Java NullPointerException due to indexing array

Question

If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23. Find the sum of all the multiples of 3 or 5 below 1000.

So it gives null pointer exception at arr[i] = i;. I am new to Java programming so I wanted to try arrays

 public class Multiples
    {
        private int arr[];
        private int sum = 0;

    public void multiples1()
      {
        for(int i= 0 ; i <= 1000; i++)
            {
                arr[i] = i ;
                if((i%3 == 0 && i%5==0))
                    {
                        sum = sum + arr[i];                         
                    }                   
            }   
        System.out.println(sum);
      }


}

You need to declare the size of your array! It's not enough to leave it blank. — Floris, Sep 17 '13 at 02:33
@Floris - No, you don't declare the size of arrays in Java. But you do need to create the arrays -- they don't create themselves/ — Hot Licks, Sep 17 '13 at 02:35
@HotLicks - initialize, declare... I was sloppy in my wording. It is late here (and Java is not "my" language). Thanks for pointing out the correct phrase. — Floris, Sep 17 '13 at 03:08
"multiples of 3 OR 5" - implies that you should use the `||` operator, not the `&&` operator, in your `if` statement! — Floris, Sep 17 '13 at 03:10
@Floris - I see related mistakes made in Objective-C quite often. In some case people believe they must "declare" a pointer by initializing it with an object (which they immediately discard by overwriting with a new object). Or they believe that simply declaring a pointer is equivalent to initializing it with an object of that type. — Hot Licks, Sep 17 '13 at 03:23
@HotLicks - I have no argument with you there. The simple lack of making sure that the compiler assigns valid memory locations for the objects you want to manipulate in your code must account for half the bugs in the known universe. — Floris, Sep 17 '13 at 03:31

Prasad Kharkar · Answer 1 · 2013-09-17T02:44:26.327

4

It should be private int arr[] = new int[arraySize];
An array is an Object so if you don't initialize it using new int[size], it will be null.
If you perform operation of a reference variable that is referring to null, you will face NullPointerException

Your revised code would be:

public class Multiples
    {
        private int arr[] = new int[1000];
        private int sum = 0;

    public void multiples1()
      {
        for(int i= 0 ; i < 1000; i++)
            {
                arr[i] = i ;
                if((i%3 == 0 && i%5==0))
                    {
                        sum = sum + arr[i];                         
                    }                   
            }   
        System.out.println(sum);
      }


}

edited Sep 17 '13 at 02:44

answered Sep 17 '13 at 02:33

Prasad Kharkar

12,721
3
35
55

You mean `int[1000]` right...? – Floris Sep 17 '13 at 02:34
@Floris, yes actually, thats why I edited and just said `arraySize` – Prasad Kharkar Sep 17 '13 at 02:35
Only if he doesn't want an array indexing exception. – Hot Licks Sep 17 '13 at 02:36
@HotLicks, yes of course :) – Prasad Kharkar Sep 17 '13 at 02:37
Thanks for the prompt reply everyone. But it gives java.lang.ArrayIndexOutOfBoundsException error – John Smith Sep 17 '13 at 02:38
@JohnSmith, becauese of your for loop `for(int i= 0 ; i <= 1000; i++)`, here it is trying to access 1001st index so change it to `for(int i= 0 ; i < 1000; i++)` after you create array as `new int[1000]` – Prasad Kharkar Sep 17 '13 at 02:40
@John Smith Java arrays are zero based, hence, a 10 element array has indexes 0 to 9. As Prasad said, you'll need the end condition of your loop to be < 1000 rather than <= 1000 – Ray Stojonic Sep 17 '13 at 02:40
Thanks! it works. I am new to programming as a lot of other people and thanks to you all, newbies like us learn. Thanks again! – John Smith Sep 17 '13 at 02:42
@JohnSmith we are all learners :) – Prasad Kharkar Sep 17 '13 at 02:42
@PrasadKharkar - Nah, I haven't learned anything new since 1973 -- it seems like I just run into the same stuff again and again in different contexts. (And, unfortunately, I seem to make the same mistakes again and again as well.) – Hot Licks Sep 17 '13 at 03:07
@HotLicks, oops, I should have written I'm a learner. – Prasad Kharkar Sep 17 '13 at 03:10
You need OR, not AND, in the if statement, in order to achieve what OP wants. – Floris Sep 17 '13 at 03:11

score 1 · Answer 2 · answered Sep 17 '13 at 02:41

The following code snippet would do what you want to achieve.

public class Multiples
{
    private int arr[] = new int[1001];
    private int sum = 0;

public void multiples1()
  {
    for(int i= 0 ; i <= 1000; i++)
        {
            arr[i] = i ;
            if((i%3 == 0 && i%5==0))
                {
                    sum = sum + arr[i];                         
                }                   
        }   
    System.out.println(sum);
  }
}

Java NullPointerException due to indexing array

2 Answers2