Object Oriented Programming

Budditha Hettige Department of Computer Science

Overview • • • • • • • •

Introduction to OOP Class and Objects Constructors Encapsulation Composition Inheritance Polymorphism Interfaces

7/11/2015

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Budditha Hettige ([email protected])

Packages Exception handling File Handling GUI Programming OOP Software development

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Introduction (OOP) • OOP is a programming paradigm that represents concepts as "objects" that have data fields (attributes that describe the object) and associated procedures known as methods

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Advantages of OOP • Provides a clear modular structure for programs • OOP makes it easy to maintain and modify existing code (new objects can be created with small differences to existing ones) • OOP provides a good framework for code libraries • Software components can be easily adapted and modified by the programmer.

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Modular Structure

School

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Exercise • Identify suitable state and behaviors (Attributes and methods) for the following – Student – Bank Account – Book – Employee – Sales Summary – Course – Result 7/11/2015

Budditha Hettige ([email protected])

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Concepts of OOP • • • • • •

Objects Classes Encapsulation Inheritance Polymorphism Data Abstraction

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What is an Objects? • Object is a software bundle of related state and behavior • Characteristics: – state and behavior

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What is a class? • A Java class is a group of Java methods and variables • Object is an instance of a class • Example class Person { // Methods // Variables } 9

Example • Create a Java Class Account with suitable methods and variables

7/11/2015

Budditha Hettige ([email protected])

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Class Modifiers • • • •

Public Abstract Final Default (none)

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Class Modifiers • public: anyone can create an object of the defined class. – only one public class per file, must have same name as the file (this is how Java finds it!). • default: is non-public (if you don't specify "public").

Java Programming: OOP

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Class Modifiers • abstract: modifier means that the class can be used as a superclass only. – no objects of this class can be created. • final: if its definition is complete and no subclasses are desired or required – a final class never has any subclasses, the methods of a final class are never overridden

Java Programming: OOP

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Field Modifiers • • • • • •

public protected private none or package or default Static Final

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Field Modifiers • public: any method (in any class) can access the

field. • protected: any method in the same package can

access the field, or any derived class. • private: only methods in the class can access the field. • Default: is that only methods in the same package can access the field.

Java Programming: OOP

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Field Modifiers • Static: Fields declared static are called class fields (class variables). others are called instance fields. There is only one copy of a static field, no matter how many objects are created. • Final:class and instance variables (static and nonstatic fields) may be declared final. The keyword final means: once the value is set, it can never be changed – static final int BUFSIZE=100; – final double PI=3.14159; Java Programming: OOP

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Method Modifiers • • • • • • • • •

public protected none or package or default private final abstract static native synchronized 17

Method Modifiers • private/protected/public: – same idea as with fields. • abstract: no implementation given, must be supplied by subclass. – the class itself must also be declared abstract

Java Programming: OOP

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Method Modifiers • static: the method is a class method, it

doesn't depend on any instance fields or methods, and can be called without first creating an object. • final: the method cannot be changed by a subclass (no alternative implementation can be provided by a subclass). Java Programming: OOP

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Method Modifiers • native: the method is written in some

local code (C/C++) - the implementation is not provided in Java. • synchronized: only one thread at a time

can call the method.

Java Programming: OOP

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Method Overloading • You can overload methods: – same method name, different parameters. – you can't just change return type, the parameters need to be different. • Method overloading is resolved at compile time. int CounterValue() { return counter; }

Won't Work!

double CounterValue() { return (double) counter; } Java Programming: OOP

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All Possible Combinations of Features and Modifiers Modifier public protected None (default)

Class yes no yes

Variable yes yes yes

Method yes yes yes

private final abstract static native transient volatile synchronized strictfp

no yes yes no no no no no yes

yes yes no yes no yes yes no no

yes yes yes yes yes no no yes yes

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Example • Create a class “Student” with following methods – print(); – input() • Create a class “Length” with following methods – Print() – input() – Add(Lenth1, Length2)

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Example contd. • Create a class Name with 3 attribute (First name, middle name and last name) and include following methods – Print(); – input() • Create a class name Date with 3 attribute (Day, Month and year) and include following methods – print() – Input() – printFormat1() // 23.5.2015 – PrintFormat2() // 23rd May 2015 24