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Motivation Sometimes it's important to have only one instance for a class. For example, in a system there should be only one window manager (or only a file system or only a print spooler). Usually singletons are used for centralized management of internal or external resources and they provide a global point of access to themselves. The singleton pattern is one of the simplest design patterns: i..

Motivation Also known as Virtual Constructor, the Factory Method is related to the idea on which libraries work: a library uses abstract classes for defining and maintaining relations between objects. One type of responsibility is creating such objects. The library knows when an object needs to be created, but not what kind of object it should create, this being specific to the application using..

Motivation Modularization is a big issue in today's programming. Programmers all over the world are trying to avoid the idea of adding code to existing classes in order to make them support encapsulating more general information. Take the case of a information manager which manages phone number. Phone numbers have a particular rule on which they get generated depending on areas and countries. If..

Motivation The Factory Design Pattern is probably the most used design pattern in modern programming languages like Java and C#. It comes in different variants and implementations. If you are searching for it, most likely, you'll find references about the GoF patterns: Factory Method and Abstract Factory. In this article we'll describe a flavor of factory pattern commonly used nowdays. You can a..

Motivation Extending an objects functionality can be done statically (at compile time) by using inheritance however it might be necessary to extend an objects functionality dynamically (at runtime) as an object is used. Consider the typical example of a graphical window. To extend the functionality of the graphical window for example by adding a frame to the window, would require extending the w..

Motivation We can not talk about Object Oriented Programming without considering the state of the objects. After all object oriented programming is about objects and their interaction. The cases when certain objects need to be informed about the changes occured in other objects are frequent. To have a good design means to decouple as much as possible and to reduce the dependencies. The Observer ..

Strategy Motivation There are common situations when classes differ only in their behavior. For this cases is a good idea to isolate the algorithms in separate classes in order to have the ability to select different algorithms at runtime. Intent Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that us..