What is this "Java"?

• Java is a general purpose programming language. This is a strength and a requirement for success, but it's not sufficient by itself.

• It is an object-oriented language. OOLs are very popular, perhaps unjustifiablly so. Java is more or less based on C++, but there are some differences. For instance, pointers are implicit so pointers and addresses need never bother programmers. But some of the advantages of pointers are lost then, for instance, you can't travel down an array by incrementing a pointer.

• It is platform independent. It's like TeX that way: it runs exactly the same on every machine.

• It is extremely portable. Java programs are compiled once, not once for each platform. But not all platforms support Java yet.

• It is network friendly. Browsers on the web can run Java applets.

• It has a GUI, the AWT (Abstract Windows Toolkit), part of the JDK (Java Developers Kit).

• Speed? Not as fast as a real compiled program. But look at Paul Callahan's Conway's Life Game, in particular, the Life Pattern Catalog.

• Like any other programming language, you write the source program. Your program will include calls to the methods in the JDK.

• Compile your program into (either standalone applications or) applets that can run in conjunction with a web browser (Netscape)

• Debugging. Of course it's tough. The language is like C++, a little smoothed, but still prone to syntax and semantics errors. There's an appletviewer that helps to find errors, but the debugger is not helpful. It's easy to misinterpret the JDK stuff and get no output at all.

• The working applet is invoked by the APPLET tag in an HTML file when Netscape reads it

• Primitive data types. The important types are int, double, char, and boolean.
  A byte is 8 bits, short 16, int 32, long 64. A float 32 bits, and double 64. Real arithmetic adheres to the IEEE 754 standards. A char is a 16 bit Unicode char. A boolean is either true or false.

• Objects. An important class extending Object is the String class. It works really well. You can think of it as another primitive data type, but of course, it's not primitive.

• Arithemetic. The expressions and assignment statements are taken from C. So you've got +, -, *, /, %, <<, >>, =, +=, ++, -=, --, etc. But remember that booleans are not ints. Relations are ==, !=, <, >, <=, and >=. Logical operations are &&, ||, !. There are bitwise operations, too. To print, use a statement like

          System.out.println("Hello world!")
  

• Control statements. Basic control structures are if, if/else, for, while, do/while. Also break, continue, and return. And switch, case, default.

• Arrays. (Vectors are expandable arrays.) To declare an array of length 10:

          int A[] = new int[10];
  

  Indexing starts at 0. You can reassign an array a new array, but that loses the old values unless there is a reference to them. To make two references to the same array, use

  	B = A;
  

  Arrays can be higher dimensional, of course. They can also be ragged.

• Pointers. They're all hidden from the programmer. Every object (as opposed to primitive data type) is implicitly referred to by a pointer. If you need a pointer to an int, for instance, there is an Integer object (in the java.lang package) that is a "wrapper" for an int. Instances of objects are created with new (like malloc in C), but the programmer doesn't have to worry about freeing memory once the object is no longer needed. There's a garbage collector in Java.

• Methods (aka. functions, procedures, subroutines). Java is an object-oriented language. That means every function is a method of some class. It's sometimes a matter of choice which class you choose to associate a function to. For unary operations, you can use the class of the argument. For instance, the function to find the area of a circle could be encoded as a method in a Circle class. Binary operations aren't so clear. The product of two Complex numbers would probably be encoded as a "public static" method of Complex with two arguments (parameters).

• Subclasses and inheritance. Every class is an extension (subclass) of Object, sometimes through several levels. Each subclass inherits every method of its parent, and has more. Also, a subclass can override the methods of its parents (unless that method is "final" as declared in the parent).

JavaScript is a language that looks like Java that Netscape created for better communication with its browser. JavaScript appears in an html file and is interpreted by the browser. It has some built-in objects, but no more can be created by the programmer. It has loose typing and dynamic binding. It doesn't do a lot that Java does, for instance, draw a circle.

Two more pacckages, java.io and java.util, make up the core packages. In order to use Java with the web, the most important packages are java.awt and java.applet. The java.applet package really isn't very big, since it relies so much on the java.awt package.

• The best reference is Sun's Java Tutorial. But it doesn't include everything you need to know. The more arcane information involves the AWT and JDK. For documentation on that, use Java API Documentation. Documentation has been very lean for the last year, but now there are many introductory texts. Most, however, don't answer the questions you'll have when you get into advanced problems.

• Nelson Yu's New AWT Tutorial explains the Abstract Windows Toolkit in the JDK (Java Developers Kit

• The Java FAQ and Brewing Java, a tutorial

• T. J. Willis' Java for Human Beans, an outline of a talk given at the KCACM March, 1996, Meeting

• JavaSoft, Sun's official Java site

• (and Gamelon without frames, my preference)

• Java Applet Rating Service

• Java Repository

• JavaWorld on-line magazine

• Many of the refernces on the interactive page of the Mathematics Archives are Java applets.

• Of course, there are my own applets, see my Java home page.

David E. Joyce
Email: djoyce@clarku.edu