design_patterns.md

Design Patterns

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0. design patterns with 23 types from Gang of Four

Creational               Structural           Behavioral
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Abstract Factory         Adapter              Chain of Responsibility
Builder                  Bridge               Command
Factory Method           Composite            Interpreter
Prototype                Decorator            Iterator
Singleton                Facade               Mediator
                         Flyweight            Observer
                         Proxy                State
                                              Strategy
                                              Template Method
                                              Visitor

1. Singleton

   1) Private constructor to restrict instantiation of the class from other classes.
   2) Private static variable of the same class that is the only instance of the class.
   3) Public static method that returns the instance of the class, this is the global  
      access point for outer world to get the instance of the singleton class.
   4) Used for logging, driver objects, caching and thread pool.
      Used in other design patterns like Abstract Factory, Builder, Prototype, Facade etc.
      Used in core java classes also. for example java.lang.Runtime, java.awt.Desktop.
   5) Introduced 2 best versions (Bill Pugh, enum)
   

   Bill Pugh Singleton
   enum Singleton
2. Factory Method

   1) Used when we have a super class with multiple subclasses and based on input, 
      we need to return one of the sub-class.
   2) Benefit: Removed instantiation of actual implementation classes from client code, 
               making it more robust, less coupled and easy to extend.
   3) Example in JDK: java.util.Calendar, ResourceBundle, NumberFormat
   

   Computer Simulation
3. Builder

   1) Used too many arguments to pass from client program.
   2) Example in JDK: java.lang.StringBuilder#append()
   

   Builder Example
4. Abstract Factory

   1) It's "factory of factory" and can be easily extended to accommodate more products.
   2) It's robust and avoid conditional logic of Factory pattern.
   3) I used Singleton and Builder pattern also in this example.
   

   Abstract Factory Example
5. Prototype

   *Basic idea*: Situation (Object creation is a costly affair, lots of time and resources,
                            and a similar object already existing).
                 Action (clone the existing object and change it)
   

   Prototype Example
6. Observer

   *Basic idea*: Define a one-to-many dependency between objects so that when one object changes state, 
                 all its dependents are notified and updated automatically. (GoF)
   

   Observer Example
7. Strategy