loop" but I can't because the values of the array are null. I don't know why and I don't know all the errors in this code, because I am still trying to run it. I can only compile, but as I said, I have the NPE problem when I run. However, for me it's logically acceptable the constructor receiving the parameter and the "for loop" in the way I wrote. So, when I run, the message is exactly this:

java.lang.NullPointerException at ArrayLab.initialize(ArrayLab.java:19) at Driver.main(Driver.java:16)

My questions are: What's wrong with my code? Why my array is null and it doesn't have the value assigned in the driver? How can I assign values to this array? Is there anything wrong with my constructor? And what about the "for loop"? What can I do to fix it?

public class ArrayLab
{
    private int[] ArrayNum;

    public ArrayLab(int Num)
    {        
        int[] ArrayNum = new int[Num];        
    }

    public void initialize()
    {        
        for (int ANUM = 0; ANUM <= ArrayNum.length; ANUM++)
        {               
            ArrayNum[ANUM] = (int) Math.round((Math.random() * 10));          
        }        
    }    

    public void print()
    {
        System.out.println(ArrayNum);
    }

    public void printStats()
    {
        double Sum = 0;
        int Max = 0;
        int Min = 0;

        for (int ANUM = 0; ANUM < ArrayNum.length; ANUM++)
        {            
            Sum = Sum + ArrayNum[ANUM];     

            if (ArrayNum[ANUM] > ArrayNum[Max])
            {
                Max = ANUM;
            }            

            if (ArrayNum[ANUM] < ArrayNum[Min])
            {
                Min = ANUM;
            }
        }

        double Average = Sum/ArrayNum.length;
        System.out.println("Average Value: " + Average);
        System.out.println("Maximum Value: " + Max);
        System.out.println("Minimum Value: " + Min);
    }       

    public void search(int GivenNumber)    
    {
        for (int ANUM = 0; ANUM < ArrayNum.length; ANUM++)
        {            
            if(GivenNumber == ArrayNum[ANUM])
            {
                System.out.println(GivenNumber + " was found.");
            }

            else
            {
                System.out.println(GivenNumber + " was not found.");
            }
        }
    }
}

and

public class Driver
    {
        public static void main(String[] args)
        {
            //create new instance of the ArrayLab class with parameter of 10
            ArrayLab array = new ArrayLab(10);

            //print out the array created by ArrayLab constructor
            array.print();

            //initialize the array with random values
            array.initialize();

            //print the randomized array
            array.print();

            //print stats for the array
            array.printStats();

            //search for 3
            array.search(3);

            //search for 2
            array.search(2);
        }
    }

Possible duplicate of What is a Null Pointer Exception, and how do I fix it? — John3136
– John3136, Commented Oct 22, 2015 at 5:07
Debug your code and have a look closer to the lines which are specified in the error message, you have some object which is null, but you try to invoke some method from that object. Thus you get NullPointerException. — Rufi
– Rufi, Commented Oct 22, 2015 at 5:10
In your constructor, you have re declared your array. So your actual array is not being allocated any memory — SacJn
– SacJn, Commented Oct 22, 2015 at 5:14
I suspect this is a homework assignment and you two are in the same class. stackoverflow.com/questions/33272636 — OneCricketeer
– OneCricketeer, Commented Oct 22, 2015 at 5:20
Yes, this is a homework assignment. But I didn't ask for answers, since I have most of my homework done. Anyway, my problem is already solved. Thanks everyone for help. — ZessLamperouge
– ZessLamperouge, Commented Oct 22, 2015 at 5:26

SacJn · Accepted Answer · 2015-10-22 05:14:56Z

0

In your constructor, you have re declared your array. So your actual array is not being allocated any memory

private int[] ArrayNum;

public ArrayLab(int Num)
{        
    ArrayNum = new int[Num];        
}

answered Oct 22, 2015 at 5:14

SacJn

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1 Comment

ZessLamperouge Over a year ago

Thank you SacJn!! It worked for me. I can compile and run it!! Perfect.

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