Design patterns with real time example

Question

I want to learn Design patterns with real time example. So can any one suggest where I can start.

Answer 1

Note: Adding brief definition with real life and Java API examples.

Creational

How do you want to create objects?

Prototype : A fully initialized instance to be copied or cloned
Example : initial status of chess game

• java.lang.Object#clone()

Builder - Separates the construction of a complex object from its representation so that the same construction process can create different representations
Example : Kitchen is a Factory, Chef is a Builder where waiter tell to chef "pizza with cheese, onion". Here chef exposed attributes but hidden implementation.

• java.lang.StringBuilder

Singleton - A class of which only a single instance can exist
Example : President of a country

• java.lang.Runtime#getRuntime()

Factory Method - Creates an instance of several derived classes.
Example : In an organisation HR works as factory method. Here development team request type of resource need to HR. Based on request type, HR provide resource to Development team.

• java.util.Calendar#getInstance()

Abstract Factory - Creates an instance of several families of classes
Example : HP, Samsung and Dell laptops are uses Intel and AMD processor.

• javax.xml.parsers.DocumentBuilderFactory#newInstance()

Factory Method vs Abstract Factory

Structural

This design patterns is all about Class and Object composition i.e. How do you want structure the software component.

Proxy - An object representing another object
Example : check book leaf, credit card, debit card are proxy for Money and a customer representative to order a product.

• java.rmi.*, the whole API actually.

Composite - Gives an unified interface to a leaf and composite.
Example : File System in Operating Systems, Directories are composite and files are leaves. System call Open is single interface for both composite and leaf.

Decorator - Gives additional feature to leaf, while giving unified interface.
Example : 1) Adding discounts on an order 2) gun is a deadly weapon on it's own. But you can apply certain "decorations" to make it more accurate, silent and devastating.

• All subclasses of java.io.InputStream, OutputStream, Reader and Writer have a constructor taking an instance of same type.

Facade - Simplifies multiple complex components with single interface
Example : Control Panel

• javax.faces.context.ExternalContext, which internally uses ServletContext, HttpSession, HttpServletRequest, HttpServletResponse, et

Adapter - Provides different interface for an interface.
Example : Power Adapters

• java.util.Arrays#asList()

Flyweight - A fine-grained instance used for efficient sharing
Example : Dial tone

• java.lang.Integer#valueOf(int) (also on Boolean, Byte, Character, Short and Long)

Behavioral

This design patterns is all about Class's objects communication i.e How do you want a behavior in software?

Chain of Responsibility - A way of passing a request between a chain of objects
Example : Loan or Leave approval process

• javax.servlet.Filter#doFilter()

Iterator - Sequentially access the elements of a collection
Example : Next/Previous buttons on TV

• All implementations of java.util.Iterator & java.util.Enumeration

State - Alter an object's behavior when its state changes
Example : A fan wall control

Observer - A way of notifying change to a number of classes
Example : Bidding or auction

• Publish/Subscribe JMS API

Visitor - Defines a new operation to a class without change
Example : Taxi

Template - Defer the exact steps of an algorithm to a subclass
Example : A blue print

• All non-abstract methods of java.io.InputStream, java.io.OutputStream, java.io.Reader and java.io.Writer.
• All non-abstract methods of java.util.AbstractList, java.util.AbstractSet and java.util.AbstractMap.
• javax.servlet.http.HttpServlet, all the doXXX() methods by default sends a HTTP 405 "Method Not Allowed" error to the response. You're free to implement none or any of them.
  • JMSTemplate HibernateTemplate and JdbcTemplate in Spring

Command - Encapsulate a command request as an object
Example : The "Guest Check" at a diner is an example of a Command pattern. The waiter or waitress takes an order or command from a customer and encapsulates that order by writing it on the check. The order is then queued for a short order cook. Note that the pad of "checks" used by each waiter is not dependent on the menu, and therefore they can support commands to cook many different items.

• All implementations of java.lang.Runnable

Memento - Capture and restore an object's internal state
Example : save the state in a game & Undo/Redo operation in Windows

• All implementations of java.io.Serializable

Mediator - Defines simplified communication between classes
Example : Air Traffic Controller(ATC)

Strategy - A Strategy defines a set of algorithms that can be used interchangeably.
Example : Modes of transportation

• java.util.Comparator#compare(), executed by among others Collections#sort().
• javax.servlet.http.HttpServlet, the service() and all doXXX() methods take HttpServletRequest and HttpServletResponse and the implementor has to process them (and not to get hold of them as instance variables!).
• javax.servlet.Filter#doFilter()

Courtesy.
Design Patterns