Nullpointerexception in this function

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Hello CodeProject community,

I'm trying to instantiate a matrix of an element. But when I call a certain element in the matrix, it would return a Null (or a NullPointerException when calling a method). Here's a simplified version of my code which still returns Null. I wonder what causes this problem. Thank you!

Best regards

What I have tried:

Java

public class MyProgram extends ConsoleProgram
{

    public void run()
    {
        someClass[][] sc = new someClass[1][2];
        System.out.println(sc[0][0].getX());
    }
    
}

public class someClass
{
    
    private int x = 0;
    public someClass()
    {
 
    }
    
    public int getX()
    {
        return x;
    }
}

Posted 23-Jan-18 14:42pm

Yuxi Long

Updated 23-Jan-18 15:40pm

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Mohibur Rashid · Accepted Answer · 2018-01-23T15:40:00

your problem is with function run

This is what you have done

Java

public void run()
{
       someClass[][] sc = new someClass[1][2]; // you have allocated some space for the array itself;
       System.out.println(sc[0][0].getX()); // accessing here is a violation
       // since you have not initialize space for each object; hence before you start accessing you need to initialize.
}

in other word

Java

sc[0][0] = new someClass();

When you declare an array of primitive variables, java set default value for that variable. And for all classes and String default is null. Follow the example to understand better

Java

public class Klass {
    public int x=10;
    public static void main(String []args) {
        int []i = new int[2];
        char []c=new char[2];
        Klass []k=new Klass[2];
        String s[] = new String[2];
        System.out.println(i[0] + " " + i[1]);
        System.out.println(c[0] + " " + c[1]); // in this case c is set to zero
        System.out.println(s[0] + " " + s[1]);
        System.out.println(k[0] + " " + k[1]);
        try {
          System.out.println(k[0].x); // since k is null it will thow exception
        } catch(NullPointerException e) {
            e.printStackTrace();
        }
        k[0] = new Klass();
        System.out.println(k[0].x); // You will see some result
    }
}