Naming a Package - Java Tutorial 5.0 英文版

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Naming a Package

With programmers worldwide writing classes, interfaces, enums, and annotations using the Java programming language, it is likely that two programmers will use the same name for two different classes. In fact, the previous example does just that: It defines a Rectangle class when there is already a Rectangle class in the java.awt package. Still, the compiler allows both classes to have the same name. Why? Because they are in different packages, and the fully qualified name of each class includes the package name. That is, the fully qualified name of the Rectangle class in the graphics package is graphics.Rectangle, and the fully qualified name of the Rectangle class in the java.awt package is java.awt.Rectangle.

This generally works just fine unless two independent programmers use the same name for their packages. What prevents this problem? Convention.

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By Convention:  Companies use their reversed Internet domain name in their package names (for example, com.company.package). Name collisions that occur within a single company need to be handled by convention within that company, perhaps by including the region or the project name after the company name (for example, com.company.region.package).

In some cases, the internet domain name may not be a valid package name. This can occur if the domain name contains a hyphen or other special character, if the package name begins with a digit or other character that is illegal to use as the beginning of a Java name, or if the package name contains a reserved Java keyword, such as "int". In this event, the suggested convention is to use an underscore. For example:

Domain name	Package name
clipart-open.org	org.clipart_open
free.fonts.int	int_.fonts.free
poetry.7days.com	com._7days.poetry

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