Section 2: Creational Patterns in Java

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Section 2: Creational Patterns
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All of the Creational patterns deal with ways to create instances of objects This is important because your program should not depend on how objects are created and arranged In Java, of course, the simplest way to create an instance of an object is by using the new operator:
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fred = new Fred(); //instance of Fred class
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However, doing this really amounts to hard coding, depending on how you create the object within your program In many cases, the exact nature of the object that is created could vary with the needs of the program Abstracting the creation process into a special "creator" class can make your program more flexible and general The six Creational patterns follow:
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Factory Method pattern provides a simple decision-making class that returns one of several possible subclasses of an abstract base class, depending on the data that are provided We'll start with the Simple Factory pattern as an introduction to factories and then introduce the Factory Method pattern as well Abstract Factory pattern provides an interface to create and return one of several families of related objects Builder pattern separates the construction of a complex object from its representation so that several different representations can be created, depending on the needs of the program Prototype pattern starts with an instantiated class, which it copies orclones to make new instances These instances can then be further tailored using their public methods Singleton pattern is a class of which there may be no more than one instance It provides a single global point of access to that instance
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3 The Factory Pattern
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One type of pattern that we see again and again in OO programs is the Simple Factory pattern A Simple Factory pattern returns an instance of one of several possible classes depending on the data provided to it Usually all classes that it returns have a common parent class and common methods, but each performs a task differently and is optimized for different kinds of data This Simple Factory serves as an introduction to the somewhat more subtle Factory Method GoF pattern we'll discuss shortly It is, in fact, a special subset of that pattern
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How a Factory Works
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To understand the Simple Factory pattern, let's look at the diagram in Figure 31 Figure 31 A Simple Factory pattern
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In this figure, X is base class and classes XY and XZ are derived from it The XFactory class decides which of these subclasses to return depending on the arguments you give it On the right, we define a getClass method to be one that passes in some value abc, and that returns some instance of the class x Which one it returns doesn't matter to the programmer since they all have the same methods, but different implementations How it decides which one to return is entirely up to the factory It could be some very complex function, but it is often quite simple
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Sample Code
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Let's consider a simple case in which we could use a Factory class Suppose we have an entry form and we want to allow the user to enter his name either as "firstname lastname" or as "lastname, firstname" We'll make the further simplifying assumption that we will always be able to decide the name order by whether there is a comma between the last and first name This is a pretty simple sort of decision to make, and you could make it with a simple if statement in a single class, but let's use it here to illustrate how a factory works and what it can produce
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We'll start by defining a simple base class that takes a String and splits it (somehow) into two names:
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public class Namer { //base class extended by two child classes protected String last; //split name protected String first; //stored here public String getFirst() { return first; //return first name } public String getLast() { return last; //return last name } }
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In this base class, we don't compute any names as such, but we do provide implementations of the getFirst and getLast methods We'll store the split first and last names in the Strings first and last, and, since the subclasses will need access to these variables, we'll make them protected
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