cosc 1p02

COSC 1P02

Intro. to Computer Science 8.1

Cosc 1P02

Only those who attempt the absurd can achieve the impossible.

COSC 1P02


Classes Classes

entities within a system imported

e.g. Turtle of same project

write more than one class Instances

objects created via new Instance variables

object’s memory state

Methods things objects can do behaviour

System execution via interacting objects

COSC 1P02


Methods

Method calls local

methodname ( paramlist ) of another object

object . methodname ( paramlist ) this . methodname ( paramlist )

Behaviour depends on state e.g. effect of forward depends on pen state and

direction methods refer to instance variables

can base actions on values of instance variables loops, decisions

Role of constructor well-defined initial state

COSC 1P02


Data Abstraction

Procedural abstraction is limited Abstraction based on set of values and operations upon

them i.e. a class

many possible instances each can perform methods each object’s behaviour depends on its state

Don’t need to know how state recorded or how operations implemented to use object

Example: Turtle

COSC 1P02


Case Study: Payroll System

Problem weekly payroll

Analysis & design entities

employee details about employee

payroll system itself report generation

basic algorithm merger of report generation and process to EOF

encapsulate details about employee in Employee class pay rate technique to compute net pay

revised report algorithm calculatePay

employee data

COSC 1P02


Implementation—Employee Class

Class not a main class

no method main imports

Execution only when methods (constructor) called

Attributes/instance variables employee number, name pay rate year-to-date values

COSC 1P02


Implementation—Employee Class. Algorithms/methods

constructor read data of one employee

calculatePay references instance variables references local methods

Memory model instance variables parameters local variables

COSC 1P02


Implementation—Employee Class. Reading employee records

constructor initial state stream parameter

end of file process to EOF pattern

Garbage collection multiple Employee objects garbage

Writing employee records who knows employee data? stream parameter consistent with constructor

COSC 1P02


Implementation—Payroll Class

Streams Processing algorithm

process to EOF report generation prompting detail line

Testing & debugging test scripts employee data file

boundary conditions

COSC 1P02


Information Hiding Class design

Cohesion Selective disclosure

only what client needs to know hide representation and implementation

Accessor and updater methods instance variables

private if accessible, provide accessor method

Java convention: getxxx if updateable, provide updater method

Java convention setxxx pseudo attributes

Methods private vs public

COSC 1P02


The end

cosc 1p02

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