Extending class with enum - java

Question

Lets say i have two classes A and B, which have theirs internal state represented by enum State. Class B extends A to add new functionality, but it also adds new internal state. To be precise, it adds fine-grained state support, by dividing one of the A`s state into two new states.

I also need to use PropertyChangeSupport to monitor state changes.

Currently I have all states defined in class A in enum State. I have protected methods setState() and getState(). To check on state from the outside, I wrote methods isAvaible(), isComplete(), etc. These methods are present only in appropriate class, depending on their hierarchy and state support.

I realy dont like the idea, having all states defined in parent class. How do I implement this correctly in Java? As far as I know, it`s not possible to use inheritance with enums.

You can't extend an enum - it would be easier to understand if you could show the structure of your code as code instead of describing it. — assylias
– assylias, Commented Apr 8, 2012 at 19:59

emory · Accepted Answer · 2012-04-08 20:04:33Z

4

Delegation is preferred to inheritance.

enum StateA
{
       AVAILABLE , COMPLETE ;
}

class A
{
      private StateA state ;
}

enum StateB
{
     AVAILABLE_TO_PUBLIC ( StateA . AVAILABLE ) , AVAILABLE_INTERNALLY ( StateA . AVAILABLE ) , COMPLETE ( COMPLETE ) ;

     StateB ( StateA sa )
     {
          this . sa = sa ;
     }

     private StateA sa ;
}

class B
{
      private StateB state ;
}

answered Apr 8, 2012 at 20:04

emory

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6 Comments

loybert Over a year ago

isn't it so, that AVAILABLE_TO_PUBLIC and AVAILABLE_INTERNALLY would be equal in this modeling?

emory Over a year ago

@loybert, AVAILABLE_TO_PUBLIC and AVAILABLE_INTERNALLY would not be equal in this model. AVAILABLE_TO_PUBLIC . equals ( AVAILABLE_INTERNALLY ) should be false. This "adds fine-grained state support, by dividing one of the A`s state into two new states."

loybert Over a year ago

ah cool. thought this would have the same internal integer-value, because of the same mapping to StateA.AVAILABLE

James Goodwin Over a year ago

Why don't you just combine StateA and StateB into one enum?

emory Over a year ago

@JamesGoodwin Actually, I like your idea better than my answer. I interpreted Puty's "dont like the idea, having all states defined in parent class" as meaning he did not want something like that. But if it were me, I would probably do as you recommend.

|

RalphChapin · Accepted Answer · 2012-04-09 18:37:16Z

I think you want to ditch the Enum concept except as something private to class A to help you program it. A's external state is available through methods like isAvailable() and isComplete(). Class B would extend A but should know nothing of A's enums, though it may have a private one of its own. If a change needs to affect A's state, and hence A's enum StateA, do it through method calls.

The "state" of an object of either class A or B should be determined, accessed, used, or whatever through its methods, not by accessing an enum value. In other words, an instance of A or B in itself is a complete definintion of state. It may not name the state, as an enum would, but it tells you everything you need to know about what the "state" allows and can do and how it can be changed.

Alternatively, you could set up an enum visible to everybody and define it globally. Then enum StateA is completely independent of class A, solving the basic design problem. Classes A and B, and others, would then depend on StateA to provide the language they use to communicate with each other and the rest of the system, making for a better design.

I kind of like the first idea better, because, by tweaking A and B, and extending them, you can really fine tune the meaning of each state and even add new states without making a formal announcement. (Adding a value to an independent StateA is not going to be a big deal, but programmers using StateA might wonder if they should be using the new value--a minor nuisance.)

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