Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma
Table Of Content
An abstract factory offers the interface for creating a set of related or dependant objects without explicitly specifying their classes. Creational Patterns become more important as systems evolve to depend more on object composition than class inheritance. Emphasis shifts away from hardcoding fixed behaviors toward defining a smaller set of fundamental behaviors. The Prototype pattern allows developers to create new objects by cloning existing ones.
Books
The Strategy pattern enables developers to encapsulate varying algorithms and behaviors behind a common interface. The book demonstrates how the Strategy pattern promotes flexibility in selecting algorithms at runtime. Gangs of Four design patterns lay the foundation of core design patterns in programming. There are many other design patterns built on top of these patterns for specific requirements. There are 11 behavioral design patterns defined in the GoF design patterns.
GoF Design Pattern Types
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A decorator allows to add behavior to an individual object, either statically or dynamically, without affecting the behavior of other objects from the same class. This object is sent to a receiver which executes the command. The advantage of this technique is that the requests can be queued, logged or implemented to support undo/redo. The Interpreter Pattern represents a grammar as a class hierarchy and implements an interpreter as an operation on instances of classes. Use the Interpreter Pattern when there is a language to interpret and you can represent statements in the language as abstract syntax trees. Behavioral Patterns are concerned with algorithms and the assignment of responsibilities between objects.
Software design patterns
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.
Use of an interface also leads to dynamic binding and polymorphism, which are central features of object-oriented programming. The Observer pattern establishes a dependency relationship between objects, ensuring that changes in one object are reflected in its dependent objects. "Design Patterns" explores the Observer pattern's use in implementing event handling and notifications. Patterns are more like receipts – they point you to right direction but they don’t guarantee that they are the solution you are looking for.
Chapter 5: Behavioral Patterns
Same way you use receipts in cooking book – you know what you want, book gives just points you to right direction and it is up to you to get there. 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.
Chapter 3: Creational Patterns
Instead of using numerous constructors, the builder pattern uses a builder object, that instantiates and initializes objects using a multiple of steps. The Facade Pattern shows how to make a single object represent an entire subsystem. The facade carries out its responsibilities by forwarding messages to the objects it represents. Designs that use the Abstract Factory, Prototype, or Builder Patterns are even more flexible, but they’re often more complex. Using inheritance is recommended mainly when adding to the functionality of existing components, reusing most of the old code and adding relatively small amounts of new code. If you’re building an application program (such as a document editor or spreadsheet), then internal reuse, maintainability, and extension are high priorities.
Builder Patterns separate the construction of a complex object from its representation so that the same construction process can create different representations. The flyweight pattern is used to reduce the memory and resource usage for complex models containing many hundreds, thousands or hundreds of thousands of similar objects. The Facade pattern provides a simplified interface to a complex subsystem. The authors demonstrate how the Facade pattern promotes simplicity and reduces client complexity. The prototype pattern is used to instantiate a new object by copying all of the properties of an existing object, creating an independent clone.
The book outlines the benefits of the Prototype pattern in enhancing object creation flexibility and optimizing system performance. The Singleton pattern ensures that a class has only one instance and provides a global point of access to that instance. The book discusses the Singleton pattern's use cases and caveats, allowing software engineers to leverage it efficiently when needed.
There are 7 structural design patterns defined in the Gangs of Four design patterns book. The chain of responsibility pattern is a design pattern that defines a linked list of handlers, each of which is able to process requests. When a request is submitted to the chain, it is passed to the first handler in the list that is able to process it.
The clients can access and use individual objects and compositions in the same manner. Delegation is a way of making composition as powerful for reuse as inheritance. Delegation is analogous to subclasses deferring requests to parent classes.
By adopting design patterns, software engineers can create systems that are adaptable to evolving needs, resulting in more resilient and maintainable codebases. Rather than composing interfaces or implementations, structural patterns describe ways to compose objects to realize new functionality. This added flexibility of object composition comes from the ability to change the composition at run-time, which is impossible with static class composition. 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. Patterns in this book are used today in almost every program and API to keep code clean, testable and manageable.
The book empowers software engineers with the knowledge needed to evaluate when and how to apply design patterns effectively. The Composite pattern treats individual objects and compositions of objects uniformly. The book showcases how the Composite pattern simplifies the representation of hierarchical structures. The bridge pattern is a design pattern that separates the abstract elements of a class from its technical implementation.
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