April 2002 3CSG1 1

Electronic CommerceJava (1)

John WordsworthDepartment of Computer ScienceThe University of ReadingJ.B.Wordsworth@rdg.ac.ukRoom 129, Ext 6544

April 2002 3CSG1 2

Lecture objectives

Outline the place of the Java language in E-commerce

Revise the concepts of object-oriented programming

Describe the structure of a Java class and its parts

Describe the structure of a Java application, and the way it uses objects

Explain how Java implements inheritance

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What is JavaAn object-oriented programming language invented by Sun Microsystems

A collection of classes that model useful objects: windows, buttons, text areas, files, databases …

A language for developing applications that can be executed on many platforms:

Write Java source code

Compile to Java bytecodes (javac)

Execute under the control of the Java Virtual Machine (java)

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Java in e-commerce

Client programming with applets:

Classes for setting up and controlling the graphical user interface

Server programming with servlets:

Classes for managing the interface with tier 1 (JSPs)

Classes for managing the interface with tier 3 (EJBs)

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An object-oriented languageObject - an instance of a class:

variablesmethods

Message - executing a method: destination object (class)parameters

Class - a blueprint for objects:class variable (static) and class method (static)instance variable and instance methodconstructorinheritance

Interface - a collection of methods

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An interface for a simple bank account

public interface AccountIf {void deposit (int amt); void withdraw (int amt); void showBal; }

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A class for a simple bank account public class Account implements AccountIf {private String sortcode, acctno;private int balance;Account (String sc, String an){ sortcode = new String (sc); acctno = new String (an);balance = 0;}public void deposit (int amt) { balance = balance + amt; }public void withdraw (int amt) { balance = balance – amt;}public void showBal { System.out.println ( “Sort code “+sortcode+” Acct number “+acctno=“ Balance “+balance); }

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A program to use a bank account

public class AccountMain { public static void main (String [] args) { Account ac1 = new Account ("304050", "01234567"); ac1.showBal(); ac1.deposit(500); ac1.showBal(); ac1.withdraw(575); ac1.showBal(); }}

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A bit of inheritancepublic class DAccount extends Account{ DAccount (String sc, String an, int opening) { super (sc, an); balance = opening;} public void withdraw (int amt) { if (amt < balance) { balance = balance - amt; } else { System.out.println ( "Withdrawal not allowed"); } }}

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A program to use a deposit account

public class DAccountMain { public static void main (String [] args) { DAccount ac2 = new DAccount ("304050", "01234567“, 1000); ac1.showBal(); ac1.withdraw(500); ac1.showBal(); ac1.withdraw(500); ac1.showBal(); }}

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Sample output

Sort code 405060 Account number 98765432 Balance 1000

Sort code 405060 Account number 98765432 Balance 500

Withdrawal not allowed

Sort code 405060 Account number 98765432 Balance 500

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Key points

Java is an object-oriented language for writing portable applications.

Java has classes that support the construction of programs to run in Tier 1 (applets) and Tier 2 (servlets) of an e-commerce system.

Java classes have a regular structure that can be adapted to writing object-oriented applications, classes, and objects.