Objects containing Array of Objects

Question

I could assign objects (ingredients) to an array in the main class, like this:

Ingredient[] ingredient = new Ingredient[5];
Ingredient potato = new Ingredient("Potatoes");
ingredient[0] = potato;

but what I really want to do is put the array inside another object (food), so I can access it like this:

fries.ingredient[0] = potato;

so that each food has its own collection of ingredients. However, everything I've tried leads to 'NullPointerException' or 'cannot find symbol'. How do I resolve this?

Edit:
Sorry for taking a while. I don't know how to indent inside blockquotes, but here goes. This was my (failed) attempt that causes NullPointerException.

Main.java:

public class Main {
public static void main (String[] args) {
Ingredient potato = new Ingredient("Potatoes");
Food fries = new Food("Fries");
fries.ingredient[0] = potato;
} }

Food.java:

public class Food {
Ingredient[] ingredient;
String name;
public Food(String name) {
this.name = name;
Ingredient[] ingredient = new Ingredient[5];
} }

Ingredient.java

public class Ingredient {
String name;
public Ingredient(String name) {
this.name = name;
} }

We can't tell you how to fix your code without seeing neither the the code, nor the error message. — JB Nizet
– JB Nizet, Commented Dec 22, 2013 at 12:37
A NullPointerException is thrown when an application attempts to use null in a case where an object is required. You are trying to access something that doesn't exist, post your code it will help to help you. — Ojonugwa Jude Ochalifu
– Ojonugwa Jude Ochalifu, Commented Dec 22, 2013 at 12:40
You most likely didn't instantiate Ingredient. You'd have to post more code for us to see. — Space Ghost
– Space Ghost, Commented Dec 22, 2013 at 12:41

Marko Topolnik · Accepted Answer · 2013-12-22 13:50:57Z

1

In your constructor, you have this:

Ingredient[] ingredient = new Ingredient[5];

You have declared a local variable named ingredient which shadows your instance variable of the same name. Change that line to

this.ingredient = new Ingredient[5];

As a next step in your learning, consider using a List<Ingredient> instead of the array. Arrays are non-resizable, among other inconveniences. Basically, their primary use is in the internals of an implementation, not in client code.

edited Dec 22, 2013 at 13:50

answered Dec 22, 2013 at 13:28

Marko Topolnik

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1 Comment

user3127055 Over a year ago

leads to 'cannot find symbol' for me edit: nvm, works now. thanks!

Dima · Accepted Answer · 2013-12-22 12:47:36Z

0

create a class:

public class Food{
   public Object[] ingredient;
   public food(int numOfIng){
        this.ingredients=new Object[numOfIng]
   }
}

now just do:

Food fries=new Food(5);
Ingredient potato = new Ingredient("Potatoes");
fries.ingredient[0] = potato;

answered Dec 22, 2013 at 12:47

Dima

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Comments

solvator · Accepted Answer · 2013-12-22 13:34:59Z

0

You can try to do though interface, so as I think something like:

interface Food {

   void takeInfo();
}

public class FoodSelection{

private List<Food> listOfFood = new ArrayList<>();

public void selectFries(){
  initFood(new Fries());
}

// other food...

public void initFood(Food food){
food.takeInfo();
listOfFood.add(food);
}

}

Now it's time to create fries class..

class Fries implements Food{

private String[] ingredients;

public void takeInfo(){
   // declare Fries ingredients.
}

}

So, to call it do something like this:

FoodSelection takeFood = new FoodSelection();

takeFood.selectFries();

edited Dec 22, 2013 at 13:34

answered Dec 22, 2013 at 13:13

solvator

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2 Comments

user3127055 Over a year ago

I'll try, but I'm not yet familiar with those keywords

solvator Over a year ago

It will works. And btw the question in your post is answered by Marko Topolnik

S.Yavari · Accepted Answer · 2013-12-22 13:42:31Z

you should do that as the following code:

Ingredient potato = new Ingredient("Potatoes");
fries = new ClassName(); //Replace with your class Constructor method
fries.ingredient = new Ingredient[5];
fries.ingredient[0] = potato;

Edit:

Your Food class must be like this:

public class Food {
    Ingredient[] ingredient;
    String name;

    public Food(String name) {
        this.name = name;
        this.ingredient = new Ingredient[5];
    }
}

and Main class:

public class Main {
    public static void main(String[] args) {
        Ingredient potato = new Ingredient("Potatoes");
        Food fries = new Food("Fries");
        fries.ingredient[0] = potato;
    }
}

and Ingredient class

public class Ingredient {
    String name;

    public Ingredient(String name) {
        this.name = name;
    }
}

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