Design Patterns

Types: creational, structural and behavioral

Patterns: solutions to recurring design problems

Creational: addresses complexity of object creation

Structural: deals with composition of objects

Behavioral: deals with interaction between objects

Creational Design Patterns

Prototype: cloned or initial setup, fully initialized object e.g. Chessboard

Builder: separates object construction from it’s representation e.g. buffet meal builder, coffee builder. Implementation: inner static class

Singleton: single instance per JVM e.g. president of a country. Implementation: private constructor,

Factory Method: create a family of object types, adding new types to family is easier e.g. car factory. Implementation: a class with method getCar(make, model), Car abstract class, Altima class, Camry class etc

Structural Design Patterns

Proxy: an object representing another object, hides complexity in communicating with real object e.g. proxy setting, redirect URL, credit card, ejb remote objects

Decorator: add responsibilities to objects dynamically e.g. make pizza with toppings decorator, discount, Java I\O classes

Facade: a single class that represents an entire sub systems. Minimizes network calls, defines transactions boundary ( online order – invoice, payment etc)

Adapter: match interfaces of different object e.g mobile charger (US, India), Java to JSON or XML

Flyweight: A flyweight is an object that minimizes memory usage by sharing as much data as possible with other similar objects e.g. jdbc connection pool, string intern

Behavioral Design Patterns

Chain of responsibility: a way of passing a request between a chain of objects e.g. loan approval process, exceptions handling multi-catch

Iterator: sequentially access the elements of a collection

State: alter an object behavior when it’s state changes e.g. fan speed. (Next) state is decided based on current state.

Strategy: encapsulate an algorithm inside a class e.g. compare() different types of sorting algorithms

Observer: a way of notifying a change to a number of objects e.g. payment received, notify() in object class. Notifier class with register(a) method.

Visitor: define operations to class without changing code ( extend but no modifications) open/closed principle.

Clients traverse the object structure and call a dispatching operation accept(visitor) on an element — that “dispatches” (delegates) the request to the “accepted visitor object”. The visitor object then performs the operation on the element (“visits the element”).

Template: defines the program skeleton of an algorithm in an operation, deferring some steps to subclasses. E.g. AbstractController.handleRequestInternal() class in Spring MVC

Command: An object is used to encapsulate all information needed to perform an action or trigger an event at a later time. Four terms always associated with the command pattern are command, receiver, invoker and client. E.g. order meal in restaurant (paper with order detailse is the command). E.g. HttpRequest, Runnable interface,

Memento: capture and restore an objects internal state e.g. Undo\Redo operations, Deserialization

Mediator: defines simplified communication between classes e.g. Air Traffic Controller, ESB bus,

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