Chapter 2.1

INTRODUCING JAVA
APPLICATIONS AND APPLETS
Chapter 2.1:

Introduction
 Java is the Internet programming language
 It also can be used to develop standalone
applications
 Java is cross-platform, object-oriented,network-
based, and multimedia-ready.

The History of Java
 1991 - developed by James Gosling and
colleagues at Sun Microsystems. The language,
initially called Oak. Designed for use in embedded
consumer electronic applications and intended to
replace C++
 1995- renamed Java designed for Internet
applications.

Characteristics of Java
–Simple
–Object Oriented
–Distributed
–Interpreted
–Robust
– Secure
– Portable
– Architecture
neutral
– High-
performance
– Multithreaded
– Dynamic

The Basics of Java Program
 There are 2 types of Java programs:
 Applications
○ Standalone programs
 Applets
○ Programs designed to run on a Web browser

 Applications
 single plain-looking terminal window
or
 with a graphical user interface (GUI) use one or more
windows filled with user interface components such as
buttons, text input fields, and menus

Single plain-looking ApplicationGUI Application

 Basic unit of a Java program – class
 Therefore application is a collection of one or more
classes.
 A class is a collection of methods and data members
 One of the classes in a Java application program
must have only one method main.

Packages, Classes,
Methods
 Package: A collection of related classes.
 Class: Consists of methods.
 Method: Designed to accomplish a specific task.

Import Statement
 Used to import the components of a package into
a program.
 Reserved word.
 import java.io.*;
Imports the (components of the) package java.io into the
program.
 Primitive data types and the class String:
 Part of the Java language.
 Don’t need to be imported.

Creating a Java Application
Program
 Syntax of a class:
 Syntax of the main method:

Programming Style
 Know common syntax errors and rules.
 Use blanks appropriately.
 Use a semicolon as a statement terminator.
 Important to have well-documented code.
 Good practice to follow traditional rules for naming
identifiers.

Skeleton of Java Program
import statements if any
public class ClassName
{
declare named constants and/or stream objects
public static void main(String[] args) throws
IOException
{
variable declaration
executable statements
}
}

import java.io.*;
/*This class is created to manipulate arithmetic operations*/
public class ArithmeticProgram {
static final int NUMBER = 12;
static Scanner in = new Scannner (System.in);
public static void main(String[] args) throws IOException {
int firstNum, secondNum;
firstNum=18;
System.out.println(“Enter an integer:”);
secondNum = in.nextInt();
System.out.println(“Value of firstNum:”+firstNum);
System.out.println(“Value of secondNum:”+secondNum);
firstNum=firstNum+NUMBER+2*secondNum;
System.out.println(“New value of firstNum:”+firstNum);
}
}
Example #1

Skeleton of Java Program/*This class is created to perform arithmetic operation*/
public class Count{
private int num1,num2,addResult,multiplyResult; //list of attributes
public void setData(int n1, int n2){ //to assign a value to num1 and
num2
num1 = n1;
num2 = n2;
}
//to perform addition between num1 and num2
public void performAddOperation(){
addResult = num1 + num2;
}
//to perform multiplication between num1 and num2
public void performMultiplyOperation(){
multiplyResult = num1 * num2;
}
//to display the result of addition and multiplication
public void displayResult(){
System.out.println(num1 + " * " + num2 + " = " +
multiplyResult);
System.out.println(num1 + " +" + num2 + " = " + addResult);
}
Example #2

/*This class is created to test/manipulate the Count class
public class TestCount{
public static void main(String[]args){
Count count=new Count();//create an instance of the Count class
count.setData(12, 6);
count.performAddOperation();
count.performMultiplyOperation();
count.displayResult();
}//end main
}//end class
Example #2
Output:
12 * 6 = 72
12 +6 = 18

How to use Java?
 Understand files and folders/directories
 Locate the Java compiler/ Install J2SE
 Set path & Java class path
 Write a simple program (later)
 Save your work
 Compile & run
 Use Dos Command Prompt or IDE

Creating a Java Program
 Use any text editor to create or edit a Java source
code.
 Save the file. Use the same class name. For
example class name: Count, file name should be
Count.java
 Compile the program to translate the source code
into bytecode. If no syntax errors, compiler will
generate file named Count.class
 Execute the program.

An Integrated Development
Environment

 Basic elements of a Java program include:
 The main method
 Reserved words
 Special symbols
 Identifiers
 Data types
 Expressions
 Input
 Output
 Statements

 To create a Java application, it is important to
understand:
 Syntax rules.
 Semantic rules.
 How to manipulate strings and numbers.
 How to declare variables and named constants.
 How to receive input and display output.
 Good programming style and form.

File Hello.java
1 public class Hello
2 {
3 public static void main(String[] args)
4 {
5 // display a greeting in the console window
6 System.out.println("Hello, World!");
7 }
8 }

Java Program Elements
 A Java program is made up of class definitions.
 A class definition must contains a header and a
body.
 A class contains zero or more methods
 A method is a named section of code that also has
a header & body
 A method contains program statements
 Single-line (starts with //) and multi-line (enclosed
by /* and */) comments are used to document the
code

Java Program Structure
public class Hello
{
}
// comments about the class
class header
class body
//Comments can be placed almost anywhere

Java Program Structure
public class MyProgram
{
}
// comments about the class
public static void main (String[] args)
{
}
// comments about the method
method header
method body

Compiling and Running
 Type program into text editor
 Save (file name must be similar to class name)
 Open Dos Window
 Change directory to saved file directory
 Compile into byte codes
javac Hello.java
 Execute byte codes
java Hello

JVM
Create/modify source code
Source code
Compile source code
Byte code
Run byte code
Output
Syntax errors
Runtime errors or
incorrect results

Applets
 Applet: a Java program that is embedded within a
Web page and executed by a Web browser
 is a program written in the JavaTM programming
language that can be included in an HTML page.
When you use a Java technology-enabled browser
to view a page that contains an applet, the applet's
code is transferred to your system and executed
by the browser's Java Virtual Machine (JVM).
 Create an applet by extending the class JApplet.
 class JApplet is contained in package javax.swing.

Applets
 A Java application is a stand-alone program with a
main method (like the ones we've seen so far)
 A Java applet is a program that is intended to be
transported over the Web and executed using a
web browser
 An applet also can be executed using the
appletviewer tool of the Java SDK
 An applet doesn't have a main method
 Instead, there are several special methods that
serve specific purposes

Applet Methods
 init method:
 Initializes variables.
 Gets data from user.
 Places various GUI components.
 paint method:
 Performs output.

Applets
 The paint method is executed automatically
whenever the applet’s contents are drawn
 The paint method accepts a parameter that is an
object of the Graphics class
 A Graphics object defines a graphics context on
which we can draw shapes and text
 The Graphics class has several methods for
drawing shapes

//********************************************************************
// Einstein.java Author: Lewis/Loftus
//
// Demonstrates a basic applet.
//********************************************************************
import javax.swing.JApplet;
import java.awt.*;
public class Einstein extends JApplet
{
//-----------------------------------------------------------------
// Draws a quotation by Albert Einstein among some shapes.
//-----------------------------------------------------------------
public void paint (Graphics page)
{
page.drawRect (50, 50, 40, 40); // square
page.drawRect (60, 80, 225, 30); // rectangle
page.drawOval (75, 65, 20, 20); // circle
page.drawLine (35, 60, 100, 120); // line
page.drawString ("Out of clutter, find simplicity.", 110, 70);
page.drawString ("-- Albert Einstein", 130, 100);
}
}

The HTML applet tag
 An applet is embedded into an HTML file using a
tag that references the bytecode file of the applet
 The bytecode version of the program is
transported across the web and executed by a
Java interpreter that is part of the browser
<html>
<head>
<title>The Einstein Applet</title>
</head>
<body>
<applet code="Einstein.class" width=350 height=175>
</applet>
</body>
</html>

Skelaton of a Java Applet
import java.awt.Graphics;
import javax.swing.Japplet;
public class WelcomeApplet extends JApplet
{
}

<HTML>
<HEAD>
<TITLE>WELCOME</TITLE>
</HEAD>
<BODY>
<APPLET code = "GrandWelcome.class" width="440" height="50">
</APPLET>
</BODY>
</HTML>
import java.awt.*;
import javax.swing.JApplet;
public class GrandWelcome extends JApplet{
public void paint(Graphics g) {
super.paint(g);
g.setColor(Color.red);
g.setFont(new Font("Courier",Font.BOLD, 24));
g.drawString("Welcome to Hava
Programming",30,30);
}
}

Java Platform
 Platform is a hardware or software environment to
execute a program.
 It also can be defined as a combination of
operating system and hardware.
 The popular platform are Windows 2000, Linux,
Solaris, and MacOS.
 Java platform is different with other platform
because it is only a software to execute an
application on different hardware platform.

Java Platform
 It has 2 components :
 Java Virtual Machine (JVM)
 Java Application Programming Interface (Java API)
 (source : Website http://java.sun.com).

Java Virtual Machine (JVM)
 Also called a "Java Interpreter"- Converts bytecode into
OS specific commands. In addition to governing the
execution of an application's bytecodes, JVM handles
related tasks such as managing the system's memory,
providing security against malicious code, and
managing multiple threads of program execution. JVM
sits on top of a native environment (OS) and allows for
portability of applications across various hardware as
long as the hardware has the JVM on it.
 JVM executes instructions that a Java compiler
generates. This run time environment, is embedded in
various products, such as web browsers, servers, and
operating systems.
java.sun.com/developer/onlineTraining/new2java/programming/
learn/unravelingjava.html

Chapter 2.1

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Chapter 2.1