CODEWITHSUNDEEP
javaarraysnullpointerexception2d
user3002066
user3002066

Reputation: 1

Nullpointerexception while checking an array

I am pretty new to java but i have to initialize an 2d-array size n, in this example 10. After initialization i want to check the diagonal entries if they are false, and if set them to true. After i want to return value of i.

This is what i coded:

First of all initialization of the array:

public static void init(int n) {
        boolean friendship[][] = new boolean[n][n];}

and after i tried this:

public static int addUser(String name) {
        int id=0;
        for ( int i=0;i<friendship.length;i++) {
            if ( friendship[i][i] = false) {
                friendship[i][i] = true;
                id = i;
            }
        }
        return id;
    }

Sadly its throwing:

Exception in thread "main" java.lang.NullPointerException
    at x.SocialNetwork.addUser(SocialNetwork.java:18)
    at x.SocialNetwork.main(SocialNetwork.java:53)

What can i do to fix this?

PS: Sorry for bad english and formatting.

Upvotes: 0

Views: 69

Answers (1)

Sotirios Delimanolis
Sotirios Delimanolis

Reputation: 279880

I assume you have a static field called friendship. In this method

public static void init(int n) {
     boolean friendship[][] = new boolean[n][n];
}

you are declaring a new local friendship variable, that is shadowing the static member. Therefore, the static friendship field remains null and when you try to access it in addUser you get a NullPointerException.

Use

public static void init(int n) {
     friendship = new boolean[n][n];
}

assuming again that you have something like

public static boolean[][] friendship;

In this

if ( friendship[i][i] = false) {

you are actually setting friendship[i][i] to false. The equality operator is ==.

This is how I see your class

public class Test {
    /* visibility identifier doesn't matter */ static boolean[][] friendship;

    public static void init(int n) {
        // this is a different variable from the member declared above
        // it is a local variable
        boolean friendship[][] = new boolean[n][n]; 
    }

    public static int addUser(String username) {
        int id=0;
        for ( int i=0;i<friendship.length;i++) {
            if ( friendship[i][i] = false) { // referring to static field, not the local variable in init()
                friendship[i][i] = true;
                id = i;
            }
        }
        return id;
    }
}

Upvotes: 3

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