CSC 205Java Programming II

Defining & Implementing Classes

Topics

Objects and their responsibilities Define a class Implement and test a class Use a class

A Dummy Class

Write a class can be simple A dummy class with only one line of code

class Dummy {} A default constructor will be provided: it’s equivalent to

Dummy(){} You can create a Dummy object

Dummy d = new Dummy(); But such a class is not useful

You can do very little with object d

Responsibilities

An object should Maintain its own state: know something

A String object should know its length A Date class should know the year, month, day

associated with it Be able to provide certain services: do something

A String object should be able to return a portion of itself (a substring) as required

A JOptionPane object should be able to show itself as a input or message dialog as required

Classes & Their Members

A class defines what its instances (or objects) should know and should be able to do Data members: used to maintain state Method members: define behaviors

Example: the Throttle class

Throttletop:intposition:int

getFlow():doubleisOn():booleanshift(amt:int)shutOff()

Class name

Data members

Methods

Define A Class

Classes Entity classes (or problem domain class)

Usually don’t have a main method Main classes: classes with a main method

Only one such class in an application Possible members in a class

entity class main classconstructor V V* main method X Vmethods V V*variables V V*

Encapsulation

Information hiding Make instance variables private Provide public accessors (or getters) Provide public mutators (or setters) only when needed Example: when designing a Person class

Define the age variable as private Provide a public getAge() method What about a public setAge(int n) method? NO! Provide a updateAgeAnnualy() method

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Information Hiding

topposition

getFlow

isOn

shift

shuttOff

A Throttle object

Each object is identified by a unique (hash-)code

A handle

Private data membersPublic methodsServe as interfaces

The Client (or External) View

getFlow isOn shutOff

Writing Methods

An entity class needs to interact with other classes Message passing

Message: receiver + operation name + parameter list Return value is optional

Contracts between client and server objects Accessibility modifier Return type, maybe void Method name Parameter list, maybe none

Specification Format

Signature and description Parameters Returns Precondition Postcondition Throws

The Throttle Class

See source code files for the class and a test driver class under our class folder

How to test an entity class? Write a test driver class, that has a main method Testing is trying to break the application

Use extreme values Use values beyond the normal ranges

Creating a Throttle object with a negative size Trying to shift it to illegal positions