Chapter 1 Introduction Design Patterns: Elements of Reusable Object-Oriented Software Book
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Creational patterns are ones that create objects, rather than having to instantiate objects directly. This gives the program more flexibility in deciding which objects need to be created for a given case. The authors further distinguish between 'Aggregation', where one object 'has' or 'is part of' another object (implying that an aggregate object and its owner have identical lifetimes) and acquaintance, where one object merely 'knows of' another object. Sometimes acquaintance is called 'association' or the 'using' relationship. Acquaintance objects may request operations of each other, but they are not responsible for each other. Acquaintance is a weaker relationship than aggregation and suggests much looser coupling between objects, which can often be desirable for maximum maintainability in designs.
Gunnar Peipman – Programming Blog
Experienced object-oriented designers will tell you that a reusable and flexible design is difficult if not impossible to get “right” the first time. Before a design is finished, they usually try to reuse it several times, modifying it each time. Design Patterns was the first book that gathered together and defined solutions in the form of patterns to most common problems in technical design of software.
Object-Oriented Design Patterns in Life
The XMLToJSONAdapter class is used to convert the XML data from EmployeeAPI into JSON format, making it compatible with the rest of the system that expects JSON data. Suppose you have a music player in your car that only supports cassettes, but you want to listen to music from your smartphone. The adapter has an interface compatible with the cassette player and provides an interface for your smartphone to use. The prototype pattern is used to instantiate a new object by copying all of the properties of an existing object, creating an independent clone.
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A singleton class ensures that only one instance of the class can be created. The pattern is often called an anti-pattern because it may lead to high coupling of components. The proxy pattern is used to provide a surrogate or placeholder object, which references an underlying object.
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The bridge pattern is a design pattern that separates the abstract elements of a class from its technical implementation. This provides a cleaner implementation of real-world objects and allows the implementation details to be changed easily. The adapter pattern is a design pattern that is used to allow two incompatible types to communicate. Where one class relies upon a specific interface that is not implemented by another class, the adapter acts as a translator between the two types. The Adapter Design Pattern is useful for ensuring compatibility between classes with incompatible interfaces.
Patterns in this book are used today in almost every program and API to keep code clean, testable and manageable. So we can say adapter design patterns are just like a real-life adapter which to used for different purposes. They use inheritance to compose interfaces and define ways to compose objects to obtain new functionality.
Erich Gamma: A pattern of success ZDNET - ZDNet
Erich Gamma: A pattern of success ZDNET.
Posted: Tue, 18 Jul 2006 07:00:00 GMT [source]
Table of Contents
Any other regularity in the code is a sign, to me at least, that I'm using abstractions that aren't powerful enough-- often that I'm generating by hand the expansions of some macro that I need to write. The authors also discuss so-called parameterized types, which are also known as generics (Ada, Eiffel, Java, C#, VB.NET, and Delphi) or templates (C++). These allow any type to be defined without specifying all the other types it uses—the unspecified types are supplied as 'parameters' at the point of use. The Adapter design pattern follows the Single Responsibility Principle (SRP) and Open-Closed Principle (OCP).
Use of an interface also leads to dynamic binding and polymorphism, which are central features of object-oriented programming. The state pattern is used to alter the behaviour of an object as its internal state changes. The pattern allows the class for an object to apparently change at run-time. The composite pattern describes a way to create tree structures using objects and object groups.
Today we see these patterns in many programs and developers on different platforms are more and more aware of these first defined design patterns. The authors refer to inheritance as white-box reuse, with white-box referring to visibility, because the internals of parent classes are often visible to subclasses. Designing object-oriented software is hard, and designing reusable object-oriented software is even harder. You must find pertinent objects, factor them into classes at the right granularity, define class interfaces and inheritance hierarchies, and establish key relationships among them. Your design should be specific to the problem at hand but also general enough to address future problems and requirements.
Each pattern also includes code that demonstrates how it may be implemented in object-oriented programming languages like C++ or Smalltalk. N2 - Capturing a wealth of experience about the design of object-oriented software, four top-notch designers present a catalog of simple and succinct solutions to commonly occurring design problems. Each pattern describes the circumstances in which it is applicable, when it can be applied in view of other design constraints, and the consequences and trade-offs of using the pattern within a larger design. Patterns allow designers to create more flexible, elegant, and ultimately reusable designs without having to rediscover the design solutions themselves. Design Patterns is golden classics of software design books written by Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides (aka Gang of Four – GoF). When this book was published first it was revolutionary because it contained still undocumented knowledge from software development field that needed some systematic work and organization on it.
The proxy provides the same public interface as the underlying subject class, adding a level of indirection by accepting requests from a client object and passing these to the real subject object as necessary. The authors employ the term 'toolkit' where others might today use 'class library', as in C# or Java. In their parlance, toolkits are the object-oriented equivalent of subroutine libraries, whereas a 'framework' is a set of cooperating classes that make up a reusable design for a specific class of software. They state that applications are hard to design, toolkits are harder, and frameworks are the hardest to design. They warn that the implementation of a subclass can become so bound up with the implementation of its parent class that any change in the parent's implementation will force the subclass to change. Furthermore, they claim that a way to avoid this is to inherit only from abstract classes—but then, they point out that there is minimal code reuse.
According to the book "Design Patterns, Elements of Reusable Object-Oriented Software," the Adapter design pattern converts the interface of a class into another interface clients expect. The adapter lets classes work together that couldn't otherwise because of incompatible interfaces. The adapter design pattern, also known as Wrapper, is a popular structural design pattern in software development. If you're unsure what a design pattern is, have a look at my earlier articles on the design pattern series.
Instead of classes communicating directly, and thus requiring knowledge of their implementation, the classes send messages via a mediator object. When I see patterns in my programs, I consider it a sign of trouble. The shape of a program should reflect only the problem it needs to solve.
AB - Capturing a wealth of experience about the design of object-oriented software, four top-notch designers present a catalog of simple and succinct solutions to commonly occurring design problems. They then go on to systematically name, explain, evaluate, and catalog recurring designs in object-oriented systems. All patterns are compiled from real systems and are based on real-world examples.
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