Design Patterns | Object Oriented Design
By definition, Design Patterns are reusable solutions to commonly occuring problems(in the context of software
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By definition, Design Patterns are reusable solutions to commonly occuring problems(in the context of software design). Design patterns were started as best practices that were applied again and again to similar problems encountered in different contexts. They become popular after they were collected, in a formalized form, in the Gang Of Four book in 1994. Originally published with c++ and smaltalk code samples, design patterns are very popular in Java and C# can be applied in all object oriented languanges. In functional languages like Scala, certain patterns are not necesary anymore. Creational Design Patterns:Click to zoom 
Singleton – Ensure that only one instance of a class is created and Provide a global access point to the object.
When to Use Singleton pattern should be used when we must ensure that only one instance of a class is created and when the instance must be available through all the code. A special care should be taken in multi-threading environments when multiple threads must access the same resources through the same singleton object.
There are many common situations when singleton pattern is used: Click to zoom 
Factory(Simplified version of Factory Method) – Creates objects without exposing the instantiation logic to the client and Refers to the newly created object through a common interface.
When to Use
Factory pattern should be used when:
Along with singleton pattern the factory is one of the most used patterns. Almost any application has some factories. Here are a some examples in java: Click to zoom  Document Application Example 
Factory Method – Defines an interface for creating objects, but let subclasses to decide which class to instantiate and Refers to the newly created object through a common interface.
When to Use
Factory Method pattern should be used when:
– a framework delegate the creation of objects derived from a common superclass to the factory – the base factory class does not know what concrete classes will be required to create – delegates to its subclasses the creation of concrete objects – factory subclasses subclasses are aware of the concrete classes that must be instantiated Factory method pattern, compared to Factory pattern replace the factory with an abstract class and a set of concrete factories subclasses. The subclasses are responsible for creating concrete product objects; for factory method is possible adding new product classes without changing the abstract factory. The same result can be achieved for simplified factory pattern if reflection is used.
Along with singleton pattern the factories are the most used patterns. Almost any application has some factories. Here are a some examples: Click to zoom  Look & Feel Example 
Abstract Factory – Offers the interface for creating a family of related objects, without explicitly specifying their classes.
When to Use
Abstract Factory should be used when:
Examples of abstract factories:
Click to zoom  Text Converter Example 
Builder – Defines an instance for creating an object but letting subclasses decide which class to instantiate and Allows a finer control over the construction process.
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Prototype – Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.
Click to zoom  Database Example 
Object Pool – reuses and shares objects that are expensive to create..
When to Use Basically, we’ll use an object pool whenever there are several clients who needs the same stateless resource which is expensive to create.
The most common situations when object pool pattern is used: Behavioral Design Patterns:Click to zoom 
Chain of Responsibiliy – It avoids attaching the sender of a request to its receiver, giving this way other objects the possibility of handling the request too.
– The objects become parts of a chain and the request is sent from one object to another across the chain until one of the objects will handle it.
Click to zoom  Restaurant Example 
Command – Encapsulate a request in an object, Allows the parameterization of clients with different requests and Allows saving the requests in a queue.
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Interpreter – Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language / Map a domain to a language, the language to a grammar, and the grammar to a hierarchical object-oriented design
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Iterator – Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
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Mediator – 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.
Click to zoom  News Publisher Example 
Observer – Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
Click to zoom  Robot Example 
Strategy – Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.
Click to zoom  Travel Example 
Template Method – 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 letting them to change the algorithm’s structure.
Click to zoom  Customers Example 
Visitor – Represents 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.
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Null Object – Provide an object as a surrogate for the lack of an object of a given type. / The Null Object Pattern provides intelligent do nothing behavior, hiding the details from its collaborators.
Structural Design Patterns:Click to zoom 
Adapter – Convert the interface of a class into another interface clients expect. / Adapter lets classes work together, that could not otherwise because of incompatible interfaces.
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Bridge – Compose objects into tree structures to represent part-whole hierarchies. / Composite lets clients treat individual objects and compositions of objects uniformly.
Click to zoom  Shapes Example 
Composite – Compose objects into tree structures to represent part-whole hierarchies. / Composite lets clients treat individual objects and compositions of objects uniformly.
Click to zoom  Wargame Example 
Flyweight – use sharing to support a large number of objects that have part of their internal state in common where the other part of state can vary.
Click to zoom  Calculator Example 
Memento – capture the internal state of an object without violating encapsulation and thus providing a mean for restoring the object into initial state when needed.
Click to zoom  Image Viewer 
Proxy – provide a “Placeholder” for an object to control references to it.
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