Java Glossary : package

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package

The most important thing to know about packages is that you can leave them out only for tiny experiments when all your code is in the nameless default package. As soon as you put any of your code into named packages, all of it has to be put into named packages. My rule of thumb is any code I plan to keep more than 24 hours gets put into packages right off the bat. Starting with Java 1.4, you can't import a class unless it belongs to some named package.

You can't use classes in the default package from a named package. So except for tiny experiments, always put your classes in a named package of the form com.mindprod.xxx or org.hans.xxx i.e. your domain name backwards, all lower case.

When a source file gets too large, you can split it up, and still have it behave much as if it were one file by declaring all the parts as belonging to the same package. In C++ terminology, all classes in the same package are friends of each other. You just add a line such package com.mindprod.business; at the top of each source *.java file. You use your website name backwards to ensure global uniqueness. The more qualification you have specified in your CLASSPATH the less qualification you are allowed to specify in your import statements. There is no redundancy permitted. It is not enough to simply have your class files on the classpath. They must be in particular locations or inside jars on the CLASSPATH with just the right embedded path information. If you can't understand my handwaving, try reading Sun's explanation.

Sun tends to put the core classes in the java.* package, the slightly more peripheral classes such as Swing and JavaMail in javax.* and the classes they don't want users invoking directly in com.sun.*. The core distribution JDK contains all three types, though it excludes some of the javax and com.sun classes.

Traditionally packages have all lower case names, and like all Java names they are case sensitive. In contrast, Sun coding standards require classes to begin with a capital letter and ordinary variables and methods with a lower case letter. Embedded words are capitalised.

According the the naming conventions you should choose your package name to match your website. e.g. com.mindprod or ca.mindprod with the TLD first. This ensures globally unique package naming.

When you decide to bring your existing work into alignment with this naming convention, make sure you update:

matching directory names, including case.
package names in the package statements.
package names the import statements.
package names in your BAT files.
package names in your project files.
package names in all code that references your package.

It might be a good time to tidy your imports while you are at it.

CLASSPATH ¤ import ¤ jar ¤ java.exe ¤ package scope ¤ Sudsy's explanation of packages

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