Parameterized class in Java

Question

What am I doing wrong in this Java class? clazz field is allways null. Shouldn't clazz be automatically populated with the type defined on the concrete class?

Thanks!

public abstract class AbstractDAO<E extends Domain, T extends Number> {

    protected EntityManager em;
    private Class<E> clazz;

    public AbstractDAO(final EntityManager em) {
        this.em = em;
    }

    public E find(T id) {
        return em.find(clazz, id);
    }

    public List<E> findAll() {
        CriteriaBuilder cb = em.getCriteriaBuilder();
        CriteriaQuery<E> cq = cb.createQuery(clazz);
        Root<E> from = cq.from(clazz);
        CriteriaQuery<E> select = cq.select(from);
        return em.createQuery(select).getResultList();
    }

    // other methods
}

You should be asking this question: stackoverflow.com/questions/18707582/… — BalusC
– BalusC, Commented Sep 18, 2015 at 20:18
wouldn't that be nice :) or if we could do E.class ... unfortunately we can't do that in java. — ZhongYu
– ZhongYu, Commented Sep 18, 2015 at 20:23

bmargulies · Accepted Answer · 2015-09-18 20:15:24Z

2

No, nothing in Java auto-populates a Class<T> field in a generic class. If your generic class needs to know the type of one of the type parameters, you must add a constructor argument of type Class<T> and initialize it from there. See, for example, the class EnumMap in the JDK.

answered Sep 18, 2015 at 20:15

bmargulies

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1 Comment

Joao Brito Over a year ago

What I meant by auto-populate was inferred from the concrete class. Issue solved with part of this: stackoverflow.com/a/18709327/1831948 Thanks!!

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