Java resources
  The clock
  Mandel and its source
  Java Tutorial
  First programs
  Java version of algorithmic image gallery
  Online java compiler FAQ
  Interesting applets
  How to table
  JDK demo
  test applet
  Applet collection
  Digital Unix java
  Applet Proxy Server



  Hooked On Java

  Teach yourself Java in 21 days

  The Java tutorial

  Using Java

  Java Quick Reference

  List of Java Books

Using other people's applets
Material to get started
Definition of some terms
The first step
OOP in a nutshell
Learning programming : step 2
Using Netscape to run applications
The actors on stage metaphor
Using predefined Classes
Learn Java in a hour
Java packages
Data Types
Frequently used constants and methods
Java I/O tutorial
Strings tutorial
Abstract data types

Learning Java

Using other people's applets

First of all why not trying to use in your pages applets developped by other people calling them like routines.
Of course you can use the
Gamelan Java directory.
As a first try, see here a page with a clock.
The code shows that I use a Clock2.class applet (this has been copied on my disk but I could have used the original) with parameters width=170 and height=150. I can call the applet changing the parameters like I call a library subroutine.
In this second example I have to copy first the applet on my disk, since it uses some data from disk and a remote applet cannot access data on my disk. Animator animates a set of static gifs stored on disk.

Files with a name that ends in class contain a compiled piece of Java code. Sometime the source code is also available and you can copy it and try to compile it producing the compiled code that you then try to use as applet. If everything works, you can then try to make some changes to the source in order to adapt it to your needs.
This is what I have done with this Mandelbrot generator. This is the source. Note that the source ends with .java .

Some material on the Web good to get started

The book Hooked on Java has put online all the applets used in the book and other interesting information. These applets show how to perform some basic function. Material from the book Teach yourself Java is also available.
This instead is a Java tutorial

Some terms with definition

  • Applications are normal programs that you use outside the browser.
  • Applets are special programs (objects) that you use inside a Web document with the <APPLET> tag.
  • The java compiler would compile a source generating bytecodes:these can run on any platform that has a Java interpreter.
  • Being Java object oriented,each program is a class of objects.
  • In a class we have definitions of methods (functions) that act on the class variables.
  • To use a class you must create an instance of it with instance variables that define the attributes of this object.
  • JDK indicates the Java Developer's Kit available from Sun with a compiler(javac) a appletviewer(appletviewer) and an interpreter (java).
  • The constructor is a special method called when the object is initialized.
  • Member data and member functions are other way to call instance variables and instance methods.
  • Abstract classes cannot be instantiated and are created only to serve as a basis for making subsclasses.They exist only to express the common properties of all its subclasses. They can have methods defined but left blank (they do nothing) since it is understood that you must define them in the subclasses:these are called an abstract methods.
  • An interface is a set of empty methods definitions:these methods must be defined in the class that implements that interface. For example:
           public interface Drawable {
    public void draw();

The first step

Download and install the JDK on your computer. Write the "hello world" application and applet, compile them and
see them.

Object-oriented programming in a nutshell

In OOP programming programs are sets of objects wich communicate through messages. An object is defined by variables and methods(functions). A class is a prototype describing variables and methods of all objects of the same type. You program by defining the classes of objects necessary to perform a task.Classes are organized in hierarchies with classes lower in hierarchy inheriting state and behaviour from parent classes. This inheritance mechanism provides a natural way of structuring programs.This also means that you define new classes by specifying how these are different. A class inherits all the methods and variables from its parent class. You rewrite a method if necessary, overriding it. With the JDK you get some libraries of classes that can be used to perform some frequent tasks (for example graphics programming).

Learning programming:step 2

If this is not your first programming language, then learning to program is more like learning to write in a foreign language. You start by copying (simple) programs written by other people. Although now you can easily paste them (or ftp them) I prefer to actually write them:this make more easy the understanding of the program.
This is a table pointing to programs copied from various sources in this way.

Using Netscape to view applications

You can use netscape (netscape2 on Unix platforms) to view applications compiled with the command: netscape -java classname but first you should define CLASSPATH with setenv CLASSPATH .:/usr/local/lib/netscape/ Apparently this should also work to execute the compiler with the command netscape -java [filename].java but I didn't have luck. More info

Object oriented graphics programming;the AWT class library;the actors on stage metaphor

You can think about graphics objects (but also other kind of objects) as actors on stage that play toghether directed by messages. One special message (which has the same name as the class) will make the actor appear:this is the constructor. It also gives a name A,B, the actor. For example you say : Panel A = new Panel(); and a new actor of the class Panel is created with name A. After that you call the actor by its name saying A.dothis(parameter) and the actor will perform the action you ordered. For example A.add(slider1);.
Now actors on stage will perform in parallel (i.e. doing things in the same time). You can do the same with objects by using threads.Threads are themselves objects (as everything in Java) which respond to the messages run (i.e. memorize a script to be executed), start (i.e. start running the script) and stop (stop executing the script).
Follows a partial AWT class hierarchy
  • Component
    • Label
    • Button
    • Checkbox
    • Choice
    • Textfield
    • Scrollbar
    • Container
      • Panel
        • Applet


Although you define in paint what to draw inside your applet,to request the actual drawing you call repaint(). When repaint is processed, before calling paint, there is a call to update() which will erase the zone to be painted and then calls paint. If you don't want the drawing zone to be erased, then you have to modify update.

Using predefined classes

To use predefined classes in a applet, you just add a command: import classname;
at the beginning of your applet. You put the .class file in the same directory where you have the applet. I had a few problems compiling an applet with an external class. I was not able to compile the applet using the remote compiler. Instead on a pc, after some trials, it worked if I put the class at the same level in the JDK where you have the "java" and "sun" directory.

Learn Java in a hour

small Java course on the Datamation site, is an interesting tutorial.Following the instructions you will develop a non trivial applet from design(object oriented) to implementation. What you really understand depends on your previous preparation:
  • Have you ever done programming writing a simple small program ?
    If yes, then you at least know what means:
    • data types
    • to compile,interpret,execute,link(use libraries)
    • simple statements
  • Have you done some complex program (more than a subroutine) in any language?
    If yes then you understand
    • program development(top down or bottom-up)
    • call of subroutines/methods
    • definition of global data structures
  • Have you done object oriented programming?
    If yes then you understand:
    • That a program is a set of classes
    • Each class deals with a object defined by some data structure composed from instance variables and a set of methods to process this data structure.
    • how to define subclasses extending already existing classes in libraries.
    • how to override methods
  • Have you done GUI programming?
    If yes then you understand:
    • Set of predefined graphics objects with their methods to manipulate them.
    • The need of writing methods(subroutines) to process events sent to you from the event server when the user does some action like clicking with the mouse.
  • Have you ever done multithreaded programming?
    If yes then you know what means having many threads running in parallel and how to synchronize them to access shared resources.
Each of these five items is a new layer of increasing complexity. You need years to master them .Anyway by following the lesson you can follow the expert seeing step by step how the final program develops.

Java Packages

The JDK contains:
  • directory bin:basic Java language tools
  • directory demo:Demo programs
  • directory lib:Class libraries
    • java.lang:basic language classes (String,etc)
    • java.applet:classes for applet manipulation.
    • java.awt:Abstract windowing toolkit (the classes for GUI)
    • input/output classes
    • protocol API
    • java.util:Miscellanea like random-numbers,date,etc

Java basic data types

  • boolean:true or false
  • byte:8-bit integer
  • short:16-bit integer
  • int:32 bit integer(this is the normal integer.
  • char:used to represent single characters
  • float:32 bit IEEE float
  • double:64-bit IEEE float (the normal float type).
  • String:class used to represent character strings.

Frequently used constants and methods

  • Integer: wrapper class used to manipulate integer.
    For example:
    x=Integer.parseInt(s) converts a string to integer
    s=Integer.toString(x) converts a int to string
  • System.out.println(s);:used to write a message on the standard output stream.
  • main: the method first called from the "system" in a application.It must be declared like this:
    public static void main(String args[]){ }
  • paint: method called from "netscape" to (re)paint a graphic object.
  • Applet: Netscape requires the program to run inside the browser to be a subclass of this class
    public class CiaoATutti extends Applet {
  • Color.redConstant indicating the red color.Similar constants can be used to indicate all main colors.

Java I/O tutorial

In most languages the basic reading/writing loop goes like this:
OPEN file filename for I/O
DO while(there are new records)
read/write record
In Java this is slightly more complex because you have to take care of exceptional conditions in a explicit way with the syntax:
try {   }
catch (Exception e) { handle exception};
In addition, you have to know what are the objects connected to i/o in order to use the read/write methods. These are the important things to know:
  1. The standard input and output are the and System.out
  2. System.out is an object PrintStream which has the methods println(string) to print a new line or print(string) to simply add a new string to the current line.
  3. To direct your print output to a file "temp.out" you have to create a PrintStream object connected to this file:let's call it myOutput. Then you can send your output to this file with myOutput.print( ) or myOutput.println( );
  4. Copy the writing loop from the following code:
    try {
    FileOutputStream out = new FileOutputStream("temp.out");
    PrintStream myOutput = new PrintStream(out);
    /* here the printing loop */
    catch (IOException e) {
    System.out.println("Error: "+e);
  5. is instead an object of the class FileInputStream This class has a method nc=read(vector of byte) that allows for reading characters from standard input(nc is the number of read characters read).
  6. To read from use the following code:
    byte line[] = new byte[100];
    int nc = 0;
    try {
    nc =;
    catch (IOException e){
    System.out.println("Error in reading line");
  7. Note that in order to create a PrintStream object, we created also a FileOutputStream object and this has a method write(array_of_bytes,nc) that allows to write nc characters in output.This can be used also with System.out . So we can echo the input line by writing:
  8. The method string=readLine() allows to read complete lines from input in strings but this works only with objects DataInputStream So to use readLine from you should use the following code:
    String line;
    DataInputStream in = new DataInputStream(;
    try {
    line = in.readLine();
    catch (Exception e){}
  9. Reading from a file "temp.dat" using readLine will involve the following code:
    String line;
    try {
    FileInputStream in = new FileInputStream("temp.dat");
    try {
    DataInputStream myInput = new DataInputStream(in);
    try {
    while((line=readLine()) != null){
    /* process line */
    catch (Exception e){
    System.out.println("Error: "+ e);
    catch (Exception e){
    System.out.println("Error: "+ e);
    catch(Exception e) {
    System,out.println("failed to open file");
    System.out.println("Error: "+ e);
  10. If the line contains more then one data, you should use the StreamTokenizer object to get the single data.

    Strings tutorial

    In other languages to handle strings you have functions like: length(s),index(s,s1),substring(s,i,n),s(i:j)= , if(s1==s2),etc
    In Java, being a string s an object you do everything with methods connected with this object:
    • n= length(s) becomes n=s.length()
    • s1=substr(s,11,7) becomes s1=s.substr(11,17)
    • i=index(s,'d') i=s.indexOf("d")
    • if(s1 == s2) if (s1.equals(s2))

    Abstract data types

    Another way to think about object oriented languages like Java is that they allow you to extend the language by defining new data types (abstract data types) and the operations that you can do with them.


    If you are using JDK setting an incorrect CLASSPATH seems to be the more frequent cause of error.I have found this relatively simple solution. In the directory where I am working I define a "mylogin.bat" file with the command:
    set CLASSPATH=C:\java\lib;C:\website\htdocs\telnet;C:\website\java
    Here I list all directories with classes needed by my applications including the directory where I am now. So when I start working with Java I open a Ms-DOS window, redirect myself on the directory and then ,as first thing, I give the command "mylogin". Then I can start giving the commands "java" and "javac".
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