Java Reference
In-Depth Information
Part I: Commonly Used Patterns
Chapter 1. Creational Patterns
Introduction to Creational Patterns
These patterns support one of the most common tasks in object-oriented programming—the creation of objects in
a system. Most OO systems of any complexity require many objects to be instantiated over time, and these
patterns support the creation process by helping to provide the following capabilities:
Generic instantiation - This allows objects to be created in a system without having to identify a specific class
type in code.
Simplicity - Some of the patterns make object creation easier, so callers will not have to write large, complex
code to instantiate an object.
Creation constraints - Some patterns enforce constraints on the type or number of objects that can be created
within a system.
The following patterns are discussed in this chapter:
Abstract Factory - To provide a contract for creating families of related or dependent objects without having to
specify their concrete classes.
Builder - To simplify complex object creation by defining a class whose purpose is to build instances of another
class. The Builder produces one main product, such that there might be more than one class in the product, but
there is always one main class.
Factory Method - To define a standard method to create an object, apart from a constructor, but the decision of
what kind of an object to create is left to subclasses.
Prototype - To make dynamic creation easier by defining classes whose objects can create duplicates of
themselves.
Singleton - To have only one instance of this class in the system, while allowing other classes to get access to
this instance.
Of these patterns, the Abstract Factory and Factory Method are explicitly based on the concept of defining
flexible object creation; they assume that the classes or interfaces to be created will be extended in an
implementing system. As a result, these two patterns are frequently combined with other creational patterns.
