Java create array of classes within class from outside static class

Question

I would like to create an instance of a class, that has an array of class members within it where the array is defined in length upon initialization. The code I have written does not contain any errors precompile, but after running returns nullPointerException. I want to be able to access products of class storeA by typing storeA.products[productnumber].(product variable), is this possible?

package tinc2;

public class FirstProgram {

    public static void main(String[] args) {
        store storeA = new store();
        storeA.name = "Walmart";
        storeA.products = new store.product[3];
        storeA.products[0].name = "Horses";
        System.out.println(storeA.products[0].name);
    }

    public static class store{
        String name;
        product products[];
        static class product{
            String name;
            int quantity;
            double price;
        }
    }

}

Grzegorz Górkiewicz · Accepted Answer · 2017-01-27 20:41:40Z

1

Go for

public static void main(String[] args) {
    store storeA = new store();
    storeA.name = "Walmart";
    storeA.products = new store.product[3];
    storeA.products[0] = new store.product();
    storeA.products[0].name = "Horses";
    System.out.println(storeA.products[0].name);
}

instead.

Besides you should place those classes in separate files. You should follow naming conventions in Java, e.g. Store instead of store. You should use getters and setters.

I would avoid statics, if it is possible.

edited Jan 27, 2017 at 20:41

answered Jan 27, 2017 at 20:25

Grzegorz Górkiewicz

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3 Comments

tincopper2 Over a year ago

that's the first syntax I tried, nullPointerException.

Grzegorz Górkiewicz Over a year ago

No, it's modified ;)

tincopper2 Over a year ago

it appears that each individual product as a piece of the array needs to be declared individually to be used, and that is what I was missing. I added these two line to your code and now it works exactly how it needs to, thank you. for(int i=0;i<storeA.products.length;i++) storeA.products[i] = new store.product();

tonythewest · Accepted Answer · 2017-01-27 20:32:46Z

0

You should not be instantiating a static class. Your Product class should not be defined as static. I recommend:

package tinc2;

public class FirstProgram {

    public static void main(String[] args) {
        Store.name = "Walmart";
        Store.products = new Product[1];
        Store.products[0] = new Product();
        Store.products[0].name = "Horses";
        System.out.println(Store.products[0].name);
    }

    public static class Store{
        String name;
        Product products[];
    }

    public class Product{
        String name;
        int quantity;
        double price;
    }
}

edited Jan 27, 2017 at 20:32

answered Jan 27, 2017 at 20:27

tonythewest

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5 Comments

tincopper2 Over a year ago

That would imply that I only have one store.

tonythewest Over a year ago

This is just a sample. You can size the array any way that you want. Alternatively, you could use a data structure with a flexible size like a linked list.

tincopper2 Over a year ago

I am trying to make it so the class Store is more of a data type, that contains an array of products that can be initialized whenever, however, and numerously.

tonythewest Over a year ago

Indeed. Do you think that the Product class definition has to be an internal class of Store for that to work? It does not. I would recommend that you define Product separately.

tincopper2 Over a year ago

You suggest without reasoning.

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