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Everything is an object, Part 2

Build your first Java program

By Bruce Eckel, JavaWorld.com, 10/06/00

There are several issues you must understand before seeing your first Java program.

TEXTBOX: TEXTBOX_HEAD: Everything is an object: Read the whole series!

• Part 1. Learn to write your first Java program with these Java basics

• Part 2. Build your first Java program
• :END_TEXTBOX
  
  

Name visibility

A problem in any programming language is the control of names. If you use a name in one module of the program, and another programmer uses the same name in another module, how do you distinguish one name from another and prevent the two names from "clashing?" In C, this is a particular problem because a program is often an unmanageable sea of names. C++ classes (on which Java classes are based) nest functions within classes so they cannot clash with function names nested within other classes. However, C++ still allowed global data and global functions, so clashing was still possible. To solve this problem, C++ introduced namespaces using additional keywords.

Java was able to avoid all of this by taking a fresh approach. To produce an unambiguous name for a library, the specifier used is not unlike an Internet domain name. In fact, the Java creators want you to use your Internet domain name in reverse since those are guaranteed to be unique. Since my domain name is BruceEckel.com, my utility library of foibles would be named com.bruceeckel.utility.foibles. After your reversed domain name, the dots are intended to represent subdirectories.

In Java 1.0 and Java 1.1, the domain extensions com, edu, org, net, and so forth were capitalized by convention, so the library would appear as COM.bruceeckel.utility.foibles. Partway through the development of Java 2, however, it was discovered that this caused problems, and so now the entire package name is lowercase.

This mechanism means that all of your files automatically live in their own namespaces, and each class within a file must have a unique identifier. So you do not need to learn special language features to solve this problem -- the language takes care of it for you.

Using other components

Whenever you want to use a predefined class in your program, the compiler must know how to locate it. Of course, the class might already exist in the same source code file that it's being called from. In that case, you simply use the class -- even if the class doesn't get defined until later in the file. Java eliminates this forward referencing problem so you don't need to think about it.

What about a class that exists in some other file? You might think that the compiler should be smart enough to simply go and find it, but there is a problem. Imagine that you want to use a class of a particular name, but more than one definition for that class exists (presumably these are different definitions). Or worse, imagine that you're writing a program, and as you're building it you add a new class to your library that conflicts with the name of an existing class.

To solve this problem, you must eliminate all potential ambiguities. This is accomplished by telling the Java compiler exactly what classes you want using the import keyword. import tells the compiler to bring in a package, which is a library of classes. (In other languages, a library could consist of functions and data as well as classes, but remember that all code in Java must be written inside a class.)

 

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