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Chapter 2
Classes and Objects
CMPU-101: Problem-Solving and Abstraction Marc Smith (sect. 52) Vassar College Fall 2006 2
Object-Oriented Programming
Idea: Programs should closely model the real world. Claim: Such programs are easier to write & maintain. Must be able to translate between:
- Real World! Model! classes
- Information! Data! fields
- Behavior! Interrogation/! methods Manipulation
Example: A Bank
Object-Oriented Design:
- Study the problem
- Identify the major classes of objects involved Relevant classes:
- Account - place where money is kept
- Person - who the money belongs to
- Employee - person who facilitates bank transactions
Wait--we were speaking of Objects…
What is a Class?
- Class
- a set of like objects, or
- a template for creating objects
- Objects
- instances of classes,
- created by instantiation. An object fills in the open slots of the template.
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Unified Modeling Language (UML)
ClassName data methods A visual language for describing and designing classes and class relationships Two types of class relationships: is-a, has-a 6
Class Account
Account owner balance accountNumber Account(…) — constructor getBalance deposit withdraw addInterest
A constructor is a special method
used to instantiate a new object
Account owner balance accountNumber Account(…) getBalance deposit withdraw addInterest
owner is of type Person ,
so Account has-a Person
has-a relationship between classes
is-a relationship between classes
Person name ssn dateOfBirth address Person(…) — constructor setName setAddress ! Employee dateHired monthlySalary
An Employee is-a Person
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A Given Class Allows Only Certain Methods
Account allows getBalance and withdraw. PrintStream allows print and println. 14
Another example
Student name ssn dateOfBirth gpa Sometimes in a UML diagram, we show only the fields of the class, depending on our purpose
A Section of Students
Section roster instructor room timeSlot Section(…) — constructor addStudent(Student) removeStudent(Student) contains(Student) intersect(Section) In this case, roster is a collection of Student s - - a one-to-many, has-a relationship! (one Section has many Student s)
Naming conventions: Identifiers
Composed of: letters digits underscore ("_") Must begin with a letter. Examples: timeToLive bistro U MAXIMUM_VALUE
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Naming conventions: Capitalization
Classes — all words are capitalized Account BankAccount ClosedCurve Matrix3By Instances — the first word is not capitalized cs numberOfStudents averageAge taxRate 18
Categories of Identifiers
Reserved words class, public, private, static, … (see Appendix Two) Names defined by other programmers in the API System.out, println, PrintStream Names we select
Brainstorm!
Given a problem, we must answer the questions: What are the classes? What are the methods? What are the instance variables? Written assignment: choose from Chapter 2 Exercises
