Question

My main method:

public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    String string1;
    string1 = input.next();

    LargeInteger firstInt = new LargeInteger(string1);

    System.out.printf("First integer: %s \n", firstInt.display());
}

LargeInteger class:

public class LargeInteger {

    private int[] intArray;

    //convert the strings to array
    public LargeInteger(String s) {
        for (int i = 0; i < s.length(); i++) {
            intArray[i] = Character.digit(s.charAt(i), 10); // in base 10
        }
    }

    //display the strings
    public String display() {
        String result = "";

        for (int i = 0; i < intArray.length; i++) {
            result += intArray[i];
        }
        return result.toString();
    }
}

Answer 1

You are not initialize the array intArray, that way you are getting error, here is the complete program

import java.util.Scanner;

class  TestForNull {

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        String string1;
        string1 = input.next(); 

        LargeInteger firstInt = new LargeInteger(string1);

        System.out.printf ("First integer: %s \n", firstInt.display());
    }

}

and this is LargeInteger

public class LargeInteger {

    private int[] intArray;
    //convert the strings to array
    public LargeInteger(String s) {
        intArray = new int[s.length()];
        for (int i = 0; i < s.length(); i++) {
            intArray[i] = Character.digit(s.charAt(i), 10); // in base 10
        }
    }

    //display the strings
    public String display() {           
          String result="";

          for (int i = 0; i < intArray.length; i++) {     
            result += intArray[i];
          }
          return result.toString();
        }   
}