Design Pattern - Factory Method Pattern

Factory Method Pattern
From definition to implementation
17-Dec-14 Mudasir Qazi - mudasirqazi00@gmail.com 1

Contents / Agenda
• Definition
• Advantages and Usage
• Implementation – Class diagram
• Implementation – Step 1 to 3
• Implementation – Test and Result
• Difference between Factory and Abstract Factory Patterns

Definition
• A Factory Pattern or Factory Method Pattern says that just define an
interface or abstract class for creating an object but let the subclasses
decide which class to instantiate.
• In other words, subclasses are responsible to create the instance of
the class.
• The Factory Method Pattern is also known as Virtual Constructor.
• Factory Method pattern comes under “Creational Design Patterns”.
• In Factory pattern, we create object without exposing the creation
logic to the client and refer to newly created object using a common
interface.

Advantages and Usage
• Advantages:
1. Factory Method Pattern allows the sub-classes to choose the type of objects
to create.
2. It promotes the loose-coupling by eliminating the need to bind application-
specific classes into the code. That means the code interacts solely with the
resultant interface or abstract class, so that it will work with any classes that
implement that interface or that extends that abstract class.
• Usage:
1. When a class doesn't know what sub-classes will be required to create
2. When a class wants that its sub-classes specify the objects to be created.
3. When the parent classes choose the creation of objects to its sub-classes.

Implementation - Overview
• We're going to create a Shape interface and concrete classes
implementing the Shape interface. A factory class ShapeFactory is
defined as a next step.
• FactoryPatternDemo, our demo class will use ShapeFactory to get a
Shape object. It will pass information (CIRCLE / RECTANGLE / SQUARE)
to ShapeFactory to get the type of object it needs.

Implementation – Class diagram
• We are going to implement following class diagram with step by step
explanation.

Step 1 : Create Interface
• Our interface is simple, just contains a method to draw a shape.

Step 2 : Concrete classes

Step 3 : Shape Factory
To keep the
implementation simple,
we can define our factory
class as shown in picture.
This will return
appropriate object.

Step 4 : Test
We can easily test our pattern by
just writing this code in our main
function.
First, create a shape factory and
use its getShape method to draw
new objects.

Step 5 : Results

Factory Method vs Abstract Factory
• The methods of an Abstract Factory are implemented as Factory Methods.
Both the Abstract Factory Pattern and the Factory Method Pattern
decouples the client system from the actual implementation classes through
the abstract types and factories. The Factory Method creates objects
through inheritance where the Abstract Factory creates objects through
composition.
• With the Factory pattern, you produce implementations (Apple, Banana,
Cherry, etc.) of a particular interface -- say, IFruit. With the Abstract Factory
pattern, you produce implementations of a particular Factory interface --
say, IFruitFactory. Each of those knows how to create different kinds of fruit.
• Factory method: You have a factory that creates objects that derive from a
particular base class.
Abstract factory: You have a factory that creates other factories, and these
factories in turn create objects derived from base classes. You do this
because you often don't just want to create a single object (as with Factory
method) - rather, you want to create a collection of related objects.

Example
• Imagine you are constructing a house and you approach a carpenter
for a door. You give the measurement for the door and your
requirements, and he will construct a door for you. In this case, the
carpenter is a factory of doors. Your specifications are inputs for the
factory, and the door is the output or product from the Factory.
• Now, consider the same example of the door. You can go to a
carpenter, or you can go to a plastic door shop or a PVC shop. All of
them are door factories. Based on the situation, you decide what kind
of factory you need to approach. This is like an Abstract Factory.

Design Pattern - Factory Method Pattern

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