Comments on: How to create sealed classes in Java

By: dibblego

dibblego — Tue, 11 Sep 2007 01:55:00 +0000

Hi David,
This trick is well known. It is called a Closed Algebraic Data Type. I once wrote about how to emulate these in Java here:

http://blog.tmorris.net/maybe-in-java

By: David R. MacIver

David R. MacIver — Thu, 06 Sep 2007 07:25:00 +0000

Well, note that Java enum instances can be made anonymous classes, so you can already add custom logic to enums as long as you don’t need to add custom method signatures as well (but yes, this trick would work if you wanted custom method signatures).

Glad you like it. :-)

By: James Iry

James Iry — Thu, 06 Sep 2007 02:40:00 +0000

D'oh! What's sad is I had accidentally come awefully close to this before because I used it in an elaboration of the old "Java type safe enum pattern" wherein I had not only a collection of unique values, but each value had to have some custom logic. The result was that each enum was an instance of a unique subclass. I had just totally forgotten about it and never thought to generalize it for sealed classes which might have multiple instances. Thanks David!

By: David R. MacIver

David R. MacIver — Wed, 05 Sep 2007 15:50:00 +0000

Fair enough. It seemed too simple to be original to me, but I hadn’t seen it anywhere and no one else seemed to have heard of it, so I thought I’d post a note. :-)

(I guess more generally this is the observation that nested classes allow you to create an approximation to C++’s ‘friend’ status)

By: hiker

hiker — Wed, 05 Sep 2007 15:35:00 +0000

This actually is a well known trick in the C++ community as the language doesn’t offer sealed classes. Other classes can inherit from such ‘sealed’ class only if they are declared as friends to it. Nice find, though.