Design patterns are solutions to recurring problems
- Deal with the creation (construction) of objects
- Explicit (constructor) vs. implicit (DI, reflection, etc.)
- Wholesale (single statment) vs. piecewise (step-by-step)
Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.
Provide an interface for creating families of related or dependent objects without specifying their concrete classes.
Separate the construction of a complex object from its representation so that the same construction process can create different representations.
Specify the kind of objects to create using a prototypical instance, and create new objects by copying this prototype.
Ensure a class has only one instance and provide a global point of access to it.
- Concerned with the structure (e.g., class members)
- Many patterns are wrappers that mimic the underlyiing class' interface
- Stress the importance of good API design
Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.
Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces.
Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.
Provide a unified interface to a set of interfaces in a subsystem. Façade defines a higher-level interface that makes the subsystem easier to use.
Decouple an abstraction from its implementation so that the two can vary independently.
Use sharing to support large numbers of fine-grained objects efficiently.
Provide a surrogate or placeholder for another object to control access to it.
- They are all different; no central theme
Memento is a behavioral design pattern that lets you save and restore the previous state of an object without revealing the details of its implementation.
Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.
Iterator is a behavioral design pattern that lets you traverse elements of a collection without exposing its underlying representation (list, stack, tree, etc.).
Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.
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.
Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.
Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
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.
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.
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.
