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singleton^2
subject			fact

singleton^2	has definition A class for which only one instance should exist
	is a subtopic of 6.5 - The Singleton Pattern
	is a kind of class
class	can have instances
	contains all of the code that relates to its objects including code describing how the objects of the class are structured - i.e. the data stored in each object that implement the properties The procedures, called methods, that implement the behaviour of the objects
	contains data associated with each object
	declares a list of variables, called instance variables, corresponding to data that will be present in each instance
	has part class name
	has part class variable
	has part code
	has part constructor
	has part instance variable
	has part method
	has part variables
	is an abstract representation of all the instances of that class that may ever exist
	is probably useless if it has no responsibilities attached to it
	is the unit of data abstraction in an object-oriented program
	is divided up into methods
	is drawn as a box with the name of the class inside in a UML class diagram
	is needed in a domain model if you have to store or manipulate instances of it in order to implement a requirement
	represents several similar objects
	see also class^2
	should be created to hold a responsibility if the responsibility cannot be attributed to any of the existing classes
	should be named after things their instances represent in the real world
	should have a comment at the top describing the purpose of the class, how it should be used, its authors and its history of modification
	should not be named after the internals of a computer system such as 'Record', 'Table', 'Data', 'Structure', or 'Information'
data abstraction	groups the pieces of data that describe some entity, so that programmers can manipulate that data as a unit
	helps a programmer to cope with the complexity of data
	hides the details of data

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