Java: Pattern for updating all equal objects (in the same context) that don't share the same reference

Question

In my java application I am using equal objects multiple times at different places. That means the equals method returns true, when comparing theses objects. Now I want to update one object and make the changes to all objects that are equal. Do you know if there is a pattern for that?

My concrete use case is: I am using JSF, JPA and CDI. A user is on web page that allows him to edit the detached entity EntityA. The page is sessionscoped. EntityA has two references to an EntityB (also detached). These objects can be same. Not the same reference, but they may be equal.

@Entity
public class EntityA {
   @OneToOne()
   private EntityB entity1;
   @OneToOne();
   private EntityB entity2;
}

The JSF view lets the uses select entity1 and entity2 from a selection list. It also shows some details of theses EntityBs and the user is allowed to edit entity1 and entity2 seperately. Everything works fine, except the user has choses the same (equal) EntityB for entity1 and entity2. Then, only the references to these objects are updated. Of course entity1 and entity2 are two different JPA entites, and are not the same reference. But I want to distribute the changes to all detached instances of EntityB. I have this situation hundreds of times in my application, so I dont want to take care about, which objects have to be updated in which situations. I need some solutation the does it for me automatically. One Idea was to keep all objects I use in this session in special list and every time a request was submitted and processed iterate over this map and change alle equal objects. But his sounds very dirty. Maybe there is a build in JPA function to make all equal objects the same reference. I dont know if this is possible. Do you have a solution for this? Thanks.

Answer 1

I'm going to abstract your problem out a bit here: if a change to one object requires changing a number of other objects, consider putting the field that you're changing in a separate object, and have all those objects reference it.

For example, if you have:

class MyClass {
  String info;
  int id;
}

and two instances of MyClass with the same 'id' should both be updated when the 'info' field changes then use this:

class myClass {
  myInfoClass info;
  int id
}

class myInfoClass {
  String value;
}

and give all instances of myClass that are equal the same instances of myInfoClass. Changing myClass.info.value will effectively change all instances of myClass, because they all hold the same instance of myInfoClass.

Sorry if I've got the syntax slightly wrong, I jump between languages a lot.

I use this technique in a game I wrote recently where a switch activates a door- both the switch and door have a Circuit object that holds a boolean powered field. The doors 'isOpen()' method simply returned circuit.powered, and when the switch is activated I just call switch.circuit.powered = true, and the door is automatically considered 'open'. Previously, I had it searching the game's map for all doors with the same circuit id, and changing the powered field on each.