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Message:

Public Methods in Non Public Classes

Posted by M Nelson on January 16, 2001 at 4:19 PM

In many cases, it is valuable for a package to expose a public interface
without exposing the details of how the interface is implemented.

For example, some of our applications have a "persistence"
package. This package contains:

1. A public interface that declares a set of methods for saving and retrieving
objets to and from persistent storage.

2. Some classes which have only package access that
implement the public interface with public methods.

3. A public class which implents the factory pattern. This
class has public methods that allow classes in other packages to
request an instance of an object that implements the public
interface.

When objects in the application want to save data, they
ask the factory (a public class) for an instance of a class which
implements the (public) interface. The factory can construct a new instance of
the (package scope) concrete class that implements the interface, and
return it to the requesting class. The requesting class is
restricted from relying on any knowledge of what specific
implementing class is being returned. Since the concrete classes
in the persistence package are package visible only, the
calling class cannot create instances of them directly. They
have to ask the public factory for an instance.

This enforces an abstraction that is very valuable. If we want
to install a different version of our application that uses
filesystem files instead of a database to save data, we can
modify the factory to instantiate and return a different set
of classes that implement the public interface. Since the
actual concrete classes in the package are only visible within
the package, we can rely on the assumption that we can replace
them without any other code having to change. As long as the
new classes implement the public interface,
they can be substitued, and the other classes
don't even know they have changed.

:-)

Mark