Java Static Problem

see also JavaSingleton

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 public class Registry {
     private static List register = new ArrayList();
     public static void register(Object obj){
          register.add(obj);
     }
     public static boolean isRegistered(Object obj){
          return register.contains(obj);
     }
 }

The above Registry class makes use of a private static List to hold objects. There are many cases where this is fine. However, What would happen if Registry were loaded by two different ClassLoaders. I think there would be two instances of the class, and therefore two instances of the register list. -- EricHerman

More generally, since static methods are not polymorphic, you may find using them to encourage a less objecty (and perhaps more smelly) style.

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There's also the trap that I fall into, that static members are not inherited. So if you have a FooRegistry which extends Registry, it needs its own static ArrayList and you have to re-write the register etc. methods not in terms of super().

Though personally I don't find these kinds of "static classes" objectionable, especially where they're used for Factory or registries or in-general keeping track of instances of a set of related classes. I'm also happy that there exists the Collections class (I like this 'pluralization' naming convention, too) in the Java library for its methods like #sort(), but there are others there - especially things like #unmodifiableSet() that would benefit from MoveMethod refactoring.

In ZoIsite? I use "static classes" to implement a kind of Facade - that's how I think of it anyway. Whenever I write one of these, I always include something like

 private ClassName() {} // static class

-- KarlKnechtel

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Interesting. Where would you move 'Collections.unmodifiableSet(<Set>)' to?

'new UnmodifiableSet?(<Set>)' That's what happens behind the scenes in Collections.unmodifiable<Blah>() anyway.

Something like that. and Collections.singleton(Object) - I think that was the name; the one that makes an immutable set containing just the specified Object - definitely needs that kind of treatment. Say, new UnmodifiableSet?(Object[]) with a one-element array.

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Static methods in Java aren't really class methods. They're really global methods, since they're bound at compile-time and can't be overridden. In the past I've found myself writing bizarre hacks to get around this limitation in language design, though these days I just use another language if I have a choice. -- francis

CategoryComputerLanguage?