Thursday, December 5, 2013

23 Gang of Four Design Patterns

Creational Patterns
  1. Abstract Factory:  Creates an instance of several families of classes, the factory. Provide an interface for creating families of related or dependent objects without specifying their concrete classes.
  2. Builder: Separates object construction from its representation. Separate the construction of a complex object from its representation so that the same construction processes can create different representations.
    • The intention of builder is to avoid a huge list of constructor. For example
      We have a Car class. The problem is that a car has many options. The combination of each option would lead to a huge list of constructors for this class.
      class Car is
        Can have GPS, trip computer and various numbers of seats. Can be a city car, a sports car, or a cabriolet.
      class CarBuilder is
        method getResult() is
            output:  a Car with the right options
          Construct and return the car.
        method setSeats(number)
        method setCityCar()
        method setCabriolet() is
        method setSportsCar() is
        method setTripComputer() is
        method unsetTripComputer() is
        method setGPS() is
        method unsetGPS() is
      Construct a CarBuilder called carBuilder
      car := carBuilder.getResult()

      Another example here, to make a meal

  3. Factory Method: Creates an instance of several derived classes. Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses. an example of polymorphism
  4. Prototype: A fully initialized instance to be copied or cloned. Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.
  5. Singleton: A class of which only a single instance can exist. Ensure a class only has one instance, and provide a global point of access to it.
Structural Patterns
  1. Adapter: Match interfaces of different classes.Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn’t otherwise because of incompatible interfaces.
  2. Bridge: Separates an object’s interface from its implementation. Decouple an abstraction from its implementation so that the two can vary independently.
    • JDBC-ODBC Bridge
      another example
      interface Engine
          method go
      abstract class Vehicle
          constructor Vehicle(engine)
          method drive() { engine.go();}
      Two classes implemented Engine
      Two solid classes extend Vehicle
      Vehicle car = new CivilCar(new NormalEngine()); –> actually it is alling normalEngine.go()
      The implementation is on normalEngine, decoupled from vehicle.
  3. Composite: A tree structure of simple and composite objects. Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.
  4. Decorator: Add responsibilities to objects dynamically.  Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.
  5. Facade: A single class that represents an entire subsystem. Provide a unified interface to a set of interfaces in a system. Facade defines a higher-level interface that makes the subsystem easier to use.
  6. Flyweight: A fine-grained instance used for efficient sharing. Use sharing to support large numbers of fine-grained objects efficiently. A flyweight is a shared object that can be used in multiple contexts simultaneously. The flyweight acts as an independent object in each context — it’s indistinguishable from an instance of the object that’s not shared.  Here is the code example When the factory is asked for an instance, it is checking if the instance is created before. If yes, return the instance otherwise create the instance and keep the reference. The instance here shall be immutable as it is shared. It is like a cache. 
    • java.lang.Integer#valueOf(int) (also on Boolean, Byte, Character, Short, Long, Float and Double)
    • Java String creation. (Read about string creation in Java specification)
    • Swing borders
  7. Proxy: An object representing another object. Provide a surrogate or placeholder for another object to control access to it.
    • Spring AOP creates proxies for supplied objects
    • java.lang.reflect.Proxy
    • java.rmi.*, the whole API actually.
    • proxy vs decoration: proxy controls on the method call of the target class; while decoration adds more functions on the target class
Behavioral Patterns
  1. Chain of Resp. : A way of passing a request between a chain of objects. 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.
  2. Command: Encapsulate a command request as an object. Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.  
  3. Interpreter: A way to include language elements in a program. Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language. (recognizeable by behavioral methods returning a structurally different instance/type of the given instance/type; note that parsing/formatting is not part of the pattern, determining the pattern and how to apply it is)  //TODO
  4. Iterator: Sequentially access the elements of a collection. Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
  5. Mediator: Defines simplified communication between classes. 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.
  6. Memento: Capture and restore an object's internal state. Without violating encapsulation, capture and externalize an object’s internal state so that the object can be restored to this state later. Memento pattern is implemented with two objects – Originator and Caretaker. The originator is some object that has an internal state. The caretaker is going to let the originator save the state or restore the saved state.
  7. Observer: A way of notifying change to a number of classes. Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
  8. State: Alter an object's behavior when its state changes. Allow an object to alter its behavior when its internal state changes. The object will appear to change its class. 
    • javax.faces.lifecycle.LifeCycle#execute() (controlled by FacesServlet, the behaviour is dependent on current phase (state) of JSF lifecycle)
    • example, an order, state new –> placed –> shipped –> finished –> returned and cancelled. For each status, the behavior is different. For example, an order which is new or placed can be cancelled, but cannot be returned; an order which is shipped or finished cannot be cancelled, the finished order can be returned, nothing shall be done on an cancelled order.
  9. Strategy: Encapsulates an algorithm inside a class. Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it. Make an algorithm's behaviour can be selected at runtime
  10. Template: Defer the exact steps of an algorithm to a subclass. 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. (recognizeable by behavioral methods which already have a "default" behaviour definied by an abstract type)
  11. Visitor: Defines a new operation to a class without change. 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. The pattern should be used when you have distinct and unrelated operations to perform across a structure of objects. This avoids adding in code throughout your object structure that is better kept separate.

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