Design Patterns

Creational Patterns

In software engineering, creational design patterns are design patterns that deal with object creation mechanisms, trying to create objects in a manner suitable to the situation. The basic form of object creation could result in design problems or added complexity to the design. Creational design patterns solve this problem by somehow controlling this object creation.

Creational design patterns are composed of two dominant ideas. One is encapsulating knowledge about which concrete classes the system uses. Another is hiding how instances of these concrete classes are created and combined.^[1]

Creational design patterns are further categorized into Object-creational patterns and Class-creational patterns, where Object-creational patterns deal with Object creation and Class-creational patterns deal with Class-instantiation. In greater details, Object-creational patterns defer part of its object creation to another object, while Class-creational patterns defer its object creation to subclasses.^[2]

Five well-known design patterns that are parts of creational patterns are the

• Abstract factory pattern, which provides an interface for creating related or dependent objects without specifying the objects' concrete classes.^[3]
• Builder pattern, which separates the construction of a complex object from its representation so that the same construction process can create different representation.
• Factory method pattern, which allows a class to defer instantiation to subclasses.^[4]
• Prototype pattern, which specifies the kind of object to create using a prototypical instance, and creates new objects by cloning this prototype.
• Singleton pattern, which ensures that a class only has one instance, and provides a global point of access to it.^[5]

Structural Patterns

• Adapter pattern: 'adapts' one interface for a class into one that a client expects
  • Adapter pipeline: Use multiple adapters for debugging purposes.^[1]
  • Retrofit Interface Pattern:^[2][3] An adapter used as a new interface for multiple classes at the same time.
• Aggregate pattern: a version of the Composite pattern with methods for aggregation of children
• Bridge pattern: decouple an abstraction from its implementation so that the two can vary independently
  • Tombstone: An intermediate "lookup" object contains the real location of an object.^[4]
• Composite pattern: a tree structure of objects where every object has the same interface
• Decorator pattern: add additional functionality to a class at runtime where subclassing would result in an exponential rise of new classes
• Extensibility pattern: aka. Framework - hide complex code behind a simple interface
• Facade pattern: create a simplified interface of an existing interface to ease usage for common tasks
• Flyweight pattern: a high quantity of objects share a common properties object to save space
• Pipes and filters: a chain of processes where the output of each process is the input of the next
• Private class data pattern: restrict accessor/mutator access
• Proxy pattern: a class functioning as an interface to another thing

Behavioral Patterns

In software engineering, behavioral design patterns are design patterns that identify common communication patterns between objects and realize these patterns. By doing so, these patterns increase flexibility in carrying out this communication.

Examples of this type of design pattern include:

• Chain of responsibility pattern: Command objects are handled or passed on to other objects by logic-containing processing objects
• Command pattern: Command objects encapsulate an action and its parameters
• "Externalize the Stack": Turn a recursive function into an iterative one that uses a stack^[1]
• Hierarchical visitor pattern: Provide a way to visit every node in a hierarchical data structure such as a tree
• Interpreter pattern: Implement a specialized computer language to rapidly solve a specific set of problems
• Iterator pattern: Iterators are used to access the elements of an aggregate object sequentially without exposing its underlying representation
• Mediator pattern: Provides a unified interface to a set of interfaces in a subsystem
• Memento pattern: Provides the ability to restore an object to its previous state (rollback)
• Null Object pattern: Designed to act as a default value of an object
• Observer pattern: aka Publish/Subscribe or Event Listener. Objects register to observe an event that may be raised by another object
  • Weak reference pattern: De-couple an observer from an observable^[2]
• Protocol stack: Communications are handled by multiple layers, which form an encapsulation hierarchy^[3]
• Scheduled-task pattern: A task is scheduled to be performed at a particular interval or clock time (used in real-time computing)
• Single-serving visitor pattern: Optimise the implementation of a visitor that is allocated, used only once, and then deleted
• Specification pattern: Recombinable business logic in a boolean fashion
• State pattern: A clean way for an object to partially change its type at runtime
• Strategy pattern: Algorithms can be selected on the fly
• Template method pattern: Describes the program skeleton of a program
• Visitor pattern: A way to separate an algorithm from an object

Concurrency Patterns

In software engineering, concurrency patterns are those types of design patterns that deal with the multi-threaded programming paradigm. Examples of this class of patterns include:

• Active Object^[1][2]
• Balking pattern
• Double checked locking pattern
• Guarded suspension
• Leaders/followers pattern
• Monitor Object
• Reactor pattern
• Read write lock pattern
• Scheduler pattern
• Thread pool pattern
• Thread-Specific Storage