Question

My title is a bit vauge, here’s the problem I’m talking about.

Say I have:

class Bill
{
   public String legislationID;
   public BillStatus currentStatus;
}

//and

class BillStatus
{
   public String description; 
}

When persisted, BillStatus corresponds to a table with one row per possible state of a piece of legislation. (e.g. “On House Floor”, “On Senate Floor”, etc).

The problem I always run into is this:

As soon as behavior starts to vary based on which instance of BillStatus is attached to Bill, the whole thing becomes hard to represent. I see two options:

1) A lot of switches in Bill that branch behavior depending on current BillStatus
2) methods in bill status that accept Bill as a parameter, and have switches that execute different logic depending on identity of instance. (so kind of mimicking classes implementing a common interface)

Both these methods seem sloppy. What I would like to be able to do is this:

class abstract BillStatus
{
    public abstract String getDescription();
    public abstract void callForVote(Bill bill);
}

Then:

@SpecialEntityTag(whenRowValue="HOUSEFLOOR")
class BillStatusHouseFloor extends BillStatus
{
     /* special overrides for some methods */     
}

class BillStatusGeneric extends BillStatus
{
    /* standard implementation of BillStatus */
}

This has to be a common problem. Am I thinking about it the wrong way? Basically the goal is to encapsulate behavior that changes with particular value of a non-primitive child property.

Answer 1

What prevents you from using your design?

Hibernate (or JPA in general) is able to handle inheritance, and you may store a whole inheritance hierarchy in a single table with a discriminator column telling which subclass to use for a particular row.

See http://docs.jboss.org/hibernate/core/3.6/reference/en-US/html_single/#inheritance-tableperclass for details.