Java Workshop for Teachers May 6, 2005 A Brief Look at the Java Programming Language

What Java Is and Is Not  Java is a full-fledged, powerful and very versatile object-oriented programming language.  Java is a “sequel” to C++ and a “prequel” to Microsoft’s C #.  Java is not Javascript.

How is Java like and unlike what you may already know?  Syntax is very similar to C and C++  Must deal with classes from the beginning  Can write stand-alone programs (old- fashioned console, as well as new-fangled GUI) and also “applets” that run via HTML pages on the WWW  A platform-dependent java compiler produces a file of platform-independent “bytecodes” that are then interpreted by a platform- dependent java interpreter

Early History of Java  Started at Sun Microsystems (1990/91)  James Gosling headed development team  Began life as ‘oak’, was later renamed ‘Java’  Designed for consumer electronic products  First Person, Inc. lost Time-Warner contract  Also worked well for internet programming  Java and HotJava (Web browser) formally announced (Sun World ’95, May 23 in San F)

Version History of Java  Java 1.0 (1995) 212 classes in 8 packages  Java 1.1 (1997) 504 classes in 23 packages  Java 1.2 (1998) 1520 classes in 59 packages  Java 1.3 (2000) 1842 classes in 76 packages  Java 1.4 (2002) 2991 classes in 135 packages  Java 5.0 (2004) 3270 classes in ? packages Java 5.0 contains the first major changes in the language itself since version 1.1.

Java Language Features  object-oriented  simple  distributed  interpreted  robust  secure  architecture neutral  portable  high performance  multithreaded  dynamic

//HelloWorld.java public class HelloWorld { public static void main(String[] args) { System.out.println("Hello, world!"); } $ javac HelloWorld.java The above command produces HelloWorld.class, which is then interpreted by the following command: $ java HelloWorld The Java “Hello, world!” Program

Some Hands-On Experience A couple of console applications: WelcomeConApp.java ConsoleWelcome.java and ConsoleWelcomeTest.java A GUI application: WelcomeGUIApp.java An applet: WelcomeApplet.java and WelcomeApplet.html

WelcomeConApp.java A typical single-file “console program”, with text output to a “console window” (also called “the screen” or “the standard output”) Compile it with the command $ javac WelcomeConApp.java Confirm that you got a file called WelcomeConApp.class Run that.class file with the command $ java WelcomeConApp

ConsoleWelcome.java and ConsoleWelcomeTest.java A typical, but very small, multi-file Java program (only two files) Consists of a class file capable of producing objects of that class, plus another “driver class” that creates and uses an object of the first class Compile both files with the single command $ javac ConsoleWelcomeTest.java Run the program with the command $ java ConsoleWelcomeTest

WelcomeGUIApp.java A small GUI application program, illustrating - programmer color choices - programmer font selection - programmer choice of window size - programmer placement of text in the window This is again a single-file program, but a later practice exercise asks you to convert it to a two-file program (and make some other changes as well).

Java Applets vs. Java Applications An applet is not a standalone program, but instead runs within a web browser An applet must be run via an HTML file which refers to the applet’s class file in an applet tag An applet can also be run by a utility called appletviewer that comes with the JDK (Java Development Kit), just like javac and java appletviewer just shows the applet and ignores any HTML code in the HTML file Applications have a main function, applets don’t

WelcomeApplet.java and WelcomeApplet.html WelcomeApplet.java shows the order in which three of the “built-in” methods of an applet are called: init() is called to do whatever needs doing once start() is called after init() and at each “restart” paint() is called to draw the Graphics object WelcomeApplet.html contains an element that refers to the applet-containing file WelcomeApplet.class which is loaded and run by the browser

Follow-Up Practice Exercises Convert WelcomeConsoleTest.java to use the first two “command-line parameters” as the text of the welcome message and the date. Convert WelcomeGUIApp.java to a two-file program and choose another font, color scheme, and message. Revise WelcomeApplet.java so that the following command will run the applet: $ appletviewer WelcomeApplet.java (assuming WelcomeApplet.class is available)

StudentMarks1.java to StudentMarks6.java A sequence of programs to illustrate incremental program development Get something that works and does a “small version” of what your ultimate goal program will do With each “iteration” add one or more new features that take you toward that goal This kind of program development is closer to the modern “extreme programming” approach than to the older “structured programming”

Some of the things you will see in the StudentMarks*.java sequence Building a GUI interface with buttons and an area to display text Reading input from a file of text Exception handling Event handling Use of a “comparator” to change the “natural” order of a list of strings Generic programming (a brand new feature in Java 5.0)

SplashScreen.java A “fun” application to promote Java Shows how to position your application in the middle of the screen independently of the screen resolution Uses randomly generated values Uses the sleep() method of the Thread class to generate a short delay Uses a “keyboard listener” to terminate the program