Understanding Objects, Classes, and Inheritance in OOP with UML and Java, Slides of Banking and Finance

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Object-Oriented Software Engineering

Practical Software Development using UML and Java

Chapter 2: Review of Object Orientation

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2.1 What is Object Orientation?

Procedural paradigm:

• Software is organized around the notion of procedures
• Procedural abstraction —Works as long as the data is simple
• Adding data abstractions —Groups together the pieces of data that describe some entity —Helps reduce the system’s complexity. - Such as Records and structures

Object oriented paradigm:

• Organizing procedural abstractions in the context of data abstractions

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Object Oriented paradigm

An approach to the solution of problems in which all computations are performed in the context of objects.

• The objects are instances of classes, which: —are data abstractions —contain procedural abstractions that operate on the objects
• A running program can be seen as a collection of objects collaborating to perform a given task

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A View of the Two paradigms

See in Umple

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2.2 Classes and Objects

Object

• A chunk of structured data in a running software system
• Has properties —Represent its state
• Has behaviour —How it acts and reacts —May simulate the behaviour of an object in the real world

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Objects

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Classes

A class:

• A unit of abstraction in an object oriented (OO) program
• Represents similar objects —Its instances
• A kind of software module —Describes its instances’ structure (properties) —Contains methods to implement their behaviour

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Is Something a Class or an Instance?

• Something should be a class if it could have instances
• Something should be an instance if it is clearly a single member of the set defined by a class Film
• Class; instances are individual films. Reel of Film :
• Class; instances are physical reels Film reel with serial number SW
• Instance of ReelOfFilm Science Fiction
• Instance of the class Genre. Science Fiction Film
• Class; instances include ‘Star Wars’ Showing of ‘Star Wars’ in the Phoenix Cinema at 7 p.m .:
• Instance of ShowingOfFilm

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Naming classes

• Use capital letters —E.g. BankAccount not bankAccount
• Use singular nouns
• Use the right level of generality —E.g. Municipality, not City
• Make sure the name has only one meaning —E.g. ‘bus’ has several meanings

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2.3 Instance Variables

Variables defined inside a class corresponding to data present in each instance

• Also called fields or member variables
• Attributes —Simple data —E.g. name, dateOfBirth
• Associations —Relationships to other important classes —E.g. supervisor, coursesTaken —More on these in Chapter 5

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Variables vs. Objects

A variable

• Refers to an object
• May refer to different objects at different points in time

An object can be referred to by several different variables at the same time

Type of a variable

• Determines what classes of objects it may contain

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Class variables

A class variable’s value is shared by all instances of a class.

• Also called a static variable
• If one instance sets the value of a class variable, then all the other instances see the same changed value.
• Class variables are useful for: —Default or ‘constant’ values (e.g. PI) —Lookup tables and similar structures

Caution: do not over-use class variables

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2.4 Methods, Operations and Polymorphism

Operation

• A higher-level procedural abstraction that specifies a type of behaviour
• Independent of any code which implements that behaviour —E.g. calculating area (in general)

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Methods, Operations and Polymorphism

Method

• A procedural abstraction used to implement the behaviour of a class
• Several different classes can have methods with the same name —They implement the same abstract operation in ways suitable to each class —E.g. calculating area in a rectangle is done differently from in a circle

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Polymorphism

A property of object oriented software by which an abstract operation may be performed in different ways in different classes.

• Requires that there be multiple methods of the same name
• The choice of which one to execute depends on the object that is in a variable
• Reduces the need for programmers to code many if-else or switch statements

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2.5 Organizing Classes into Inheritance

Hierarchies

Superclasses

• Contain features common to a set of subclasses

Inheritance hierarchies

• Show the relationships among superclasses and subclasses
• A triangle shows a generalization

Inheritance

• The implicit possession by all subclasses of features defined in its superclasses

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An Example Inheritance Hierarchy

Inheritance

• The implicit possession by all subclasses of features defined in its superclasses

See in Umple

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The Isa Rule

Always check generalizations to ensure they obey the isa rule

• “A checking account is an account”
• “A village is a municipality”

Should ‘ Province ’ be a subclass of ‘ Country ’?

• No, it violates the isa rule —“A province is a country” is invalid!

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A possible inheritance hierarchy of

mathematical objects

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Make Sure all Inherited Features Make

Sense in Subclasses

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2.6 Inheritance, Polymorphism

and Variables

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Some Operations in the Shape Example