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) Observers (report about state) This lecture is about state only

What is “State”? Key notion Definition: Java example: 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.

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)

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

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