Java: The Good Parts

Chapter 4. Packages

One of the least appreciated good parts of Java can be found at the beginning of every Java file. This feature is so ubiquitous that most experienced Java programmers don’t even notice it, much less take the time to think about what it does for them and how to use it correctly. I am speaking of the package feature of Java, along with the notion of importing from some other package. This feature, and the protection mechanisms that are part of it, is one of the simple tools that allow large-scale software to be developed in the language.

It is easy to ignore the packaging system, to use it carelessly, or to simply not give it much thought until it causes you trouble. But rightly understood, the package mechanism allows the design of the overall software system to be disentangled and made independent in a number of important ways. If your package declarations don’t reflect your design decisions, you aren’t using the system correctly (although you will hardly be singled out for that sin). If they do, you have introduced another mechanism to allow understanding and isolation to your system, making it easier to comprehend, develop, and maintain.

The Basics

The first bit of information that the compiler sees in any Java source file is the package declaration. There can be lots of commentary prior to this declaration (for example, it is common practice to put any licensing or copyright information in a comment at the very beginning of a source file), but the compiler discards ...