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# What is the Singleton Pattern?

Article Details
• Written By: G.W. Poulos
• Edited By: Daniel Lindley
2003-2018
Conjecture Corporation
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The mathematical concept of a singleton is a number set that contains only one single number. Software engineers and programmers have translated this concept into something called a singleton pattern, in which a specific set of information is accessed by various programs only once and always contains the same set of instructions or information. The singleton pattern is used in programming languages such as Java to define a global variable, a piece of information used across an entire system that remains the same and needs to be defined only once instead of many times.

In most programming languages, before a program can run, it requires additional pieces of information about its environment. Most of the information specific to the program is contained within its own program code; however, a program often also requires variable information about things such as the version of the operating system in which it is running, the hardware it is being run on, and other specific information not unique to the program itself. Usually this information is contained in something called a class or object, which is an independent piece of programming code often contained in a file separate from the program so that it can be available to many different programs simultaneously.

If a programming language does not use the singleton pattern, each program that requires information contained in an object accesses it by opening the object with a set of parameters that specify what information in the object the program needs. Therefore, each time a different program opens the object, a different instance of the object is created in the computer’s memory. Each instance of the object can contain different information, depending on the specific needs of each program that opens it.

When a programming language that uses the singleton pattern, such as Java, begins running a program, the program first searches for the objects from which it needs information. If an object is not already open, it opens the object without any specific parameters, making all the information contained in the object available to all programs. If the object is already open, it simply reads the information in the object and continues. In this way, the information contained in an object using a singleton pattern is always the same, so there is no need to open many different versions of the object simultaneously. This saves space in the computer's memory and can also increase speed because other programs can access the information without having to spend time creating a new version of the object.