Purpose of passing factory object in Simple Factory Design Pattern (Head-First)?

Question

I am from C++ background, recently started learning Design Patterns.

I am facing problems with this code from Head First Design Patterns:

public class PizzaStore { SimplePizzaFactory factory; public PizzaStore(SimplePizzaFactory factory) { this.factory = factory; } public Pizza orderPizza(String type) { Pizza pizza; pizza = factory.createPizza(type); pizza.prepare(); pizza.bake(); pizza.cut(); pizza.box(); return pizza; } }

Please help me with the following doubt:

What is the relevance of passing a factory object in the Constructor of PizzaStore class ?

PizzaStore class already contains a SimplePizzaFactory object
The passed-on object is not initialized with any data (which needs to be copied by PizzaStore Constructor):
```
public PizzaStore(SimplePizzaFactory factory) { this.factory = factory; 
```
}

Thanks

Alejandro Goñi · Accepted Answer · 2017-11-28 15:19:59Z

PizzaStore class already contains a SimplePizzaFactory object

Well, PizzaStore have a non-initialized attribute of type SimplePizzaFactory. In the PizzaStore constructor, that attribute is initialized with a reference to the constructor argument. This is an usual pattern in Java code.

The passed-on object is not initialized with any data (which needs to be copied by PizzaStore Constructor)

There is no need to add initialization code in the constructor, is asumed that the SimplePizzaFactory is already initialized. That factory is not copied, is referenced by the PizzaStore attribute, so the factory inside the PizzaStore, after the constructor is executed, is an already initialized object.

Leo Aso · Accepted Answer · 2017-11-28 15:20:57Z

This is one place where Java differs from C++. When you type

SimplePizzaFactory factory;

It's not like in C++ where this implicitly calls a no-argument constructor and creates an object. In Java, factory will simply be null. This is because variables in Java are more similar to pointers in C++. The above is similar to

SimplePizzaFactory *factory;

You'll recognize that this doesn't create a value at all. It just makes a pointer that does not point to anything useful. If you wanted to give it a value, you would have to do

factory = new SimpleFactory(...);

or, if you have another pointer, you can pass that pointer's value to it, so it will point to the same value.

SimpleFactory *otherFactory = ...; ... factory = otherFactory;

That's how it works in Java. In short, Java variables are just references to objects, just like C++ pointers (but with garbage collection). To use a reference, first you need to assign it a value.

Diyarbakir · Accepted Answer · 2017-11-28 15:25:05Z

Have a look at PizzaTestDrive class where the PizzaStore gets its SimplePizzaFactory.

SimplePizzaFactory factory = new SimplePizzaFactory(); PizzaStore store = new PizzaStore(factory);

This is where the actual SimplePizzaFactory is created and passed on to the constructor. If you do not do this, then the factory will not be initialized, i.e. null and the following line of code will throw a NullPointerException:

pizza = factory.createPizza(type);

Wilfredo · Accepted Answer · 2017-11-28 15:28:54Z

In Java "everything is an object" and needs to be instantiated explicitly, that's why you can think of the SimplePizzaFactory inside the PizzaStore class as a pointer that needs to be instantiated (needs to have the new SimplePizzaFactory();) the same with the argument passed into the constructor, imagine a c++ pointer passed as an argument to the constructor, the pointer should already be instantiated somewhere else prior to calling the PizzaStore's constructor.

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