Design Patterns — Singleton

Here’s my second post on Design Patterns. So, this post is going to throw some light on the Singleton Patterns.

In software engineering, the Singleton Pattern is a design pattern that restricts the instantiation of a class to one object

So one might wonder why would you need to implement a Singleton? There are many scenarios where we’d rather have just one instance of a particular class. For eg.

• The class which instantiates the DB connection
• It can be used as a replacement to a Global Variable

Here are some of the advantages of using a Singleton

• Only one instance of the class will exist
• This saves up on memory
• Allows one to share data amongst classes without actually passing around objects

Here are a few examples where you must have used a Singleton

Runtime.getRuntime().exec(new String[] {"echo", "Hello, world!"});

[UIApplication sharedApplication].delegate;

Here’s an implementation of the Singleton Pattern in Java

And here’a an implementation in Objective-C

So thats how you make use of Singletons. This is one Design Pattern you will use on a regular basis. Will update this article with more samples and issues related to thread safe creation of singletons.

EDIT : Added a slightly better way to manage Singletons in Objective C. A more detailed explanation and variations to this can be found in the next post.

Originally published at mariostallone.github.io on January 14, 2013.