Classes: Identifying Classes and Candidates for Classes, Attributes, and Services

Definition

A class is a user-defined data type that represents a group of objects sharing common structure and behavior.
Attributes are the properties or data fields that describe the state of a class.
Services are the actions or behaviors that a class provides, usually implemented as methods.

Identifying classes means analyzing a problem domain to determine which entities should be modeled as classes, which properties should become attributes, and which responsibilities should become services.

Main Content

1. Identifying Classes

What a class represents

• A class usually represents a real-world object, person, place, event, role, or conceptual entity.
• Examples include Student, BankAccount, Car, Invoice, Library, and Order.
• A class should have a clear purpose in the system and a well-defined responsibility.
• A good class is usually cohesive, meaning its attributes and services all relate to one central idea.

How to identify candidate classes

• A common technique is to read the problem statement carefully and look for nouns and noun phrases.
• Nouns may indicate possible classes, but not every noun should be turned into a class.
• Ask questions such as:
  • Does this noun represent a meaningful entity in the problem domain?
  • Does it have its own identity?
  • Does it need its own data and behavior?
  • Will it be useful in the software model?
• Example: In a school management system, words like Student, Teacher, Course, Classroom, and Exam are likely candidates for classes.
• However, words like name, address, or date are usually attributes, not classes.

Choosing the best candidate classes

• Candidate classes should be:
  • Relevant to the domain
  • Stable over time
  • Distinct in responsibility
  • Useful for abstraction
• Avoid creating classes for:
  • Purely descriptive words
  • Temporary values
  • Actions that should be services
  • Overly detailed internal implementation objects unless necessary
• For example, in an online shopping system:
  • Customer, Product, Cart, Order, and Payment are strong class candidates.
  • Quantity may be an attribute of OrderItem, not a standalone class in many cases.

2. Identifying Attributes

What attributes are

• Attributes are the data members or properties that store the state of a class.
• They describe what an object is like rather than what it does.
• Example:
  • Student may have attributes such as studentId, name, age, department, and email.
  • BankAccount may have attributes such as accountNumber, balance, and accountType.

How to identify candidate attributes

• Look for descriptive words in the problem statement, often adjectives or data values associated with a class.
• Ask:
  • Does this property describe the object?
  • Should this value be stored inside the object?
  • Is it essential to the object’s identity or state?
• Example: For Car, attributes may include model, color, speed, fuelLevel, and registrationNumber.
• For Book, attributes may include title, author, ISBN, publisher, and price.

Characteristics of good attributes

• Attributes should be:
  • Meaningful
  • Required for the class’s purpose
  • Specific to the class
  • Not redundant
• Good design avoids storing derived data unnecessarily.
• For example, in a Rectangle class:
  • length and width are good attributes.
  • area is usually better as a computed property or service result, not stored permanently, because it can be calculated from length and width.
• This reduces inconsistency and improves maintainability.

Common attribute types

• Simple attributes: single values such as name, age, salary
• Composite attributes: made of smaller parts, such as address containing street, city, and postal code
• Derived attributes: computed from other attributes, such as age derived from dateOfBirth
• Optional attributes: may or may not have a value, such as middleName
• Example:
  • In a Customer class:
  • customerId may be a simple attribute
  • address may be composite
  • age may be derived
  • faxNumber may be optional

3. Identifying Services

What services are

• Services are the actions, operations, or behaviors that a class can perform.
• They represent what an object does.
• Services are usually implemented as methods.
• Example:
  • BankAccount may have services such as deposit(), withdraw(), and checkBalance().
  • Student may have services such as enrollCourse(), updateProfile(), and calculateGPA().

How to identify candidate services

• Look for verbs and verb phrases in the problem statement.
• Ask:
  • What actions are required?
  • Which class should be responsible for each action?
  • Does the action belong to the object itself?
• Example:
  • In a library system:
  • borrowBook(), returnBook(), and reserveBook() are services.
  • In an e-commerce system:
  • addToCart(), removeFromCart(), placeOrder(), and processPayment() are services.

Types of services

• Accessors: methods that retrieve information, such as getName() or getBalance()
• Mutators: methods that change state, such as setAddress() or deposit()
• Business services: methods that perform domain-specific operations, such as calculateDiscount() or issueFine()
• Control services: methods that coordinate tasks, such as login(), submitForm(), or generateReport()
• Example:
  • In a Employee class:
  • getSalary() is an accessor
  • setDepartment() is a mutator
  • calculateTax() is a business service

Relationship between attributes and services

• Services often operate on attributes.
• A class should protect its attributes by allowing controlled access through services.
• This is a key idea in encapsulation.
• Example:
  • BankAccount should not allow direct external modification of balance.
  • Instead, deposit() and withdraw() should change the balance under controlled rules.
• This ensures valid state and prevents errors such as negative balances or invalid updates.

Working / Process

1. Study the problem statement carefully

• Read the domain description and identify the important nouns, noun phrases, and verbs.
• Understand the real-world scenario before designing classes.
• Separate meaningful domain concepts from irrelevant words.

2. List candidate classes, attributes, and services

• Mark nouns as possible classes or attributes.
• Mark verbs as possible services.
• Organize the extracted items into groups.
• Example:
  • Problem: “A student enrolls in a course and pays fees.”
  • Possible classes: Student, Course, Fee
  • Possible attributes: studentId, courseCode, amount
  • Possible services: enroll(), payFees()

3. Evaluate and refine the model

• Check whether each class has a clear responsibility.
• Remove unnecessary or duplicate candidates.
• Decide whether a noun is better as a class, attribute, or service.
• Make sure the final design supports encapsulation and abstraction.
• Example:
  • Address may be a class in a shipping system, but only an attribute in a simple contact system.
• Finalize the class diagram or object model with the selected classes, attributes, and services.

Advantages / Applications

• Helps in building software that reflects the real-world domain accurately
• Improves encapsulation by hiding internal data and exposing controlled services
• Makes systems easier to understand, maintain, test, and extend
• Supports reuse by creating well-defined classes that can be used in multiple parts of a program
• Widely applied in system analysis, object-oriented programming, UML class diagram design, and software architecture

Summary

• Classes are identified by analyzing important entities in the problem domain.
• Attributes describe the state of a class, and services describe its behavior.
• Careful selection of classes, attributes, and services leads to clean and effective object-oriented design.
• Important terms to remember: class, object, attribute, service, candidate class, encapsulation, abstraction