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attribute
subject			fact

attribute	can be identified by looking at the description of the system and searching for information that must be maintained about each class
	has definition A simple data item present in all the instances of a class
	is a subtopic of 9.2 - Principles Leading to Good Design
	is implemented as an instance variable in Java
	is used in the analysis and design stage before it is known how the attribute will be implemented
	is a kind of data abstraction
	is a kind of variable
	must not be added to a model unless it is relevant to the application - it will be needed to implement some requirement
	must not have a plural name
	represents the properties of an object
	should be a simple variable - typically an integer or string, or a one-to-one composition
	should be private
	should not have an implicit internal structure
	should not normally represent a variable number of things
	should only be accessed through public methods so that attributes are only given valid values and so that you can change the internal design of the class without affecting how users of the class interact with it
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
variable	can contain different classes of objects depending on the type of the variable
	can refer to a particular object, several different objects during the execution of a program, or no object at all
	has scope
	has type

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