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Abstraction Functions and Representation Invariants

Abstraction Functions and Representation Invariants. CS/SWE 332 Paul Ammann. Data Abstraction. Abstract State (Client State) Representation State (Internal State) Methods (behavior) Constructors (create objects) Producers (return immutable object) Mutators (change state)

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Abstraction Functions and Representation Invariants

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  1. Abstraction Functions and Representation Invariants CS/SWE 332 Paul Ammann

  2. Data Abstraction • Abstract State (Client State) • Representation State (Internal State) • Methods (behavior) • Constructors (create objects) • Producers (return immutable object) • Mutators (change state) • Observers (report about state) • This lecture is about state only

  3. What is “State”? • Key notion • Definition: • 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: • Variables: • List list; • int x; • Possible States: • list = [], x = 5 • list = null; x = 0 • list = [“cat”, “dog”]; x = -8 • list = [“cat”, 1, null]; x = 0 • etc.

  4. Motivation • Why hide implementation from client? • Makes reimplementation (maintenance) possible! • Protects implementation from client • Lecture covers two key notions • Abstraction function • Implemented by toString() in Java • Representation Invariant • Not standard in Java, but fits assertion mechanisms • Is available in C# (VisualStudio Code Contracts)

  5. Abstraction Function • Abstract state • What the client sees • Examples given in class • Representation state • What the implementation manipulates • Examples given in class • Abstraction function simply maps representation states to abstract states • Required for any implementation • Difference here – we’re documenting it! • Code examples: in-class exercises

  6. Rep Invariant • Rep invariant captures constraints on implementation variables • “Why my code works” • English descriptions are fine • But needs to be coded to be effective • Can check in Junit tests • Code examples: in-class exercises

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