Generic method without type argument

Question

I was wondering if there's a way to, in Java, use a generic type within a generic method without requiring an argument specifying the class type. Here's the normal way of doing it:

public <T> T doSomethingGeneric(Class<T> type, int a) {
    return (T) doSomethingElse(type, a);
}

I would like this as an alternative:

public <T> T doSomethingGeneric(int a) {
    return (T) doSomethingElse(/* some magic with T here */, a);
}

Background: I'm writing utility methods with Hibernate. Here's an example of what I'm actually trying to do, and you can infer how it applies to this problem.

public static <M extends Model> M get(Class<M> type, Serializable id) {
    Session session = newSession();
    Transaction t = session.beginTransaction();
    Object model = null;
    try {
        model = session.get(type, id);
        t.commit();
    } catch (HibernateException ex) {
        t.rollback();
        throw ex;
    }
    return (M) model;
}

Is this possible? If so, how is it done? Thanks!

Out of curiosity, why are you writing your own `get()` method and not using [`EntityManager`](http://docs.oracle.com/javaee/6/api/javax/persistence/EntityManager.html)? — Matt Ball, Feb 28 '13 at 20:50
Because I'm new to Hibernate and didn't know that EntityManager existed. Thanks for the tip. :) — Blake Beaupain, Feb 28 '13 at 20:51
In that case, I strongly recommend at least skimming the relevant part of the Java EE tutorial: http://docs.oracle.com/javaee/6/tutorial/doc/bnbpy.html — Matt Ball, Feb 28 '13 at 20:54

score 4 · Accepted Answer · edited May 23 '17 at 11:49

4

You want to get a Class<T> at runtime to pass to doSomethingElse()? No, that is not possible.

If you describe why doSomethingElse needs a Class<T>, perhaps we could suggest a workaround.

edited May 23 '17 at 11:49

Community

1
1

answered Feb 28 '13 at 20:39

Matt Ball

354,903
100
647
710

I'm writing utility methods with Hibernate. Here's an example of what I'm actually trying to do, and you can infer how it applies to this problem: https://gist.github.com/blakeman8192/de57273f472ceaf1b7d8 – Blake Beaupain Feb 28 '13 at 20:42
1

Yeah, this isn't possible. That's why `Session#get()` needs a `Class` in the first place. – Matt Ball Feb 28 '13 at 20:44

score 2 · Answer 2 · answered Feb 28 '13 at 20:39

2

This isn't possible: the code of doSomethingGeneric is only compiled once, unlike the instantiation of a template in C++. The type T is erased to Object in the process.

Notice also that (T) will be flagged as an unchecked downcast.

answered Feb 28 '13 at 20:39

Marko Topolnik

195,646
29
319
436

Generic method without type argument

2 Answers2