Any nice way to make two immutable objects refer to eachother?

Question

Take these two Java classes:

class User {
   final Inventory inventory;
   User (Inventory inv) {
       inventory = inv;
   }
}

class Inventory {
   final User         


        

Answer 1

B: "Inventory created by the User is our last hope".
Y: "No, there is another."

If you abstract the references to a third party, you can control the relationship therein.

For example.

public class User
{
    private final String identifier;  // uniquely identifies this User instance.

    public User(final String myIdentifier)
    {
        identifier = myIdentifier;

        InventoryReferencer.registerBlammoUser(identifier); // Register the user with the Inventory referencer.
    }

    public Inventory getInventory()
    {
        return InventoryReferencer.getInventoryForUser(identifier);
    }
}

public interface Inventory // Bam!
{
    ... nothing special.
}

// Assuming that the Inventory only makes sence in the context of a User (i.e. User must own Inventory).
public class InventoryReferencer
{
    private static final Map referenceMap = new HashMap();

    private InventoryReferencer()
    {
        throw ... some exception - helps limit instantiation.
    }

    public static void registerBlammoUser(final String identifier)
    {
        InventoryBlammo blammo = new InventoryBlammo();
        referenceMap.add(indentifier, blammo);
    }

    public static void registerKapowUser(final String identifier)
    {
        InventoryBlammo kapow = new InventoryKapow();
        referenceMap.add(indentifier, kapow);
    }

    public static Inentory getInfentoryForUser(final String identifier)
    {
        return referenceMap.get(identifier);
    }
}

// Maybe package access constructors.
public class InventoryBlammo implements Inventory
{
    // a Blammo style inventory.
}

public class InventoryKapow implements Inventory
{
    // a Kapow style inventory.
}