Transcript Protocol - Computer Science

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
Outline
Protocols: what and why?
 Documenting protocols
 Process and guidelines

2
Review

Responsibility?

Contract?
3
Definitions


Signature: method name, input and output
parameters and their types, and return type
Protocol: set of signatures for methods to be
implemented
4
Why Write Protocols?

Define an interface to a class.

(Why?)
5
Protocol Structure

Signature
 Method
name
 Type of return value
 Type of input and output parameters
 Description of input, output, input-output
parameters



Purpose
Pre-condition
Post-condition
6
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?
7
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-byContract notations
pre: not c->isEmpty()
post: c->incldues(result) and c->forAll(e: Integer | result >= e)
 Strength
and weakness?
8
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.
9
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 such drawing element in List
else returned is a null object
Maintain the ordering between elements
elementAfter(DrawingElement) returns DrawingElement
uses List
…
10
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
11
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
12
What’s the Value of Pre- and Postconditions?

(Pair) 2 minutes
13
Process for Writing Protocols
For each class
For each contract
For each responsibility
Specify complete protocol (set of signatures)
to support the responsibility
14
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
15
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
16
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
17
Example






display()
display(device)
display(region)
display(device, region)
display(device, region, rule)
display(device, region, rule, transform)
18
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
19
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
else return -1
post: if c appears in this string
then result is the index of the first occurrence of c
else result is -1
post: result is the index of the first occurrence if c appears in this
string; otherwise it 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
21
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.
22
In-class


Identify operations for Stack
(Pair) 2 minutes
23
In-class


Specify protocols for isEmpty() and pop()
(Pair) 5 minutes
24
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