JAVA- NullPointerException

Question

The following code shows the following error:

Exception in thread "main" java.lang.NullPointerException at cdtWorkspace.swap.main(swap.java:25)

PS: I have initialized a scanner object globally as static.

15 public static void main(String args[] ) throws Exception {
16    
17            int n=scn.nextInt();
18            int[][] param=new int[n][2];
19            int[][] arr=new int[n][];
20            for(int i=0; i<n;i++){
21                param[i][0]=scn.nextInt();
22                param[i][1]=scn.nextInt();
23                int j=0;
24                while (j<param[i][0]){
25                    arr[i][j]=scn.nextInt();
26                    j++;
27                }
28            }
29 }

I am getting the same error each time i try to fill in an array using a for loop! Any suggestions?

@Quirliom No, that's not the problem. The error is on line 25, not line 17. — John Kugelman, Aug 17 '14 at 06:13
You have to initialize scanner object as, Scanner scn = new Scanner(System.in); — MrYo, Aug 17 '14 at 06:17
@Quirliom and Sanjaya Pandey , yes i do intialize the Scanner object. the error is in line 25.. — Maverick, Aug 17 '14 at 06:34
First of all, show your entire (reproducible) error-prone code, and second of all, stop typing in all caps. — David Titarenco, Aug 17 '14 at 06:51
possible duplicate of [What is a Null Pointer Exception, and how do I fix it?](http://stackoverflow.com/questions/218384/what-is-a-null-pointer-exception-and-how-do-i-fix-it) — user2864740, Oct 11 '14 at 21:28

score 3 · Accepted Answer · answered Aug 17 '14 at 06:35

3

Change 23 - 27 to

arr[i] = new int[param[i][0]];
for( int j = 0; j < param[i][0]; j++ ){  // prefer for to while
    arr[i][j] = scn.nextInt();
}

It should be obvious?

answered Aug 17 '14 at 06:35

laune

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committedandroider · Answer 2 · 2014-08-17T06:52:26.453

1

Well

           int[][] arr=new int[n][];

Makes it so arr is an array of n arrays. However those arrays are empty

When you do this

 arr[i][j]=scn.nextInt();

You're accessing columns(because the array at that row has size 0) that don't exist, hence null pointer exception `

edited Aug 17 '14 at 06:52

answered Aug 17 '14 at 06:43

committedandroider

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Watch your language! An "empty array" is what you get when you say `new int[0]`, but after that `new int[n][]` there aren't any arrays *within* `arr` at all; `arr` contains `n` times `null`. – laune Aug 17 '14 at 07:09
when you do matrix[1].length, i saw it was 0 so I assumed it was just an empty array – committedandroider Aug 17 '14 at 22:07
Well, `int[][] a = new int[3][]; System.out.println( a[1].length );` does throw a NPE. Please fix your answer. `int[][] a = new int[3][0];` gives you "empty" arrays. – laune Aug 18 '14 at 05:29

JAVA- NullPointerException

2 Answers2