Java generics type parameters

Question

Consider the following code:

public abstract class MyObject {
}

public interface Dao<T extends MyObject> {
}

public abstract class JsonDao<T extends MyObject> implements Dao<T> {
}

public abstract class SqliteDao<T extends MyObject> implements Dao<T> {
}

public interface DaoFactory<*????*> {
    public *????* getDao(Class<? extends MyObject> objectClass);
}

Now what I'd like is for getDao to be able to instanciate both JsonDao and SqliteDao, but parametrized with a class extending MyObject, basically something like this:

public interface DaoFactory<T<? extends MyObject> extends Dao<? extends MyObject>> {
    public <U extends MyObject> T<U> getDao(Class<U> objectClass);
}

Is something similar even possible or does type erasure prevent it?

Answer 1

Does this fit your needs?

public interface DaoFactory<O extends MyObject, D extends Dao<O>> extends Dao<O> {
    public D getDao(Class<O> objectClass);
    //or for any subclass
    public <RO extends O, RD extends D<RO>> RD getSubDao(Class<RO> objectSubclass);

    //or maybe this
    public <RO extends O> Dao<RO> getSimpleSubDao(Class<RO> objectSubclass);
}

Answer 2

You could declare DaoFactory as follows:

public interface DaoFactory<D extends Dao<? extends MyObject>> {

    <T extends MyObject> D getDao(Class<T> type);
}

Then, JsonDaoFactory could be implemented as follows:

public class JsonDaoFactory implements DaoFactory<JsonDao<? extends MyObject>> {

    private final Map<Class<? extends MyObject>, JsonDao<? extends MyObject>> daos =
            new HashMap<>();

    public JsonDaoFactory() {
        daos.put(Contact.class, new JsonContactDao());
        daos.put(Customer.class, new JsonCustomerDao());
    }

    @Override
    @SuppressWarnings("unchecked")
    public <T extends MyObject> JsonDao<T> getDao(Class<T> type) {
        return (JsonDao<T>) daos.get(type);
    }
}

I've used a Map to make every descendant of MyObject match its corresponding JsonDao (here I've used Contact and Customer , and JsonContactDao and JsonCustomerDao , respectively). I've suppressed an unchecked cast warning because I'm explicitly filling the map in the constructor, so I'm sure there won't be a ClassCastException .

For completeness, here's the code for SqliteDaoFactory :

public class SqliteDaoFactory implements DaoFactory<SqliteDao<? extends MyObject>> {

    private final Map<Class<? extends MyObject>, SqliteDao<? extends MyObject>> daos =
            new HashMap<>();

    public SqliteDaoFactory() {
        daos.put(Contact.class, new SqliteContactDao());
        daos.put(Customer.class, new SqliteCustomerDao());
    }

    @Override
    @SuppressWarnings("unchecked")
    public <T extends MyObject> SqliteDao<T> getDao(Class<T> type) {
        return (SqliteDao<T>) daos.get(type);
    }
}

The code for both factories is almost duplicated, but I couldn't find a way to solve this.

Here's how to use these factories:

JsonDaoFactory jsonDaoFactory = new JsonDaoFactory();
JsonDao<Contact> contactJsonDao = jsonDaoFactory.getDao(Contact.class);
JsonDao<Customer> customerJsonDao = jsonDaoFactory.getDao(Customer.class);

SqliteDaoFactory sqliteDaoFactory = new SqliteDaoFactory();
SqliteDao<Contact> contactSqliteDao = sqliteDaoFactory.getDao(Contact.class);
SqliteDao<Customer> customerSqliteDao = sqliteDaoFactory.getDao(Customer.class);