Abstraction Functions and Representation Invariants - PowerPoint Presentation

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Abstraction Functions and Representation Invariants

Paul Ammann

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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

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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.

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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)

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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

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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