More Features of the Java Language

Creating a Package

To create a package, you simply put a class or interface in it. To do this, you put a package statement at the top of the source file in which the class or interface is defined. For example, the following code appears in the source file Circle.java and puts the Circle class in the graphics package:

package graphics;
class Circle extends Graphic implements Draggable {
    . . .
}

The Circle class is a member of the graphics package.

You must include a package statement at the top of every source file that defines a class or interface that is to be a member of the graphics package. So you would also include the statement in Rectangle.java and so on:

package graphics;
class Rectangle extends Graphic implements Draggable {
    . . .
}

The scope of the package statement is the entire source file, so all classes and interfaces defined in Circle.java and Rectangle.java are also members of the graphics package. If you do not use a package statement, your class or interface ends up in the default package, which is a package that has no name. Generally speaking, the default package is only for small or temporary applications or when you are just beginning development. Otherwise, classes and interfaces belong in named packages.

Naming a Package

With programmers all over the world writing Java classes and interfaces, it is conceivable and even 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. Yet, the compiler allows both classes to have the same name. Why? Because they are in different packages and the actual name of each class includes the package name. That is, the name of the Rectangle class in the graphics package is really graphics.Rectangle, and the name of the Rectangle class in the java.awt package is really java.awt.Rectangle.

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

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By Convention: Companies use their reversed Internet domain name in their package names, like this: 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 project name after the company name, for example, com.company.region.package.

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More Features of the Java Language