Design pattern to enhance an existing data structure in Java with additional information at runtime

Question

I will start with a small example:

Imagine an application with a couple of entities.

EntityA -1---n-> EntityB -1---n-> EntityC

Let's say we have a service method that returns a list of EnityC instances. In the UI we want to display EntityC but also add some additional information to the entity that is only relevant for the UI (maybe a css class or so). A common way to solve this would be to create a wrapper arround EntityC that can also carry the additional information.

public class EntityCWrapper implements EntityC, AdditionalInfo { ...}

or maybe use a transfer object as simple data structure:

public EntityTO {
  public EntityC entity;
  public AdditionalInfo info;
}

But what if the service returns a list of EnitityA instances and we need to attach AdditionalInfo to all entities in the instance graph (including the referenced entity instances of EntityB and EntityC)?

Does anyone have an idea or can point me to a design pattern suitable in this situation?

just an example of an object graph, could be sth. like: Category hasmany Items hasmany ItemDetails — sprehn
– sprehn, Commented Feb 27, 2012 at 20:56
btw. the first idea could be implemented nicly using the Decorator pattern. — sprehn
– sprehn, Commented Feb 27, 2012 at 21:47

Ralph · Accepted Answer · 2012-02-27 21:11:09Z

1

Have a look at the Role Object Pattern.

It descripes how an object can be extended with different stuff (called Roles). The pattern is more about how to add Bussiness Logic (that is why it is called roles) but may the idea helps you.

answered Feb 27, 2012 at 21:11

Ralph

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3 Comments

sprehn Over a year ago

This was a good hint. In this case the classes in the graph need to be prepared with such an interface that behavior can be added, removed, and queried for. I guess it is not too intrusive. (much like the preperation required to implement the visitor pattern). For this additional info usecase the delegation back to the core object would not be required.

sprehn Over a year ago

and this very thing is called Extension Objects Pattern :-)

Ralph Over a year ago

@sprehn you are right, Extension Objects Pattern was the second pattern I was searching for, but I forget its name.

Kashyap · Accepted Answer · 2012-02-27 20:59:22Z

0

Assuming AdditionalInfo is instance specific (i.e. each instance of EntityC has a unique 121 relation to an AdditionalInfo instance) best option would be to enhance EntityC definition to include AdditionalInfo inside it as member/..., which is optional and filled up by you.

If you don't have control over EntityC then between teh two options you have given, I would say TransferObject seems better. You won't have to create new objects (of EntityC) that way.

answered Feb 27, 2012 at 20:59

Kashyap

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1 Comment

sprehn Over a year ago

I've seen solutions where the entity was enhanced like this (it probably is the easiest way). However, in some cases it is not appropriate for it would break the layer architecture. So my question is what else can we do in such a case.

Woot4Moo · Accepted Answer · 2012-02-27 21:05:43Z

So your data model is essentially 1 to n using the following object as an example (contrived as it may be)

class Thing  
{  
   Collection<DooDad> dooDads;
}    

class DooDad  
{  
   Collection<DooDadDetail> details;  
}  

class DooDadDetail  
{  
   ... 
}

So I would treat this as my model, in terms of n-tier architecture and therefore this should map back to my database exactly. What comes up is you want your model to do something, whatever that may be. So you should create an object that is composed of Thing that can interact with business logic or outside services. Creating a transfer object would be correct in this case as it ensures your model remains valid and that the interfaces between both you and the outside service are not dependant on your model. It is sort of a Facade pattern and a Data Transfer Object.

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