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What is a design pattern? Programming with Behnood “Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice.” A Pattern Language by Christopher Alexander 

What is a design pattern in software engineering? Programming with Behnood “Each design pattern focuses on a particular object-oriented design problem or issue. It describes when it applies, whether it can be applied in view of other design constraints, and the consequences and trade-offs of its use.” Design Patterns: Elements of Reusable Object-Oriented Software by The Gang of Four 

What is a design pattern in software engineering? Programming with Behnood “Design patterns provide the cores of ready-made solutions that can be used to solve many of software’s most common problems. Some software problems require solutions that are derived from first principles. But most problems are similar to past problems, and those can be solved using similar solutions, or patterns.” Code Complete by by Steve McConnell 

What is a design pattern in software engineering? Programming with Behnood ● The design patterns are descriptions of communicating objects and classes that are customized to solve a general design problem in a particular context. ● The design pattern identifies the participating classes and instances, their roles and collaborations, and the distribution of responsibilities. ● Each design pattern focuses on a particular object-oriented design problem or issue. It describes when it applies, whether it can be applied in view of other design constraints, and the consequences and trade-offs of its use. Source: Design Patterns – Elements of Reusable Object-Oriented Software by The Gang of Four (GoF) 

An Introduction to Design Patterns Programming with Behnood Creational Creational Patterns Patterns Structural Structural Patterns Patterns Behavioral Behavioral Patterns Patterns Design Pattern Types Design Pattern Elements Pattern Name Problem Solution Consequences 

An Introduction to Design Patterns Programming with Behnood Creational Patterns Creational Patterns Structural Patterns Structural Patterns Behavioral Patterns Behavioral Patterns Purpose Factory Method Abstract Factory Builder Prototype Singleton Adapter (class) Adapter (object) Bridge Composite Decorator Facade Flyweight Proxy Interpreter Template Method Chain of Responsibility Command Iterator Mediator Memento Observer State Strategy Visitor Scope Object Class Source: Design Patterns – Elements of Reusable Object-Oriented Software by The Gang of Four (GoF) Design Pattern Space 

An Introduction to Design Patterns Programming with Behnood Design Pattern Relationships Source: Design Patterns – Elements of Reusable Object-Oriented Software by The Gang of Four (GoF) Copyright © 2021 Behnood Eghbali Copyright © 2021 Behnood Eghbali 

Design Patterns: Creational Patterns Programming with Behnood Abstract Factory Factory Method Builder Prototype Singleton Factory Method: Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses. Abstract Factory: Provide an interface for creating families of related or dependent objects without specifying their concrete classes. Builder: Separate the construction of a complex object from its representation so that the same construction process can create different representations. Prototype: Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype. Singleton: Ensure a class only has one instance, and provide a global point of access to it. Source: Design Patterns – Elements of Reusable Object-Oriented Software by The Gang of Four (GoF) 

Design Patterns: Creational Patterns Programming with Behnood Dependency Injection Object Pool RAII Lazy Initialization Lazy Initialization: Tactic of delaying the creation of an object, the calculation of a value, or some other expensive process until the first time it is needed. This pattern appears in the GoF catalog as "virtual proxy", an implementation strategy for the Proxy pattern. Dependency Injection: A class accepts the objects it requires from an injector instead of creating the objects directly. Object Pool: Avoid expensive acquisition and release of resources by recycling objects that are no longer in use. Can be considered a generalisation of connection pool and thread pool patterns. Resource Acquisition Is Initialization (RAII): Ensure that resources are properly released by tying them to the lifespan of suitable objects. Multiton: Ensure a class has only named instances, and provide a global point of access to them. Source: Wikipedia Multiton 

Design Patterns: Structural Patterns Programming with Behnood Adapter Bridge Decorator Composite Adapter/Wrapper/Translator: Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces. Bridge: Decouple an abstraction from its implementation so that the two can vary independently. Decorator: Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality. Composite: Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly. Facade: Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use. Flyweight: Use sharing to support large numbers of fine-grained objects efficiently. Proxy: Provide a surrogate or placeholder for another object to control access to it. Source: Design Patterns – Elements of Reusable Object-Oriented Software by The Gang of Four (GoF) Flyweight Facade Proxy 

Design Patterns: Structural Patterns Programming with Behnood Front Controller Twin Module Extension Object Extension Object: Adding functionality to a hierarchy without changing the hierarchy. Twin: Twin allows modeling of multiple inheritance in programming languages that do not support this feature. Front Controller: The pattern relates to the design of Web applications. It provides a centralized entry point for handling requests. Marker: Empty interface to associate metadata with a class. Module: Group several related elements, such as classes, singletons, methods, globally used, into a single conceptual entity. Source: Wikipedia Marker 

Design Patterns: Behavioral Patterns Programming with Behnood Iterator Visitor Memento Command Source: Design Patterns – Elements of Reusable Object-Oriented Software by The Gang of Four (GoF) Chain of Responsibility State Strategy Mediator Template Method Observer Interpreter 

Design Patterns: Behavioral Patterns Programming with Behnood Source: Design Patterns – Elements of Reusable Object-Oriented Software by The Gang of Four (GoF) Memento: Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later. Mediator: Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently. Strategy: Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it. Visitor: Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates. Observer (Publish/Subscribe): Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. Chain of Responsibility: Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it. Command: Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations. Interpreter: Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language. Iterator: Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation. State: Allow an object to alter its behavior when its internal state changes. The object will appear to change its class. Template Method: Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm's structure. 

Design Patterns: Behavioral Patterns Programming with Behnood Blackboard: Artificial intelligence pattern for combining disparate sources of data. Null Object: Avoid null references by providing a default object. Servant: Define common functionality for a group of classes. The servant pattern is also frequently called helper class or utility class implementation for a given set of classes. The helper classes generally have no objects hence they have all static methods that act upon different kinds of class objects. Specification: Recombinable business logic in a Boolean fashion. Source: Wikipedia Black- board Specifica-tion Null Object Servant 

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