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The pattern concept

Initially, you can think of a pattern as an especially clever and insightful way of solving a particular class of problems. That is, it looks like a lot of people have worked out all the angles of a problem and have come up with the most general, flexible solution for it. The problem could be one you have seen and solved before, but your solution probably didn’t have the kind of completeness you’ll see embodied in a pattern.

Although they’re called “design patterns,” they really aren’t tied to the realm of design. A pattern seems to stand apart from the traditional way of thinking about analysis, design, and implementation. Instead, a pattern embodies a complete idea within a program, and thus it can sometimes appear at the analysis phase or high-level design phase. This is interesting because a pattern has a direct implementation in code and so you might not expect it to show up before low-level design or implementation (and in fact you might not realize that you need a particular pattern until you get to those phases).

The basic concept of a pattern can also be seen as the basic concept of program design: adding a layer of abstraction. Whenever you abstract something you’re isolating particular details, and one of the most compelling motivations behind this is to separate things that change from things that stay the same . Another way to put this is that once you find some part of your program that’s likely to change for one reason or another, you’ll want to keep those changes from propagating other changes throughout your code. Not only does this make the code much cheaper to maintain, but it also turns out that it is usually simpler to understand (which results in lowered costs).

Often, the most difficult part of developing an elegant and cheap-to-maintain design is in discovering what I call “the vector of change.” (Here, “vector” refers to the maximum gradient and not a collection class.) This means finding the most important thing that changes in your system, or put another way, discovering where your greatest cost is. Once you discover the vector of change, you have the focal point around which to structure your design.

So the goal of design patterns is to isolate changes in your code. If you look at it this way, you’ve been seeing some design patterns already in this book. For example, inheritance can be thought of as a design pattern (albeit one implemented by the compiler). It allows you to express differences in behavior (that’s the thing that changes) in objects that all have the same interface (that’s what stays the same). Composition can also be considered a pattern, since it allows you to change – dynamically or statically – the objects that implement your class, and thus the way that class works.

You’ve also already seen another pattern that appears in Design Patterns : the iterator (Java 1.0 and 1.1 capriciously calls it the Enumeration; Java 1.2 collections use “iterator” ). This hides the particular implementation of the collection as you’re stepping through and selecting the elements one by one. The iterator allows you to write generic code that performs an operation on all of the elements in a sequence without regard to the way that sequence is built. Thus your generic code can be used with any collection that can produce an iterator.

The singleton

Possibly the simplest design pattern is the singleton, which is a way to provide one and only one instance of an object. This is used in the Java libraries, but here’s a more direct example:

//: SingletonPattern.java
// The Singleton design pattern: you can
// never instantiate more than one.
package c16;

// Since this isn't inherited from a Cloneable
// base class and cloneability isn't added,
// making it final prevents cloneability from
// being added in any derived classes:
final class Singleton {
  private static Singleton s = new Singleton(47);
  private int i;
  private Singleton(int x) { i = x; }
  public static Singleton getHandle() { 
    return s; 
  }
  public int getValue() { return i; }
  public void setValue(int x) { i = x; }
}

public class SingletonPattern {
  public static void main(String[] args) {
    Singleton s = Singleton.getHandle();
    System.out.println(s.getValue());
    Singleton s2 = Singleton.getHandle();
    s2.setValue(9);
    System.out.println(s.getValue());
    try {
      // Can't do this: compile-time error.
      // Singleton s3 = (Singleton)s2.clone();
    } catch(Exception e) {}
  }
} ///:~

The key to creating a singleton is to prevent the client programmer from having any way to create an object except the ways you provide. You must make all constructors private, and you must create at least one constructor to prevent the compiler from synthesizing a default constructor for you (which it will create as “friendly”).

At this point, you decide how you’re going to create your object. Here, it’s created statically, but you can also wait until the client programmer asks for one and create it on demand. In any case, the object should be stored privately. You provide access through public methods. Here, getHandle( ) produces the handle to the Singleton object. The rest of the interface ( getValue( ) and setValue( )) is the regular class interface.

Java also allows the creation of objects through cloning. In this example, making the class final prevents cloning. Since Singleton is inherited directly from Object, the clone( ) method remains protected so it cannot be used (doing so produces a compile-time error). However, if you’re inheriting from a class hierarchy that has already overridden clone( ) as public and implemented Cloneable, the way to prevent cloning is to override clone( ) and throw a CloneNotSupportedException as described in Chapter 12. (You could also override clone( ) and simply return this, but that would be deceiving since the client programmer would think they were cloning the object, but would instead still be dealing with the original.)

Note that you aren’t restricted to creating only one object. This is also a technique to create a limited pool of objects. In that situation, however, you can be confronted with the problem of sharing objects in the pool. If this is an issue, you can create a solution involving a check-out and check-in of the shared objects.

Classifying patterns

The Design Patterns book discusses 23 different patterns, classified under three purposes (all of which revolve around the particular aspect that can vary). The three purposes are:

Creational: how an object can be created. This often involves isolating the details of object creation so your code isn’t dependent on what types of objects there are and thus doesn’t have to be changed when you add a new type of object. The aforementioned Singleton is classified as a creational pattern, and later in this chapter you’ll see examples of Factory Method and Prototype.
Structural: designing objects to satisfy particular project constraints. These work with the way objects are connected with other objects to ensure that changes in the system don’t require changes to those connections.
Behavioral: objects that handle particular types of actions within a program. These encapsulate processes that you want to perform, such as interpreting a language, fulfilling a request, moving through a sequence (as in an iterator), or implementing an algorithm. This chapter contains examples of the Observer and the Visitor patterns.

The Design Patterns book has a section on each of its 23 patterns along with one or more examples for each, typically in C++ but sometimes in Smalltalk. (You’ll find that this doesn’t matter too much since you can easily translate the concepts from either language into Java.) This book will not repeat all the patterns shown in Design Patterns since that book stands on its own and should be studied separately. Instead, this chapter will give some examples that should provide you with a decent feel for what patterns are about and why they are so important.

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