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PHY281 Introduction to Java Slide 1
Introduction to Java
In this section we will learn how how to use Java and write our first Java Applet:
The Java Development Kit
Running a Java Program
Java Applets
Creating an Applet
Creating the Web Page
Running the Program
Running in a Web Browser
What does it mean?
Things to remember
PHY281 Introduction to Java Slide 2
The Java Development Kit
Java was developed by Sun Microsystems in 1995.
To use Java we will use the Java 2 Platform Standard Edition (J2SE) Software Development Kit (J2SDK) (previously the Java Development Kit (JDK)).
There are several (confusing) version of Java:Java 1 : JDK 1.0.x, JDK 1.1.xJava 2 : J2SDK 1.2.x, J2SDK 1.3.x, J2SDK 1.4.x, ...
We will be using J2SDK 1.4.0
Note: Not all web browsers support all versions.
You can download the JDK free from http://java.sun.com/ However for Windows it is 35MB plus 30MB for the documentation!
PHY281 Introduction to Java Slide 3
Running a Java Application
You write Java code using an editor
javac MyProg.java
java MyProg
Java code: MyProg.java
Bytecode: MyProg.clas
s
Text Editor
Output
You save the file with a .java extension
You run the Java compiler 'javac'
You execute the bytecode with the command 'java'
This creates a file of bytecode with a .class extension
PHY281 Introduction to Java Slide 4
Java Applets
Applets are programs designed to run as part of a Web Page (Applet = little application).
Applets are similar to normal Java Applications but have extra security features to prevent a downloaded Applet damaging your computer or transmitting information from it. For instance an Applet cannot:
•Access local files•Delete local files•Run another program•Find out your name•Connect to another host
PHY281 Introduction to Java Slide 5
Running a Java Applet
You write Java code using an editor
javac MyApp.java
appletviewer MyApp.html
Java code: MyApp.java
Bytecode: MyApp.class
Text Editor
Window
You save the file with a .java extension
You run the Java compiler 'javac'
You can view the applet with the command 'appletviewer'
This creates a file of bytecode with a .class extension
Web page: MyApp.htm
l
Text Editor
Web Browser
You write a web page in html using an editor
You can view the web page from a web browser
You save the file with a .html extension
PHY281 Introduction to Java Slide 6
Creating an Applet
Open "Notepad" (Start Programs Other Notepad)
Type this in:
Save As "Greetings.java"(Put the " " round the name otherwise it adds .txt to the end!) Open a DOS Window (Start MS-DOS Prompt)
Type javac Greetings.java
G:\> javac Greetings.javaG:\>
If it gives an error check you typed it in exactly right.
import java.awt.*;import java.applet.Applet;
public class Greetings extends Applet {
public void paint(Graphics g) { g.drawString("Hello World!", 50, 50); } }
If you type dir Greetings.* you should see Greetings.java and Greetings.class
PHY281 Introduction to Java Slide 7
Creating the Web Page
In order to run an applet you have to embed it in a web page using a special <applet> tag e.g:
<applet code="name.class" width=www height=hhh></applet>
<html><head><title>Greetings Applet</title></head><body><applet code="Greetings.class" width=300 height=200 ></applet></body></html>
Using Notepad type in the following and save it as "Greetings.html": Size of the applet in
pixels
PHY281 Introduction to Java Slide 8
Running the Program
G:\> appletviewer Greetings.html
In the DOS window type appletviewer Greetings.html
You should see something like this:
PHY281 Introduction to Java Slide 9
Running in a Web Browser
In Netscape go to the File menu then Open Page ...Press Choose File...Find your file Greetings with the Netscape symbol alongside it (Greetings.html) - click on it and press Open (or double click on it)Back in the Open Page dialog press OpenYou should see something like:
Title
Your greeting
Message
PHY281 Introduction to Java Slide 10
What does it mean?
import java.awt.*;import java.applet.Applet;
public class Greetings extends Applet {
public void paint(Graphics g) { g.drawString("Hello World!", 50, 50); } }
These 2 lines tell the computer to include (import) two standard libraries awt (Abstract Window Toolkit) and applet.
This line tells the computer to display some text ( a string) on the screen.
This line announces that the program (class) can be run by anyone (public), is called Greetings and is an Applet.
This line declares what follows in the { } as a method called paint.
This is where it is displayed in pixels across and down from the top left hand corner
This is what is displayed
PHY281 Introduction to Java Slide 11
Things to rememberEverything in Java is case sensitive - Paint is not the same as paint.
The name of the class i.e. Greetings should match the name of the file Greetings.java (not greetings.java).
Curly brackets { and } are used to group parts of the program called blocks together. Blocks can be nested inside other blocks but each { must be matched with a }.
Most statements require a semi-colon ; at the end. A statement can continue on the next line if necessary.
Spaces are not important - it is recommended to indent blocks for clarity.
import java.awt.*;import java.applet.Applet;
public class Greetings extends Applet {
public void paint(Graphics g) { g.drawString("Hello World!", 50, 50); } }
PHY281 Introduction to Java Slide 12
An easier way -Java IDE's
In this section we will briefly learn how about Integrated Development Environments (IDE) for Java:
What are IDE's
Metrowerks CodeWarrior
Borland JBuilder
Eclipse
PhysEdit
PHY281 Introduction to Java Slide 13
What are IDEs?
Integrated Development Environments are programs that are designed to simplify the development of large programs. Several features they might have are:
• The ability to edit several files at once.• Syntax highlighting - different parts of the code are shown in different colours e.g. language keywords (public, class etc) in blue.• The ability to compile the code using a button or menu item and to see the messages in another window.• To run the program using a button or menu item.• To drag components from a 'toolkit' to a design window.• The ability to automatically generate lines of code.• To show a tree like view of your classes.• Extensive debugging facilities.
PHY281 Introduction to Java Slide 14
Java IDEs
Visual Basic is an IDE.
For C++ some popular IDEs are Borland C++ and Microsoft Visual C++.
For Java there are several including Metrowerks, CodeWarrior, Borland JBuilder and Eclipse. They are really designed for large complex programs and are somewhat over complicated for simple programs. They are also hard to install on the Lancaster University servers!
Steve Lloyd developed a simple IDE for a similar course called PhysEdit which has much less functionality than the others but is hopefully reasonably straightforward and sufficient for our needs. You can download it for your home computer if you wish (assuming you have Windows), although no liability is taken!
PHY281 Introduction to Java Slide 18
PhysEdit
PHY281 Introduction to Java Slide 19
PhysEdit - Creating a Project
1. Click on New Project (or File New Porject...)
2. Enter the name of the project e.g. Greetings
3. The program creates some skeleton code and a html file.
4. Add your code to the Java file.
5. Compile the code (executes javac for you).
6. View the applet (executes appletviewer for you).