Re: use case for extending enum, but this is not possible in java

On 15.06.2014 01:24, Laura Schmidt wrote:

 > I have found a need to extend enum, but this is not possible in java.
 > Take a look at the situation:
 >
 > In an application, there is an enum ListCommand that enumerates the
 > commands a user may execute on list entries:
 >
 > public enum ListCommand
 > {
 > OPEN,
 > EDIT,
 > DELETE;
 > }
 >
 > There is an interface that uses this enum:
 >
 > public interface ListCommandProcessor
 > {
 > void onListCommand (ListCommand cmd);
 > }
 >
 > And there is a generic Listing class that is used to show customized
 > lists in the GUI and which uses the above interface:
 >
 > public class Listing<T>
 > {
 > ...
 > ListCommandProcessor processor;
 > ...
 > }
 >
 > So far, so good.
 >
 > Now I am making a cut: The generic class Listing<T> should be moved into
 > a generic java library, so that it can be used by different
 > applications. So I also need to move the interface ListCommandProcesor
 > into this library, and this would implicate that I also move the enum
 > ListCommand into the library. But the enum ListCommand ist application
 > specific. If I move it to the library, it must be extendable somehow. If
 > there were no limitations in the java language, I would separate the
 > concerns of library and application like this:

 > How would you do this?

To me this is pretty clear: ListCommandProcessor is an interface and
needs an application specific implementation anyway to interpret the
argument to onListCommand(). As you do not seem to define specific
methods in ListCommand any implementation of onListCommand() is
basically totally free to do what it needs to do with the argument and
you do not have to constrain the argument type in any way. So you make
it generic. So we have

Library part:

public class Listing<T,C> {
  ...
  private ListCommandProcessor<C> processor;
  ...
}

public interface ListCommandProcessor<C> {
  void onListCommand(C cmd);
}

Specific implementation:

public class MyList {
}

public enum ListCommand {
  OPEN,EDIT,DELETE
}

public class MyApp implements ListCommandProcessor<ListCommand> {
  ...
  Listing<MyList,ListCommand> listing =
    new Listing<MyList,ListCommand> ();
  listing.setProcessor(this);

  public void onListCommand(ListCommand cmd)
  {
   switch (cmd) {
    ...
   }
  }
}

An alternative approach might be to remove ListCommandProcessor
altogether and use a more OO approach by implementing functionality
inside the ListCommand (what is an enum now but could be something else).

Library part:

public interface ListCommand<T> {
  void execute(T list);
}

public class Listing<T> {
   ...
   public void someMethod() {
     final T list = ...
     final ListCommand<T> cmd = ...
     cmd.execute(list);
   }
   ...
}

Specific implementation:

public class MyList {
}

// this could be anything, not just an enum
public enum MyListCommand implements ListCommandProcessor<MyList> {
   OPEN {
     @Override
     public void onListCommand(MyList cmd) {
       // whatever
     }
   },
   EDIT {
     @Override
     public void onListCommand(MyList cmd) {
       // whatever
     }
   }
   ...
}

Kind regards

     robert