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Java Programming ... From the Grounds Up
Part 3

by Mark C. Reynolds

This part of the tutorial is kind of a class about classes...

A Touch of Class

Note that main must occur as the class method in Java. In this example, the main method occurs within the Mclass definition, rather than at the top level as in C. This is because there are no freestanding functions within Java--every method must occur with the scope of a class. The main method is also identified as static, which means that it may not be changed within instances of this class--another departure from C, which uses static for scoping as well as permanence. The presence of a method named main identifies this code as a standalone Java application, rather than an applet.

The main method parses its command line arguments in such a way as to allow zero, one or two arguments--which, as we'll show below, will prove very useful to us. The construction argv.length extracts the length of the array argv[], and will work with any array.

Arrays are structured entities in Java; for example, an array always knows its length. If the argv[] array has at least one element, then the method parseInt in the Integer class (another predefined Java class) is called to convert that argument to an integer and store it in sta. If a second argument also is present then its value is assigned to cnt. Note that sta is initialized to 2. Since sta is a required argument for the constructor method Primes(), it must always have a value assigned to it. The next four statements actually do the work. The statement "p = new Primes(sta);" creates a new instance of the Primes class, known as p, with the private value of start set to the specified starting value sta. In Java, as in C++, the operator new is used to create new instances of classes.

If the number of arguments is greater than one, the statement on line 57 sets the class member count of the instance p to the user-specified count; otherwise, it retains its default value of 100. In either case, p.count will maintain the number of primes generated. On line 60, we allocate an integer array large enough to hold that number of elements, again using the operator "new". The primes are actually generated by the call to p.Generate(), which invokes the Generate method within the instance p of the class Primes. The results are then printed using System.out.println, which prints a string followed by a newline. It uses the println method of the out member of System (another predefined Java class) to achieve its action.

Although this listing is really not a very practical example--for starters, there's no good reason why start should be a private class member while count is public--it was mainly intended to illustrate the basic use of classes and their member variables and methods. This code demonstrated how to create a class--this example used two, Primes and Mclass--and also used the predefined Java classes System, Integer, and String. The next section provides greater detail on how these libraries provide functionality for dealing with network access and graphics.