Erro com Generics

package dao;

import java.io.Serializable;
import java.lang.reflect.ParameterizedType;
import java.util.List;

import com.db4o.ObjectContainer;
import com.db4o.ObjectSet;
import com.db4o.query.Query;

import domain.Idable;

public class Dao<T> implements GenericDAO<T> {

	private ObjectContainer session;
	private Class<T> persistentClass;

	@SuppressWarnings("unchecked")
	public Dao(ObjectContainer session) {
		this.session = session;
		this.persistentClass = (Class<T>) ((ParameterizedType) (getClass()
				.getGenericSuperclass())).getActualTypeArguments()[0];
	}

	public Class<T> getPersistentClass() {
		return persistentClass;
	}

	public T makePersistent(T u) {
		if (u instanceof Idable) {
			Idable idOb = (Idable) u;
			Db4oObjectPool.getInstance().assignId(session, idOb);
		}
		this.session.set(u);
		this.session.commit();
		return u;
	}

	@SuppressWarnings("unchecked")
	public T findById(Object id) {
		Query q = this.session.query();
		q.constrain(this.persistentClass);
		q.descend("id").constrain(id);
		ObjectSet<T> set = q.execute();
		if (set.hasNext())
			return (set.next());
		return null;
	}

	@SuppressWarnings("unchecked")
	public List<T> findByExample(T t) {
		return (List<T>) this.session.get(t);
	}

	public ObjectSet<T> findAll() {
		return this.session.query(this.persistentClass);
	}

	public void delete(T u) {
		this.session.delete(u);
		this.session.commit();
	}

	public void deleteAll() {
		for (ObjectSet<T> os = (ObjectSet<T>) findAll(); os.hasNext();) {
			T t = os.next();
			this.session.delete(t);
		}
		this.session.commit();
	}
}