Getting Null pointer exception when trying to add two string elements

Question

I was asked following question in an interview : Array a = {1,2,3,4,5}, Array b = {a,b,c,d,e}, write a program to add individual elements of both array and enter the sum in third array with output like {e1, d2, c3, b4, a5}

I was not able to come up with solution at that time and now I am trying at home and wrote following code but got null pointer exception :

public class ArrayMergeIndividualElements {

String[] a = {"1","2","3","4","5"};
String b[] = {"a","b","c","d","e"};
String s[]=null;

void mergeArrays()
{
    int k=0;
    int j=b.length-1;           

    for(int i=0;i<a.length;i++)
    {
        for(;j>=0;)
        {
            System.out.println("Number array is "+a[i]);
            System.out.println("String array is "+b[j]);                
            s[k]=a[i]+b[j]; //getting null pointer exception at this line               
            k++;
            j--;
            break;
        }
    }

    System.out.println("output is :");
    for(int l=0;l<s.length;l++)
    {
        System.out.print(s[l]);
    }
}

public static void main(String[] args) {

    ArrayMergeIndividualElements amie = new ArrayMergeIndividualElements();
    amie.mergeArrays();
}
 }

I have tried following code by searching on stackoverflow , but no luck String[] both = Stream.concat(Arrays.stream(a[i]), Arrays.stream(b[j])) .toArray(String[]::new);

Individually arrays are printing the value but when I try to add/concatenate them I am getting null pointer.

Also can we add both arrays if one is Integer array and other is String array?

Please help

Answer 1

You simply have to initialize your destination array with :
String[] s = new String[a.length];
If you didn't do that, when you try to add something to that array you obtain a NullPointerException.

Answer 2

You have not initialized your array. Try doing

String s[]= new String[a.length];