Download A First Taste of Java

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 platformdependent 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
 architecture neutral
 simple
 portable
 distributed
 high performance
 interpreted
 multithreaded
 robust
 dynamic
 secure
The Java “Hello, world!” Program
//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
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 <applet>
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