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?

This isn't really an issue with type erasure. Java just doesn't have a feature for a syntactical construct like T<U>. — Radiodef
– Radiodef, Commented Jun 9, 2017 at 15:50
interface DaoFactory<U extends MyObject, T extends Dao<U>> ? — Jorn Vernee
– Jorn Vernee, Commented Jun 9, 2017 at 15:57
@Jorn Vernee I can't because the factory should instanciate Daos for multiple classes extending MyObject. — Gaetan L.
– Gaetan L., Commented Jun 9, 2017 at 15:58

fps · Accepted Answer · 2017-06-11 03:09:54Z

1

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);

edited Jun 11, 2017 at 3:09

answered Jun 9, 2017 at 19:43

fps

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7 Comments

Gaetan L. Over a year ago

Yeah but I posted only a fragment of my class hierarchy, I use the genericity further down in the concrete classes implementing DaoFactory.

Gaetan L. Over a year ago

Yes I think you're on to something like @Aarjav below, I'll try that tomorrow and I will let you know :)

Gaetan L. Over a year ago

Okay your code works, but I'm still stuck at instanciation: I have multiple concrete implementations of JsonDao, ie.: JsonContactDao (Contact extends MyObject, JsonContactDao extends JsonDao<Contact>). I can't find a way to instantiate a JsonContactDao in getDao.

Gaetan L. Over a year ago

Same functions with differents parameters (file name for json, different fields to load in Contact and Customer). The problem is I'm trying to "inherit" both the storage type (Json, Sqlite, etc.) and the entity (Contact/Customer) and at some point I can't reconcile the two.

Gaetan L. Over a year ago

That's very nice of you. If you vanna see the whole mess my code is here: bitbucket.org/GaetanL/glace/src, in glace/app/src/main/java/com/gaetanl/gl/persistence/ for the abstract/sdk part and glace/app/src/main/java/com/gaetanl/glace/persistence/ for the concrete implementation.

|

Aarjav · Accepted Answer · 2017-06-09 16:00:29Z

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);
}

answered Jun 9, 2017 at 16:00

Aarjav

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1 Comment

Gaetan L. Over a year ago

Sounds promising! I will test this tommorow I'll let you know!

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