Packages - The Java Programming Language - page 705

protected Package definePackage(String name, String specTitle,
String specVersion, String specVendor, String implTitle, String im-
plVersion, String implVendor, URL sealBase)
Returns a Package object with the given name, and with spe-
cification and implementation values set to the corresponding
arguments. If sealBase is null the package is unsealed, other-
wise it is sealed with respect to that URL . The Package object for
a class must be defined before the class is defined and pack-
age names must be unique within a class loader. If the pack-
age name duplicates an existing name, an IllegalArgumentEx-
ception is thrown.
You can get the Package object for a given class from the getPackage
method of the Class object for that class. You can also get a Package
object by invoking either the static method Package.getPackage with the
name of the package, or the static method Package.getPackages , which
returns an array of all known packages. Both methods work with respect
to the class loader of the code making the call by invoking the getPack-
age or getPackages method of that class loader. These class loader meth-
ods search the specific class loader and all of its parents. If there is no
current class loader then the system class loader is used. Note that the
class loader methods will return null if the package is unknown because
no type from the package has been loaded.
When a shepherd goes to kill a wolf, and takes his dog along to
see the sport, he should take care to avoid mistakes. The dog has
certain relationships to the wolf the shepherd may have forgotten.
Robert Prisig, Zen and the Art of Motorcycle Maintenance