Protocols CS 4311 Wirfs Brock et al., Designing Object-Oriented Software, Prentice Hall, 1990. (Chapter 8) Meyer, B., Applying design by contract, Computer, 25(10):40-51, October 1992. 1 1

Outline Protocols: what and why? Documenting protocols Process and guidelines 2 2

Review Responsibility? Contract?

Definitions Signature: method name, input and output parameters and their types, and return type Protocol: set of signatures for methods to be implemented

Why Write Protocols? Define an interface to a class. (Why?)

Protocol Structure Signature Purpose Pre-condition Post-condition Method name Type of return value Type of input and output parameters Description of input, output, input-output parameters Purpose Pre-condition Post-condition

Notation Signature, a.k.a. syntactic interface Language neutral declaration, e.g., UML and CRC draw(in g: Graphics): void Language-specific declaration, e.g., Java and C++ void draw(Graphics g) Strength and weakness?

Notation (Cont.) Behavior, a.k.a. semantic interface Informal description or stylized texts Example: Given a non-empty list of integers, determine if it is sorted. Formal descriptions, e.g., OCL and Design-by-Contract notations pre: not c->isEmpty() post: c->incldues(result) and c->forAll(e: Integer | result >= e) Strength and weakness?

Specifying Classes Class: Drawing Superclasses: DisplayableObject Subclasses: None Class Diagram: see Figure 2-1 Collaborations Diagram: see Figure 3-5 Description: Represents the structure of the elements … Contracts 2. Maintain the elements in a drawing Know which elements are contained in the drawing addElement(DrawingElement) uses List Adds a drawing element to the front of the list of drawing elements. Pre: The element is not already contained in the list. Post: The element is the first element of the list.

Specifying Classes (Cont.) elementAt(Point) returns DrawingElement uses List, DrawingElement (3) Returns the drawing element at the given point. Pre: none Post: if boundary rectangle contains Point then returned is the first drawing element in List else returned is a null object Maintain the ordering between elements elementAfter(DrawingElement) returns DrawingElement uses List …

Pre-condition Capture the conditions that must be true before the method executes Describe the required state of the ADT or object before entering the function Written as an expression that is true or false May consist of expressions connected by logical operators (AND, OR) Use true when no pre-condition exists

Post-condition Must clearly state what is true when the method completes execution Describe the expected state upon exiting the function Should be strong enough so that only correct implementations will satisfy the condition

What’s the Value of Pre- and Post-conditions? (Pair) 2 minutes

Process for Writing Protocols For each class For each contract For each responsibility Specify complete protocol (set of signatures) to support the responsibility

Guidelines: Names Use single name for single conceptual operation, regardless of where it is found E.g., add vs. insert, remove vs. delete, contains vs. has vs. includes for collections Associate a single conceptual aspect with each method name

Guidelines: Generality Make protocols as generally useful as possible The more general a responsibility, the more messages needed to support it. E.g., size vs. enumerating elements for collections

Guidelines: Default Values Define reasonable defaults Provide most general message: user specifies all possible parameters Provide defaults for any parameter for which it is reasonable Let client specify subset of parameters

Example display() display(device) display(region) display(device, region) display(device, region, rule) display(device, region, rule, transform)

Exercise Make the indexOf method of String more general and reusable. /** Returns the index of the first occurrence of the given character. */ int indexOf(int ch) (Pair) 2 minutes

Guidelines: Pre and Post State as assertions not as actions. Why? E.g., int String.indexOf(int c) pre: none post: if c appears in this string then return the index of the first occurrence of c else return -1 then result is the index of the first occurrence of c else result is -1 20

Guidelines: Rework, rework, … Generate protocols for main responsibilities Protocols to public methods must be unambiguous since it is the interface with clients Protocols to private methods are notes to developer Common to discover holes in design at this point Repeat earlier phases of design

Example: ADT Stack Additions are restricted to one end identified as the top of the stack. Deletions are restricted to the top of the stack. Only the item at the top of the stack is accessible A stack is often referred to as a LIFO (Last In First Out) list.

In-class Identify operations for Stack (Pair) 2 minutes

In-class Specify protocols for isEmpty() and pop() (Pair) 5 minutes

In-class (Pairs) Consider a class, say Contact Manager, that manage a contacts list, where a contact contains contact information such as name, phone number, email address, etc. Identify responsibilities for Contact Manager Define protocols for adding and search operations/responsibilities. 25