Defining a Package
To create a package is quite easy: simply include a package command as the first statement
in a Java source file. Any classes declared within that file will belong to the specified package.
The package statement defines a name space in which classes are stored. If you omit the
package statement, the class names are put into the default package, which has no name.
(This is why you haven't had to worry about packages before now.) While the default package
is fine for short, sample programs, it is inadequate for real applications. Most of the time,
you will define a package for your code.
This is the general form of the package statement:
package pkg;
Here, pkg is the name of the package. For example, the following statement creates a package
called MyPackage.
package MyPackage;
Java uses file system directories to store packages. For example, the .class files for any
classes you declare to be part of MyPackage must be stored in a directory called MyPackage.
Remember that case is significant, and the directory name must match the package name
exactly.
More than one file can include the same package statement. The package statement
simply specifies to which package the classes defined in a file belong. It does not exclude
other classes in other files from being part of that same package. Most real-world packages
are spread across many files.
You can create a hierarchy of packages. To do so, simply separate each package name
from the one above it by use of a period. The general form of a multileveled package statement
is shown here:
package pkg1[.pkg2[.pkg3]];
A package hierarchy must be reflected in the file system of your Java development
system. For example, a package declared as
package java.awt.image;
needs to be stored in java\awt\image in a Windows environment. Be sure to choose your
package names carefully. You cannot rename a package without renaming the directory in
which the classes are stored.
Finding Packages and CLASSPATH
As just explained, packages are mirrored by directories. This raises an important question:
How does the Java run-time system know where to look for packages that you create? The
answer has three parts. First, by default, the Java run-time system uses the current working
directory as its starting point. Thus, if your package is in a subdirectory of the current
directory, it will be found. Second, you can specify a directory path or paths by setting the
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