The code gives NullPointerException for jagged array

Question

The code gives a NullPointerException on the commented line. I'm unable to find out the issue.

package com.lambda.classes;

import java.util.Random;

public class Lambda {

    public static void main(String []args)
    {
        int array[][]=new int[5][];
        Random r=new Random();
        Random r2=new Random();
        for(int i=0;i<5;i++){
            int x=r.nextInt(10);
            for(int j=0;j<x;j++)
            {
                int y=r2.nextInt(200);//this line gives a null pointer exception
                array[i][j]=y;
            }
        }

        for (int[] is : array) {

            for (int i : is) {
                System.out.print(i+"\t");
            }
            System.out.println();
        }
        Random x=new Random();
        System.out.println(x.nextInt(10));
        System.out.println(x.nextInt(10));
    }
}

It is unlikely that a NPE happens there. Please, share the actual code and the full exception stack trace. — BackSlash, Jul 02 '17 at 10:16
Also read: [Syntax for creating a two-dimensional array](//stackoverflow.com/q/12231453) — Tom, Jul 02 '17 at 10:25

IgorGudkov · Answer 1 · 2017-07-02T10:35:19.973

0

In you code error in this line 'array[i][j]=y;'. Because of it 'int array[][]=new int[5][];'

You need set size for inner array too.

Something like this

public static void main(String[] args) {
    int array[][] = new int[5][];
    Random r = new Random();
    Random r2 = new Random();
    for (int i = 0; i < 5; i++) {
        int x = r.nextInt(10);
        array[i] = new int[x];
        for (int j = 0; j < x; j++) {
            int y = r2.nextInt(200);
            array[i][j] = y;
        }
    }
    for (int[] is : array) {
        for (int i : is) {
            System.out.print(i + "\t");
        }
        System.out.println();
    }
    Random x = new Random();
    System.out.println(x.nextInt(10));
    System.out.println(x.nextInt(10));
}

edited Jul 02 '17 at 10:35

answered Jul 02 '17 at 10:23

IgorGudkov

143
1
10

*"You need set size for inner array too."* Not during the initialization of the 2d array. – Tom Jul 02 '17 at 10:36
Changed the code second massive init in loop. – IgorGudkov Jul 02 '17 at 10:38
There is also no need for a second `Random` object. – Tom Jul 02 '17 at 10:51
i did not see your answer before – shubham verma Jul 02 '17 at 16:47

score 0 · Accepted Answer · answered Jul 02 '17 at 15:29

Actually i am trying to implement a jagged array for that matter so whats the point of initialising the array with some thing like that a[5][100], and if i am not wrong in java declaration of some thing like int array[][] = new int[5][]; is completely fine and thanks for the answers (y) :)

score 0 · Answer 3 · answered Jul 02 '17 at 15:42

now i have made it worked without using extra space as if i could have don like int[5][100]; i might end up using extra space but thanks for that initialization point and it can be done easily using an array of list instead..... thanks guys :)

int array[][]=new int[5][];
        Random r=new Random();
        for(int i=0;i<5;i++){
            int x=r.nextInt(10);
            array[i]=new int[x];
            for(int j=0;j<x;j++)
            {
                array[i][j]=r.nextInt(200);
            }
        }

    for (int[] is : array) {
        for (int i : is) {
            System.out.print(i+" ");
        }
        System.out.println();
    }

The code gives NullPointerException for jagged array

3 Answers3