Download Java - John Wordsworth

Electronic Commerce
Java (1)
John Wordsworth
Department of Computer Science
The University of Reading
[email protected]
Room 129, Ext 6544
April 2002
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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 Java
An 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 language
Object - an instance of a class:
variables
methods
Message - executing a method:
destination object (class)
parameters
Class - a blueprint for objects:
class variable (static) and class method (static)
instance variable and instance method
constructor
inheritance
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 inheritance
public 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.
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