Any good Design Patterns for extending Value Objects?

[dave taubler]

Hi,

I am using one estbalished Design Pattern, the Value Object pattern, to pass data from my EJB tier to my Web tier. The issue I seem to constantly come across is that depending on the situation, I somtimes want a little extra information sent along with a partiuclar Value Object. A simplified example:

Say I have a User object. Pretty much anytme I pass a User object from the EJBs to my Web app, I want that user's:
- ID
- First Name
- Last Name
- Email address
In some situations, however, I want to pass along a special "Display Name" value that is constructed by my EJBs. In other, rare, situations I want information about the relationship between this User and another User. Maybe in another very specific circumstance, I want information about how long the user has been online.

So I could of course add these fields to my User object, and wind up with a huge Value Object that passes along a bunch of fields that are, in most cases irrelevant. But that would get messy, cumbersome to maintain, and somewhat ineffecient in terms of network data transfer.

So I am looking to see if there are any design patterns that exist for such a dilemna. For example, the Decorator pattern is sort of the approach I am looking for, except that it's designed more to alter the behavior of the object, not the interface (i.e. I couldn't use it to add a getDisplayName() method to the User object.) I've been looking through pattern catalogs, but haven't yet found anything useful.

Any thoughts?

[Alexandru Popescu]

Maybe another solution would imply the usage of a DynaBeans.

--
:alex |.::the_mindstorm::.

[Ilja Preuss]

How does the client side know which methods the objects will have? Does it already know from context, or will it need to probe the VO?

[dave taubler]

DynaBeans... we're not using Struts (I'm not sure if DynaBeans would work here even if we were) but maybe I will check out the general structure to see if it would work.

Ideally, I would want the client to know the VO's interface and not have to probe. Especially with the other programmers I'm working with , I'm really into type-safety these days.

[James Carman]

Originally posted by dave taubler:
DynaBeans... we're not using Struts (I'm not sure if DynaBeans would work here even if we were) but maybe I will check out the general structure to see if it would work.

Ideally, I would want the client to know the VO's interface and not have to probe. Especially with the other programmers I'm working with , I'm really into type-safety these days.

DynaBeans are not Struts-specific. They are supported in Commons BeanUtils, too.

[Geoffrey Falk]

You should use composition. Create a new value object type that includes a copy of a UserVO along with the additional information.

Example:

public final class UserVO {
  private final String name;
  private final String id;
 
  public UserVO(String name, String id) {
    this.name = name;
    this.id = id;
  }
 
  public String getName() {return name;}
  public void getId() {return id;}
}
 
public final class DisplayUserVO {
  private final UserVO user;
  private final String displayName;
 
  public DisplayUserVO(UserVO user, String displayName) {
    this.user = user;
    this.displayName = displayName;
  }
 
  public UserVO getUser() { return user; }
  public String getDisplayName() {return displayName; }
}

Note both UserVO and DisplayUserVO are immutable and cannot be extended. This pattern is preferable to subclassing in most situations.

Best regards
Geoffrey
[ March 14, 2005: Message edited by: Geoffrey Falk ]