Factory Design Pattern made simple using Enums

Writing X2J I encountered a state most of us encountered before: The need for a Factory Design Pattern. In my situation, during parsing (using SAX), there is an interface called ContentHandler (javadoc) which is required by my handler. The handler should be created according to the type of the element being currently parsed. An obvious case for a Factory.

Now, I already have an enum such as:

public enum ObjectType {
    ELEMENT,
    ARRAY;
}

I can have a factory class like this:

public class HandlerFactory() {
    /* .. singleton stuff .. */

    public ContentHandler createHandler(ObjectType type) throws X2JException {
        ContentHandler handler = handlers.get(type);

        if (handler == null) {
          switch (type) {
            case ELEMENT:
              handler = new ElementHandler();
              break;
            case ARRAY:
              handler = new ArrayHandler();
              break;
          }
          handlers.put(type, handler);
        }

        return handler;
    }
}

However, since enums can now have properties and methods as part of Tiger's new features and they're not just a group of constant integers, I can do the following:

public enum ObjectType {
    ELEMENT(ElementHandler.class),
    ARRAY(ArrayHandler.class);

    public Class<? extends ContentHandler> handlerType;

    ObjectType(Class<? extends ContentHandler> handlerType) {
        this.handlerType = handlerType;
    }
}

And the factory becomes very simple, such as:

public ContentHandler createHandler(ObjectType type) throws X2JException {
        ContentHandler handler = handlers.get(type);

        if (handler == null) {
            try {
                handler = type.handlerType.newInstance();
            } catch (Exception e) {
                throw new X2JException(String.format(\"Fail to create handler %s\", type), e);
            }

            handlers.put(type, handler);
        }

        return handler;
    }

The added bonus is that I don't have to worry about this factory class anymore - Whatever new enum values I add, the factory will not be changed. I wish I could somehow get away from calling newInstance(), but I guess it's neglegible because I do that once per enum value, which isn't a lot.

Simple, eh?

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