Organizing Java files and other practicalities - JavaTech: An Introduction to Scientific and Technical Computing with Java - page 137

This package-naming convention is so widely used in the Java industry as to be
completely standardized.
Packages can be nested to any depth. In most situations the first two or more
names reflect the domain name of the vendor or creator of the package. Many
open source packages exist as well, with package names like org.apache.ant ,
for example. Another example is the CORBA package (see Chapter 19). CORBA
is maintained by the Open Management Group, a standards body with the internet
domain name omg.org (see www.omg.org for much more information about
the OMG). Thus all the CORBA classes live in package structures that begin
with org.omg.CORBA , org.omg.Messaging , etc. As mentioned above, these
are the only packages in the entire Java class library that use uppercase letters
anywhere in the package names.
5.3.2 Import
Unless special arrangements are made, all classes must be referred to by their
complete name, which includes the full package name (such names are called
“fully qualified” names). Therefore, in TestABC.java above we specified the
full package name of the TestC class in the declaration and new expression:
mypack.extrapack.TestC testc = new mypack.extrapack.TestC ();
An exception to this rule holds for all the classes in the standard java.lang
package. Thus, whenever we use the Float class, for instance, we merely use
something like
Float someFloat = new Float (3.14);
There are nearly 3000 classes in the 135 packages in the J2SE 1.4.2 standard
class library (over 3000 classes in 165 packages in J2SE 5.0). Commercial class
libraries that one might use add many more packages. Using fully qualified class
names obviously becomes unwieldy in any non-trivial program since such a
program will involve many different classes from many different packages.
Thankfully, the Java compiler provides the special arrangements necessary
to abbreviate most fully qualified class names with just the short class name by
using the import directive. That is, the compiler understands a shortcut that
allows us to refer to something like java.awt.event.ActionEvent (see
Chapter 7) as simply ActionEvent . The appearance of an import directive
tells the compiler where to look for class definitions when it comes upon a short
class name that is not fully qualified. Every class in the java.lang package is
imported automatically. That's why we can refer to the java.lang.Float class
as just Float .Touse this abbreviation technique for classes in other packages
requires that the class name or package name appear in an import statement.
As an example in a class definition for an applet class, this import directive:
import java.applet.Applet;