School of Computer Science & Information Technology

School of Computer Science & Information Technology

G6DICP - Lecture 11G6DICP - Lecture 11

Classes & ObjectsClasses & Objects

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Classes and OO ProgrammingClasses and OO Programming An OO program consists of at least one (usually An OO program consists of at least one (usually

many more) inter-dependent classes.many more) inter-dependent classes. A class is a user-defined data type that describes A class is a user-defined data type that describes

the properties and capabilities of the real-world the properties and capabilities of the real-world objects with which the program has to deal.objects with which the program has to deal.

A class is a generic type of “thing”, whereas a A class is a generic type of “thing”, whereas a specific “thing” (an instance of the class) is an specific “thing” (an instance of the class) is an Object.Object.

A class definition consists of both the properties A class definition consists of both the properties (ie data) and the capabilities (ie program code) (ie data) and the capabilities (ie program code) of that class.of that class.

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An AnalogyAn Analogy Consider the class Pen Consider the class Pen

ie pens in general, not any ie pens in general, not any one pen in particular.one pen in particular.

Properties (variables)Properties (variables) ColourColour ThicknessThickness

Capabilities (methods)Capabilities (methods) Remove lidRemove lid Draw lineDraw line Draw circleDraw circle Replace lidReplace lid

Instances of class Pen Instances of class Pen ie specific pens, belonging ie specific pens, belonging

to specific people.to specific people. Tim’s boardwriterTim’s boardwriter Tim’s biroTim’s biro Alan’s penAlan’s pen

Instances of the class Pen Instances of the class Pen are called Objects.are called Objects.

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Using ClassesUsing Classes

DefinitionDefinition Declaration of variablesDeclaration of variables

Instance variables - specific to one instance of a classInstance variables - specific to one instance of a class Class variables - shared by all instances of that classClass variables - shared by all instances of that class

Definition of methodsDefinition of methods A method is the program code required to perform a specific task with the A method is the program code required to perform a specific task with the

class.class. Methods consist of statements.Methods consist of statements.

InstantiationInstantiation Usually an instance of a class needs to be created before it can be used.Usually an instance of a class needs to be created before it can be used. Some classes (especially utility classes) have methods and variables that Some classes (especially utility classes) have methods and variables that

can be used without instantiation.can be used without instantiation.

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The Purpose of OO ProgrammingThe Purpose of OO Programming Classes should model the real world - thus Classes should model the real world - thus

leading to efficient coding.leading to efficient coding. Classes are reusable.Classes are reusable. Libraries of reusable classes (“class libraries”) Libraries of reusable classes (“class libraries”)

make sophisticated functionality easily available.make sophisticated functionality easily available. This is a “construction kit” approach to This is a “construction kit” approach to

programming.programming. For example - consider graphical programs:For example - consider graphical programs:

Window classWindow class Text boxText box Button classButton class