Question

I am new to programming and Java and trying to write a program which takes two arrays as input and reports back their sum. I want to do this by creating a class which takes two arrays as constructor inputs and then creating a method that adds them together and a method which prints out this new sum array.

Here is my class:

public class test1 {
    int [] a;
    int [] b;
    int [] final23;

    public test1 (int x [], int y [])
    {
        int [] a = x;
        int [] b = y;
    }

    public int [] sum(int [] x, int[] y)
    {
    int [] a = x;
    int [] b = y;
    for (int i = 0; i < Math.min(x.length, y.length); i++)
    {
        final23 [0]=x[0] + y[0] ;

    }
    return final23;
    }

    public void print()
    {
    for (int i = 0; i < final23.length; i++)
    {
        System.out.println(final23[0]);
    }
    }

}

Here is my main class:

public class main1 {
    public static void main(String[] args)
    {
        int l[] = {4,7,2};
        int k[] = {4,6,2};
        test1 X = new test1(k,l);

        X.sum(k,l);
        X.print();
    }
}

I keep getting an error when I run this through:

Exception in thread “main” java.lang.NullPointerException
at test2.sum(test2.java:17)
at main1.main(main1.java:8)

I guess what I really want is for my sum method to take a test1 object as input. However, I don’t know how to do this.

Answer 1

Your variable final23 is never initialized.
In java you have to initialize an array before using it. Either you do it during the declaration (like you did with k and l) or you have to do it later with a new arrayType[arraySize];

---

Here are the way an array can be declared/initialized.

int[] iArray = {1, 2, 3}; //Declaration, Initialization, set values


int[] iArray; //Declaration
iArray = new int[3]; //Initialization
iArray[0] = 1; //Set value

int[] iArray; //Declaration
iArray = new Array[3]{1, 2, 3}; // Initialization and set values

You can of course for the two last sample put the initialization on the same line that the declaration.

---

Try this (cleaned) code :

public class test1 {
    int[] final23;

    public int[] sum(int[] x, int[] y) {
        final23 = new int[Math.min(x.length, y.length)];
        for (int i = 0; i < final23.length; i++) {
            final23[i] = x[i] + y[i];

        }
        return final23;
    }

    public void print() {
        for (int aFinal23 : final23) {
            System.out.println(aFinal23);
        }
    }


    public static void main(String[] args) {
        int l[] = {4, 7, 2};
        int k[] = {4, 6, 2};
        test1 x = new test1();

        x.sum(k, l);
        x.print();
    }
}

---

Resources :

• Oracle.com – Arrays
• JLS – Array Initializers
• JLS – Array Creation Expressions