Note51

0.0 by Claude

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Chapter 8: Specifying Operations

Overview

This chapter covers techniques for specifying operations in object-oriented systems analysis and design, focusing on Activity Diagrams and Object Constraint Language (OCL).

1.0 The Role of Operation Specifications

Operation specifications serve three critical purposes:

1.1 From Analysis Perspective

  • Ensure users’ needs are understood - Validates that requirements are correctly captured

1.2 From Design Perspective

  • Guide programmers to appropriate implementation - Provides clear direction for coding methods

1.3 From Test Perspective

  • Verify method functionality - Ensures the method performs as originally intended

2.0 Key Components of Operation Specifications

Every operation specification should include:

  • Intent or purpose of the operation
  • Operation signature including return type
  • Description of the logic
  • Other operations called (in same object or other objects)

3.0 Techniques for Describing Operation Logic

3.1 Algorithmic Techniques

  • Structure English (not covered)
  • Pseudo code (not covered)
  • Activity Diagrams ✓ (covered in this unit)

3.2 Non-algorithmic Techniques

  • Pre- and Post-Condition Pairs (not covered)
  • Decision Tables (not covered)
  • Object Constraint Language (OCL) ✓ (covered in this unit)

4.0 Activity Diagrams

Activity diagrams show the flow of control in an operation, including:

  • Sequential flow - Activities performed in order
  • Selection (branching) - Different paths based on conditions
  • Iteration (loops) - Repeated activities

Example: prepareBonusList() Operation

The diagram demonstrates:

  • Initial calculation of bonus
  • Decision points with conditions:
    • [bonus < £25] → Create warning letter
    • [bonus > £250] → Add to ‘star’ list
    • [bonus >= £25 AND bonus <= £250] → Add to regular list
  • Loop structure using [more StaffMembers] condition
  • Final formatting of the list

5.0 Object Constraint Language (OCL)

5.1 Definition

OCL is a formal language used to express constraints (约束) on UML models that:

  • Remains easy to read and write
  • Specifies constraints that cannot be expressed directly in class diagrams
  • Describes additional constraints about objects in the model

5.2 OCL Structure

OCL constraints consist of three main components:

5.2.1 Context

  • Defines the domain within which the expression is valid
  • Usually an instance of a type (e.g., object in class diagram)
  • Can also be a link (association instance)
  • Uses keyword context followed by the class name

5.2.2 Property

The property can be:

  • An attribute of the class
  • An association-end (role name)
  • A query operation
  • Referenced using self keyword

5.2.3 Operation

OCL operations applied to properties include:

  • Arithmetic operators: *, +, -, /
  • Comparison operators: >, <, =
  • Set operators: size(), isEmpty(), forAll(), select(), sum(), count()

5.3 OCL Syntax Rules

Important: When dealing with collections (sets), use the arrow operator →:

  • Example: self.employee → select(age < 60)
  • The → indicates operations on collections

5.4 Common OCL Examples

Example 1: Simple Attribute Constraint

context Company
inv: self.numberOfEmployees > 50

Interpretation: A company must have more than 50 employees

Example 2: Age Constraint

context Person
inv: self.age > 18

Interpretation: Person’s age must be more than 18 years old

Example 3: Navigation with Association

context Company
inv: self.employee.sex = male

Interpretation: Only male employees in the company (but employer can be any gender)

Example 4: Conditional Constraint

context Person
inv: self.husband → notEmpty() implies self.husband.sex = male

Interpretation: If a person has a husband, the husband must be male

Example 5: Set Operations

context Company
inv: self.employee → select(age < 60)

Interpretation: Select all employees under 60 years old

context Company
inv: self.employer → size() < 3

Interpretation: A company must have fewer than 3 employers

6.0 Applications of OCL

OCL can be used for:

  • Specifying invariants on classes and types in the class model
  • Specifying type invariants for stereotypes
  • Describing pre- and post-conditions on operations and methods
  • Specifying constraints on operations

7.0 Benefits of Using OCL

  • Code generation - Modeling tools can generate code based on OCL expressions
  • Consistency checking - Tools can verify the consistency of UML models

8.0 Practice Exercises

Write OCL constraints for the following requirements:

  1. One company has at least one employee

    context Company
inv: self.employee → size() >= 1

  2. A person who is married must be more than 18 years old

    context Person
inv: self.isMarried = true implies self.age > 18

  3. The salary of an employee cannot be greater than the salary of his/her supervisor

    context Person
inv: self.salary <= self.supervisor.salary

  4. If an employer’s age is above 60 years old, the minimum salary must be 5000

    context Person
inv: self.age > 60 implies self.salary >= 5000

Key Takeaways for Exams

  • Operation specifications serve analysis, design, and testing purposes
  • Activity diagrams show algorithmic flow with selection and iteration
  • OCL provides formal constraint specification for UML models
  • Remember the context-property-operation structure of OCL
  • Use → for set operations in OCL
  • The self keyword refers to the current instance in context
  • implies is used for conditional constraints (if-then logic)

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