CODEWITHSUNDEEP
javaarraysloopsfor-loopnullpointerexception
Jonas Natten
Jonas Natten

Reputation: 17

java - Why is this for-loop giving a nullpointer exception?

Hi there i'm creating a small game for school, but when i try to do this to fill out the x and y variables in this array i get a NPE. Could anyone help?

public class mainclass {
    public static void main(String[] args) {
        Gra piece[] = new Gra[10];

        for (int i = 0; i < piece.length; i++) {
            piece[i].x = 50;
            piece[i].y = 50;
        }
    }
}

class Gra{
    public int x = 50;
    public int y = 10;
}

Upvotes: 0

Views: 1023

Answers (5)

Subhrajyoti Majumder
Subhrajyoti Majumder

Reputation: 41220

You have initialized gra array but you did not create Object so there is no Object inside the array and by default null is initialized, so piece[i].x is actually null.x which throws NPE.

    Gra piece[] = new Gra[10];
    for (int i = 0; i < piece.length; i++) {
        piece[i] = new Gra();
        piece[i].x = 50;
        piece[i].y = 50;
    }

Upvotes: 1

TechSpellBound
TechSpellBound

Reputation: 2555

The statement 

Gra piece[] = new Gra[10];

will only initialize the array. It won't create Gra objects.

Inside the for loop, you still have to call the constructor as

for (int i = 0; i < piece.length; i++) {
    piece[i] = new Gra();
    piece[i].x = 50;
    piece[i].y = 50;
}

Also, read more about encapsulation. Its a bad idea to make instance variables public.

Upvotes: 3

Sumit Singh
Sumit Singh

Reputation: 15896

Because you are just creating Array of type Gra By 

Gra piece[] = new Gra[10];

So by default this is null .

So first initialize them like 

for (int i = 0; i < piece.length; i++) {
       pirce[i] =  new Gra();
    }

Then do 

    for (int i = 0; i < piece.length; i++) {
        piece[i].x = 50;
        piece[i].y = 50;
    }

Upvotes: 0

shazin
shazin

Reputation: 21893

You have created a an array of Gra with 10 elements.

Gra piece[] = new Gra[10];

But each element in that array is currently pointing to null.

You need to initialize those individual elements as Java's default value for Object is null.

Upvotes: 0

Nikolay Kuznetsov
Nikolay Kuznetsov

Reputation: 9579

Gra piece[] = new Gra[10];

does not initialize objects inside the array, it only creates array, so call constructor to create Gras

for (int i = 0; i < piece.length; i++) {
   piece[i] = new Gra();
   piece[i].x = 50;
   piece[i].y = 50;
}

Upvotes: 12

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