Question

I’m trying to extend a library that uses generics in entities and entities manager, so I have the following:

public class MyEntity extends ParentEntity extends BaseEntity

and

public class MyEntityManager extends ParentEntityManager extends BaseEntityManager<ParentEntity, ParentEntityDao>

Now my problem is, I’m using another class of the library to update the entity, and the method I need to call expects:

public <ENT extends BaseEntity> void update (Class<ENT> entityClass, BaseEntityManager<ENT> entityManager)

So when I try to call the method with MyEntity and MyEntityManager, it fails because my EntityManager extends from the BaseEntityManager with ParentEntity as parameter, not MyEntity, so they don’t match.

I would say that the cleanest way to solve this would be to copy the utility class that has the update method and extend it in my project, but I would like to make it generic so I can pass it EntityManagers that use any children class of ParentEntity, but I’ve been trying for a while and I cannot find the solution.

I tried changing the method signature to this:

public <ENT extends BaseEntity> void update (Class<ENT> entityClass, BaseEntityManager<? extends ENT> entityManager)

but I’m still getting a compiler exception…

EDIT: Modifying ParentEntityManager, BaseEntityManager or ParentEntity is not possible, since there are too many dependencies to those classes

Answer 1

Since it doesn’t appear you can easily fix the problem I would use a work around. You can use type erasure.

update((Class) MyEntity.class, myEntityManager);

This will compile with a warning. The warning is valid IMHO as your class structure isn’t entirely logical 😉 You can suppress this warning if you want.

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What I do is; the container class returns the type of object it contains/manages.

class BaseEntityManager<E> {
    private final Class<E> typeManaged;
    public Class<E> typeManaged() { return typeManaged; }
}

// remove duplicate class parameter
public <ENT extends BaseEntity> void update (BaseEntityManager<ENT> entityManager)

The manager know what type it manages and the update() method can ask it by calling typeManaged(). This avoid having to give a matching type or for this type to be checked.

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The problem you have is that

MyEntityManager extends BaseEntityManager<ParentEntity>

Which mean the class you have to provide is ParentEntity.class not MyEntity.class.

It appears the real solution is that you need to change

MyEntityManager extends BaseEntityManager<MyEntity>

This would be more logical IMHO and fix your problem.