无法在泛型类中调用实体保存

Question

public class JobAssetService extends GenericService<JobAssetService, JobAsset, JobAssetDao> {

}

I'm trying to provide generic save() functionality to my service layer but it doesn't seem to like what I'm passing to dao.save() . This seems like this should work...

Incompatible Types Required: M Found: java.lang.object

public class GenericService<T, M, Dao extends GenericDao> {

    protected Dao dao;
    protected EntityManager em;

    public GenericService() {

    }

    //map the dao/entity manager when instantiated
    public GenericService(Class<Dao> daoClass) {
        //map entity manager & dao
        //code removed for readability
    }

    public M save(M entity) {
        EntityTransaction tx = em.getTransaction();
        tx.begin();
        entity = dao.save(entity); //IntelliJ complains about this
        tx.commit();

        return entity;
    }
}

Answer 1

In IntellijIDEA you can put your cursor on the error, then use ALT + ENTER and intellij will probably suggest you to cast the result of

dao.save(entity)

as "M"

entity = (M) dao.save(entity); 

Answer 2

you should make GenericDao use generics too:

class GenericDao<M> {
    public M save(M entity) {
        ...
    }
}

then extend your generic service as follows:

public class GenericService<T, M, Dao extends GenericDao<M>> {