Singleton pattern

Sometimes it’s important for classes to have exactly one instance.

For example: Although there can be many printers in a system, there should be only one printer spooler. There should be only one file system and one window manager.

In such scenario Singleton Design Pattern comes to help.

Singleton pattern ensures a class only has one instance, and provides a global point of access to it.

Implementing Singleton

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To implement singleton we need do following two things.

1.    Ensure a class only has one instance:
 This can be achieved by making constructor of a class private ( or protected: In case you want to extend this class.).

2.    Provides a global point of access to instance:

This can be achieved by keeping static reference to unique instance and having method to get that instance.

Design Patterns

A brief history of design patterns:

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Christopher Alexander:

Design patterns were first described by architect Christopher Alexander in his book A Pattern Language: Towns, Buildings, Construction (Oxford University Press, 1977).

Christopher Alexander says,

"Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice"

Gang of Four (or GoF):

The concept Christopher introduced and called patterns -- abstracting solutions to recurring design problems – caught the attention of researchers in other fields, especially those developing object-oriented software in the mid-to-late 1980s.