Object referencing in Java null pointer

Question

I have a object which contains an array of doubles.

public class NumberRow {

static final int MAX_AMOUNT_OF_NUMBERS = 2500;
double[] NumberArray = new double[MAX_AMOUNT_OF_NUMBERS];

NumberRow(double[] NumberArray){
    this.NumberArray = NumberArray;
}

}

In my main program I start with creating a array of the object NumberRow in the constructor like this

NumberRow[] numberRow;

later in the program I put this code:

numberRow = new NumberRow[dataset.numberOfVariables];

After that I call a function which gives value to an numberRow:

double misc = in.nextDouble();
numberRow[k].NumberArray[i] = misc;

I did say where NumberRow is pointing to. However, eclipse gives me a null pointer pointer exception on this line:

numberRow[k].NumberArray[i] = misc;

I hope anyone can see what I did wrong? Thank you :)!

These little individual bits of code are very confusing. Please construct a simple, but complete, test-case. — Oliver Charlesworth
– Oliver Charlesworth, Commented Jun 5, 2013 at 0:15
double[] NumberArray = new double[MAX_AMOUNT_OF_NUMBERS]; is never used because you reallocate it in the constructor: this.NumberArray = NumberArray;. Are you sure that is what you intended? — assylias
– assylias, Commented Jun 5, 2013 at 0:16
NumberRow[] numberRow; isn't a constructor. It doesn't "create" anything. — Hot Licks
– Hot Licks, Commented Jun 5, 2013 at 0:18

gparyani · Accepted Answer · 2013-06-05 00:18:26Z

1

This is a common mistake I see when beginners start to use arrays of objects. When an array of object references is created, the array is initialized, but the individual elements in the array are null. So, on the statement numberRow[k].NumberArray[i] = misc;, numberRow[k] is null, which is causing the exception. Therefore, before the line, you need to put the statement

numberRow[k] = new NumberRow();

before the above statement.

answered Jun 5, 2013 at 0:18

gparyani

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Patashu · Accepted Answer · 2013-06-05 00:16:04Z

1

When you do this:

numberRow = new NumberRow[dataset.numberOfVariables];

All of the members of the array numberRow are initialized to the default value of NumberRow. NumberRow is a class, therefore its default value is null. To set values on something that is null, you must first initialize it to a new, real object or you will get a NullPointerException.

answered Jun 5, 2013 at 0:16

Patashu

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