Web Programming with Java Java - Object-Oriented Programming doc

Objects and Classes™ Basic OO concepts ƒ Object, class, state, behavior, message ™ Creating Java classes, objects, properties and methods ™ Constructors ™ Package ™ Reference data type

Web Programming with Java

Java Object-Oriented Programming

Huynh Huu Viet Email: viethh@uit.edu.vn

Department of Information Systems

ƒ Inheritance & Polymorphism

™ Some useful Java classes

™ Exception Handling, I/O

™ Readings & Exercise

™ Discussion

Objects and Classes

™ Basic OO concepts

ƒ Object, class, state, behavior, message

™ Creating Java classes, objects,

properties and methods

™ Constructors

™ Package

™ Reference data type

™ Variable scope

Basic OO concepts : Object

™ Object: any thing, which can be described by

ƒ State: property, or attribute

ƒ Behavior: function/action, or method: ask the object to perform a task

™ Example

ƒ A student, a desk, a circle, a button, and even a loan

Basic OO concepts : Class

™ Class: a set of objects with common attributes and behaviors

ƒ An object is an instance of a class

™ Class is a template or blueprint that defines what an object's data and methods will be

Basic OO concepts : Message

™ Objects interact (communicate) by

passing messages

• Defining classes and their properties

• Manipulating classes/objects properties and behaviors

• Handling objects interactions

™ At execution time

ƒ Programs are executed through predefined

manipulations and interactions

ƒ A constructor without any parameter

ƒ If a programmer doesn’t define any constructor for a class, JRE will implicitly create a default

constructor

Using Constructor

behaviors of an object when it is initially created

™ Three differences with methods:

ƒ Must have the same name as the class itself.

ƒ Do not have a return type - not even void.

object is created

ClassName() //no “void” or any other data

type {

… }

Constructor Overloading

™ Like methods, constructors can be

overloaded

™ This offers greater flexibility and

convenience of creating objects

Using Objects (1)

™ Object initialization

™ Using Member Variables

ƒ Member variable declaration

• Any where in a class, outside all methods

ƒ Used within the class

• Can be used/referenced anywhere directly

ƒ Used with objects

• Using the "." operator and preceded with object name

• Ex: myCircle.radius

ClassName objectName;

objectName = new ClassName();

Using Objects: using methods

™ Defining methods

ƒ With a return value (type)

ƒ Without a return value (type) – void

™ Calling Methods

directly

ƒ Used with objects or outside the class

• Using the "." operator and preceded with object name

• Examples:

The “main” Method

™ Following OOP guidelines, the use of the

“main” method should be deemphasized

ƒ The “main” method is merely a starting point of the application

ƒ There should not be too many statements in the main method

ƒ Using objects and methods effectively

™ Typically (and ideally), you only do these

things in the main method

ƒ Creating objects

ƒ Calling class/object methods

™ Packages are used to group classes

™ Four reasons for using packages

ƒ To locate classes

ƒ To avoid naming conflicts

ƒ To distribute software conveniently

Using Package

™ Organizing your classes into packages

ƒ No duplicate class definition in the same

referenced outside the default package

Reference Data Type

™ Reference data type stores memory

address (reference) as its value

™ Objects are reference data type

Variable Scope

™ Member variable

ƒ Something like a global

variable within the class

™ Local variable

ƒ Method parameter

ƒ Method level variable

ƒ Block level variable

™ A variable is effective at

its declaration level and

all sub-levels

Variable Overriding

™ Variables cannot be declared twice at the same

level and its sub-levels

ƒ except class member variables (overridden by local

variables)

™ Use "this" keyword to refer to the class/object

itself to access (the overridden) member variables

this.title

this.couseId

this.getCourseInfo() //”this” can be used with method

™ It is good to use “this” explicitly for member

variables

ƒ Inheritance & Polymorphism

™ Some useful Java classes

™ Exception Handling, I/O

™ Readings & Exercise

™ Discussion

™ A concept to hide information or other details from the outside programs

™ Why? Should classes and methods

properties be available to other

programs (classes)?

ƒ Cohesion, modularity, integrity

ƒ Hides complexity, consistent and clear

interaction

Access Modifiers

™ Encapsulation is implemented through the use of access modifiers/specifiers

ƒ private - completely encapsulated within the class

private int length;

ƒ default (no modifier) - within the same package

int length;

ƒ public - completely open

public int length;

ƒ (protected)

Access Modifiers for Classes

™ If a class is defined public , then it can

scope)

public class Book { … }

™ Otherwise, if it is defined “default”,

then it can be referenced only by

those in the same package

class Book { … }

™ Note: any java source file can have

only one public class

Java Access Modifiers

™ OO modeling

ƒ Class B is part of class A, or class A consists

of class B (one or many)

™ For example

Aggregation Implementation

™ Aggregation is very common in object modeling and Java programs

™ In Java, this relationship is

implemented as a class member

variable is of a class/object type,

rather than primitive type

Modeling One-to-One

™ Class A has a class B

™ A class (A)’s member property (p) is directly defined as another class (B) type

Modeling One-to-Many

™ Class A has several class B’s

™ A class (A)’s member property (p) is

defined as a collection of class (B)

™ OO modeling

ƒ “Is-A” relationship between two classes

ƒ One class (child/subclass) will inherit

properties and behaviors of another class

(parent/super class)

™ For example, if “Faculty” is a

“Employee”

ƒ “Faculty” is the subclass

Inheritance in Java

™ In Java class implementation

ƒ A subclass will inherit all available member

variables and methods of its super class

(availability is defined using access modifiers)

™ Use the statement “extends” to define inheritance

class Faculty extends Employee

{ … }

™ Using the super Keyword refers to the superclass

Overriding Methods

™ Overriding method : the method is

defined in the subclass using the

same signature and same return type

as in its superclass

™ Overriding vs Overloading

™ Example: Section 9.4

™ Polymorphism allows methods to be used

generically for a wide range of object

arguments (generic programming)

™ Example: 9.7

™ Casting

ƒ Implicit casting: Object o = new Student();

ƒ Explicit casting: Student b = (Student) o

™ Demonstrating Polymorphism and Casting :

TestPolymorphismCasting.java

Abstract Classes

™ Abstract classes

ƒ But you cannot create instances of abstract

™ An abstract method

ƒ Its implementation is provided by the

subclasses

be declared abstract

™ Example TestGeometricObject.java

Interfaces

™ An interface is a classlike construct

that contains only constants and

Interface vs Abstract class

Multiple inheritance

™ Java allows

ƒ Only single inheritance for class extension

ƒ But multiple extensions for interfaces

™ Subinterface:

ƒ An interface can inherit other interfaces using the extends keyword

public class NewClass extends BaseClass

implementes Interface1, , InterfaceN{

}

public interface NewInterface extends Interface1, , InterfaceN {

// constants and abstract methods

™ Some useful Java classes

™ Exception Handling, I/O

™ Readings & Exercise

™ Discussion

Some useful Java classes (1)

Some useful Java classes (2)

™ StringBuilder/StringBuffer Class

ƒ Use StringBuffer if it may be accessed by

multiple tasks concurrently

ƒ StringBuilder (1.5 or above) is more efficient if

it is accessed by a single task

™ Modifying Strings in the Buffer with

methods:

ƒ insert()

ƒ delete()

Some useful Java classes (3)

™ Reading text: java.util.Scanner

Scanner input = new Scanner(System.in);

Or Scanner input = new Scanner(new File(filename));

Some useful Java classes (4)

™ Some useful Java classes

™ Exception Handling, I/O

™ Readings & Exercise

™ Discussion

Exception Handling, I/O (1)

™ Run-time error occurs when a program

encounters an event that will break the

normal execution of the program

™ Traditional error handling is unstructured

and tightly binds the error handling code in the program flow

™ Java handles errors (exceptions)

ƒ in an object oriented way

ƒ with a systematical and structured mechanism

Exception Handling, I/O (2)

™ Exception handling advantages

classified – easy learning and prediction

ƒ Flexible for user application to handle the

exception

try {

statements that might cause errors }

statements to handle errors }

finally {

statements that will be executed under all conditions

}

Handling Multiple Exceptions

™ Exception handling is very easy

effective when handling multiple

potential exceptions

™ Using “java.lang.Exception” to

capture all potential exceptions and deal with them consistently

™ Using multiple “catch” block to

capture specific exceptions and deal

Typical Java Exceptions

™ Exceptions are defined using various classes

in various packages, but all inherit the

java.lang.Exception class

Understanding Exception Handling

™ Java's exception-handling model with

3 operations:

ƒ Declaring an exception,

Readings & Exercise

Discussion