CODEWITHSUNDEEP
javaclassdictionary
Mark
Mark

Reputation: 67

How to: Using a custom class as a Map parameter

I'm getting the nullpointerexception in the line 9 (the one with a comment).

I'm trying to use the method as a parametre in the shoppingCart class:

import java.util.*;

public class ShoppingCart {

    private Map<String, Purchase> ShoppingCart;

    public void add (String product, int price) {
        Purchase purchase = new Purchase(product, 1, price);
        ShoppingCart.put(product, purchase);         //this line doesn't work!
    }

    public int price() {
        Collection<Purchase> total = shoppingCart.values();

        }   
}

The constructor for the Purchase method is: 

 public Purchase(String product, int amount, int price) {

The Main tries to add the product to the shoppingCart which causes the nullpointerexception:

ShoppingCart cart = new ShoppingCart();
cart.add("milk, 3);

I suppose I'm missing something fundamental when it comes to Maps. What am I doing wrong here?

Upvotes: 0

Views: 181

Answers (3)

jbossini
jbossini

Reputation: 71

I think that the problem is that you don't initialize your shoppingCart: 

private Map<String, Purchase> ShoppingCart= new HashMap<String,Purchase>()

Upvotes: 0

Colin Hebert
Colin Hebert

Reputation: 93167

You forgot to instantiate your map ShoppingCart = new HashMap<String, Purchase>().

By the way, in java, variables always start with a lower case letter (camelCase), only classes start with an upper case (PascalCase)

Upvotes: 1

SJuan76
SJuan76

Reputation: 24885

You never create a ShoppingCart object. This is not like C++ in which the object is created when you define the variable; all variables are in fact references to objects and you need to explicity create the objects (with new).

BTW, this line causes a compilation error (shoppingCart should be lowercase)

private Map<String, Purchase> ShoppingCart;

Upvotes: 0

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