Design Patterns

Creational Patterns

Abstract Factory

Provide an interface for creating families of related or dependent objects without specifying their concrete classes.

Builder

Separate the construction of a complex object from its representation so that the same construction process can create different representations.

Factory Method

Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.

Prototype

Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.

Singleton

Ensure a class only has one instance, and provide a global point of access to it.

Structural Patterns

Adapter

Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces.

Bridge

Decouple an abstraction from its implementation so that the two can vary independently.

Composite

Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.

Decorator

Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

Facade

Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.

Flyweight

Use sharing to support large numbers of fine-grained objects efficiently.

Proxy

Provide a surrogate or placeholder for another object to control access to it.

Behavioral Patterns

Chain of Responsibility

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.

Command

Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.

Interpreter

Given a language, define a represention for its grammar along with an interpreter that uses the representation to interpret sentences in the language.

Iterator

Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.

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.

Memento

Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later.

Observer

Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

State

Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.

Strategy

Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

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 changing the algorithm's structure.

Visitor

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.

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The S.O.L.I.D. Design Principles

The S.O.L.I.D. design principles are a collection of best practices for object-oriented design.



Single Responsibility Principle (SRP)

The principle of SRP is closely aligned with SoC. It states that every object should only have one
reason to change and a single focus of responsibility. By adhering to this principle, you avoid the
problem of monolithic class design that is the software equivalent of a Swiss army knife. By having concise objects, you again increase the readability and maintenance of a system.



Open-Closed Principle (OCP)

The OCP states that classes should be open for extension and closed for modification, in that you
should be able to add new features and extend a class without changing its internal behavior. The
principle strives to avoid breaking the existing class and other classes that depend on it, which
would create a ripple effect of bugs and errors throughout your application.



Liskov Substitution Principle (LSP)

The LSP dictates that you should be able to use any derived class in place of a parent class and have it behave in the same manner without modification. This principle is in line with OCP in that it ensures that a derived class does not affect the behavior of a parent class, or, put another way, derived classes must be substitutable for their base classes.



Interface Segregation Principle (ISP)

The ISP is all about splitting the methods of a contract into groups of responsibility and assigning interfaces to these groups to prevent a client from needing to implement one large interface and a host of methods that they do not use. The purpose behind this is so that classes wanting to use the same interfaces only need to implement a specific set of methods as opposed to a monolithic interface of methods.



Dependency Inversion Principle (DIP)

The DIP is all about isolating your classes from concrete implementations and having them depend on abstract classes or interfaces. It promotes the mantra of coding to an interface rather than an implementation, which increases flexibility within a system by ensuring you are not tightly coupled to one implementation.


Source: unknown

Agile Thinking

- Analyse root causes of problems
- Plot the course to a better future
- Plan quickly and thoroughly
- Resolve conflicts without compromise

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Object Oriented Analysis and Design

A critical fundamental ability in OOA/D is to skillfully assign responsibilities to software components.

Object-oriented analysis and design (OOAD) is a software engineering approach that models a system as a group of interacting objects. Each object represents some entity of interest in the system being modeled, and is characterized by its class, its state (data elements), and its behavior. Various models can be created to show the static structure, dynamic behavior, and run-time deployment of these collaborating objects. There are a number of different notations for representing these models, such as the Unified Modeling Language (UML).

What is Analysis and Design: Analysis emphasizes an investigation of the problem and requirement, rather than a soluation.For example, if a new computerized library information system is desired, how will it be used?

“Analysis” is a board term, best qualified, as in requirements analysis (as investigation of the requirements) or object analysis (an investigation of the domain objects).

Design emphasizes a conceptual solution that fulfills the requirements, rather than its implementation. For example, a description of a database schema and software objects. Ultimately, designs can be implemented. As with analysis, the term is best qualified, as in object design or database design.

Analysis and design have been summarized in the phase do the right thing (analysis), and do the thing right (design).

Object-oriented analysis (OOA) applies object-modeling techniques to analyze the functional requirements for a system. Object-oriented design (OOD) elaborates the analysis models to produce implementation specifications. OOA focuses on what the system does, OOD on how the system does it.

What is Object Oriented Analysis and Design? : During Object Oriented analysis, there is an emphasis on finding and describing the objects- or concepts- in the problem domain. For example, in the case of the library information system, some of the concepts include BOOK.Library and Patron.

During Object-Oriented design, there is an emphasis on defining software objects and how they collaborate to fulfill the requirements. For example, in the library system, a BOOK Software object may have a title attribute and a getChapter method.

Finally, during implementation or object-oriented programming, design objects are implemented, such as a Book class in java.

Domain Concept[BOOK] à BOOK(title)[Visualization of domain concept]->

public class Book{ private String title; public chapter getChapter(int){……..}  }  [representation in an OO programming Language]

Object-oriented analysis (OOA) looks at the problem domain, with the aim of producing a conceptual model of the information that exists in the area being analyzed. Analysis models do not consider any implementation constraints that might exist, such as concurrency, distribution, persistence, or how the system is to be built. Implementation constraints are dealt during object-oriented design (OOD). Analysis is done before the Design

Abstract

Abstract classes are classes that contain one or more abstract methods. An abstract method is a method that is declared, but contains no implementation. Abstract classes may not be instantiated, and require subclasses to provide implementations for the abstract methods. Let's look at an example of an abstract class, and an abstract method.
Suppose we were modeling the behavior of animals, by creating a class hierachy that started with a base class called Animal. Animals are capable of doing different things like flying, digging and walking, but there are some common operations as well like eating and sleeping. Some common operations are performed by all animals, but in a different way as well. When an operation is performed in a different way, it is a good candidate for an abstract method (forcing subclasses to provide a custom implementation). Let's look at a very primitive Animal base class, which defines an abstract method for making a sound (such as a dog barking, a cow mooing, or a pig oinking).

public abstract Animal
{
   public void eat(Food food)
   {
        // do something with food.... 
   }

   public void sleep(int hours)
   {
        try
 {
  // 1000 milliseconds * 60 seconds * 60 minutes * hours
  Thread.sleep ( 1000 * 60 * 60 * hours);
 }
 catch (InterruptedException ie) { /* ignore */ } 
   }

   public abstract void makeNoise();
}

Note that the abstract keyword is used to denote both an abstract method, and an abstract class. Now, any animal that wants to be instantiated (like a dog or cow) must implement the makeNoise method - otherwise it is impossible to create an instance of that class. Let's look at a Dog and Cow subclass that extends the Animal class.

public Dog extends Animal
{
   public void makeNoise() { System.out.println 

("Bark! Bark!"); }
}

public Cow extends Animal
{
   public void makeNoise() { System.out.println 

("Moo! Moo!"); }
}

Now you may be wondering why not declare an abstract class as an interface, and have the Dog and Cow implement the interface. Sure you could - but you'd also need to implement the eat and sleep methods. By using abstract classes, you can inherit the implementation of other (non-abstract) methods. You can't do that with interfaces - an interface cannot provide any method implementations.