1. What is Java? A programming language Fully buzzword-compliant:
A simple, object oriented, distributed, interpreted, robust, secure, architecture neutral, portable, high performance, multithreaded, dynamic language.
From: Java: An Overview
James Gosling, Sun Microsystems, February 1995.

2. What Else is Java? According to Gosling: Java is a phenomenon
“An environment”
“A platform”
“A way of thinking”
…ok, whatever
Java is a phenomenon
Took the world by storm in 1995 when introduced with the HotJava web Browser
Quickly integrated with Netscape browser

3. Some History 1993 Oak project at Sun
small, robust, architecture independent, Object-Oriented, language to control interactive TV.
didn’t go anywhere
1995 Oak becomes Java
Focus on the web
1996 Java 1.0 available
1997 (March) Java some language changes, much larger library, new event handling model
1997 (September) Java 1.2 beta – huge increase in libraries including Swing, new collection classes, J2EE
1998 (October) Java 1.2 final (Java2!)
2000 (April) Java 1.3 final
2001 Java 1.4 final (assert)
2004 Java 1.5 (parameterized types, enum, …) (Java5!)
2005 J2EE 1.5

4. What is Java?
Java is a general-purpose, high-level programming language.
The features of Java
Java program is both compiled and interpreted.
Write once, run anywhere
Java is a software-only platform running on top of other, hardware-based platforms.
Java Virtual Machine (Java VM)
The Java Application Programming Interface (JAVA API)

5. Features of Java Simple Architecture-neutral Object-Oriented
Distributed
Compiled
Interpreted
Statically Typed
Multi-Threaded
Garbage Collected
Portable
High-Performance
Robust
Secure
Extensible
Well-Understood

6. How Will Java Change My Life?
Get started quickly
Write less code
Write better code
Develop programs faster
Avoid platform dependencies with 100% pure Java
Write once, run anywhere
Distribute software more easily

7. Java Applications and Java … lets
Stand-alone Applications
Just like any programming language
Applet
Run under a Java-Enabled Browser
Midlet
Run in a Java-Enabled Mobile Phone
Servlet
Run on a Java-Enabled Web Server
Switchlet …

8. Java Developer's Kit (I)
Java's programming environment
Core Java API
compiler
interpreter
debugger
dis-assembler
profiler
more...

9. Java Developer's Kit (II)
Interpreter
Java
Compiler
Compile
Run
Java Source
Java Bytecode
<file>.java
Java
Dis-assembler
<file>.class

10. Prepare and Execute Java
國立交通大學計算機中心柯皓仁
Prepare and Execute Java
Source Computer
Java Program
Compilation
Java ByteCode
Internet
Verification
Your computer
Java ByteCode
Execution
Restricted Env. 44 67

11. Write Once, Run Anywhere

12. ByteCode: Food for the VM
For most languages, compilation produces machine code
Java compilation produces “bytecode”
Intermediate code readable by the VM
Transferable across the Internet as applets
VM interprets BC into instructions
Partly responsible for performance lag
ByteCode produced on any platform may be executed on any other platform which supports a VM

13. execution model of Java
compiler
source
(text)
bytecode
(aka. class file)
JVML
dynamic
loading
virtual machine
verifier
JIT
compiler
compiled
code
bytecode
interpreter
CPU

14. The JIT Just-In-Time compiler
Translates bytecode into machine code at runtime
1-time overhead when run initiated
Performance increase times
Now the default for most JVM’s
Can be turned off if desired
JIT can apply statistical optimizations based on runtime usage profile

15. Not just one JVM, but a whole family
JVM (J2EE & J2SE)
Well-known Java Virtual Machine.
CVM, KVM (J2ME)
Small devices.
Reduces some VM features to fit resource-constrained devices.
JCVM (Java Card)
Smart cards.
It has least VM features.
And there are also lots of other JVMs

16. Java Platform & VM & Devices

17. Java VM and API
Java API and Virtual Machine insulate the Java program from hardware dependencies.
Java API

18. Java API
Collection of ready-made software components that provide many useful capabilities.
Grouped into libraries (packages) of related components.
Core API
Essentials: Object, String, Input and Output...
Applets
Networking
Internationalization
Security
Software Components
Object Serialization
Java Database Connectivity (JDBC)

19. The “Hello World” Application

20. Create a Java Source File
public class HelloWorld { public static void main(String[] args) { System.out.println("Hello World!"); } }

21. Compile and Run Compile
javac HelloWorld.java
One file named HelloWorld.class is created if the compilation is succeeds.
Run
java HelloWorld

22. The Simplest Java Application: Hello,World!
Since Java is object-oriented, programs are organized into modules called classes, which may have data in variables and subroutines called methods.
Each program is enclosed in a class definition.
main() is the first method that is run.
class HelloWorld
{ public static void main (String[] args)
{ System.out.println(“Hello World!”); }
Syntax is similar to C - braces for blocks, semicolon after each statement. One difference: upper and lower case matter!
The notation class.method or package.class.method is how to refer to a public method (with some exceptions). 32

23. The “Hello World” Applet

24. Create a Java Source File HelloWorldApplet.java
import java.applet.Applet; import java.awt.Graphics; public class HelloWorldApplet extends Applet { public void paint(Graphics g) { g.drawString(“Hello World!”, 5, 25); } }

25. Compile the Source File
javac HelloWorldApplet.java
One file named HelloWorldApplet.class is created if the compilation is succeeds.

26. Displaying your applet from a Web page.
Create an HTML file with an applet tag to display the results of drawing the applet.
<html><head>
<title>Simple Hello Page</title>
</head>
<body>
My Java applet says:
<applet code=“HelloWorldApplet.class” width=150 height=25>
</applet>
</body></html>
Name of your applet class.
The browser will use a rectangle of width 150 pixels and height 25 pixels to display the applet within the other html. 36

27. The Simplest Java Applet: Hello, World!
Java applets are part of the class hierarchy that can call methods to display on a screen (within the browser window). One way to draw on the screen is to call the method drawString from the standard method paint.
The import statement allows the use of methods from the Graphics class without the dot notation .
import java.awt.Graphics;
public class HelloWorldApplet extends java.applet.Applet
{ public void paint (Graphics g)
{ g.drawString(“Hello World!”, 5, 25); }
Puts this as a subclass of Applet.
The paint method displays a graphics object on the screen - one of the standard methods that takes the place of main for applets. 35