Abstraction Functions and
Representation Invariants
CS/SWE 332
Paul Ammann
Data Abstraction
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Abstract State (Client State)
Representation State (Internal State)
Methods (behavior)
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Constructors (create objects)
Producers (return immutable object)
Mutators (change state)
Observers (report about state)
This lecture is about state only
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What is “State”?
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Key notion
Definition:
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A state is an assignment of values to variables.
Need to consider all possible values for each variable
State space is cross product of possible values for each individual variable
Java example:
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Variables:
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List list;
int x;
Possible States:
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list =
list =
list =
list =
etc.
[], x = 5
null; x = 0
[“cat”, “dog”]; x = -8
[“cat”, 1, null]; x = 0
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Motivation
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Why hide implementation from client?
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Makes reimplementation (maintenance) possible!
Protects implementation from client
Lecture covers two key notions
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Abstraction function
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Implemented by toString() in Java
Representation Invariant
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Not standard in Java, but fits assertion mechanisms
Is available in C# (VisualStudio Code Contracts)
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Abstraction Function
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Abstract state
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Representation state
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What the implementation manipulates
Examples given in class
Abstraction function simply maps representation
states to abstract states
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What the client sees
Examples given in class
Required for any implementation
Difference here – we’re documenting it!
Code examples: in-class exercises
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Rep Invariant
Rep invariant captures constraints on
implementation variables
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English descriptions are fine
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“Why my code works”
But needs to be coded to be effective
Can check in Junit tests
Code examples: in-class exercises
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