Mismatching return of a query with NamedQuery

Question

Hi to all i'm new in J2EE world. I'm trying to use @Namequery to get a List of String from my model(the class is called Element), in this case the attribute name. Why when I use @NamedQuery(name="Element.findAllNames",query="SELECT e.name FROM Element e") and I use it on my ElementMgrBean using:

public List<String> getAllElementsName() {
    return  em.createNamedQuery(Element.FIND_ALLNAMES,Element.class).getResultList();
}

the result appears to be a list of elements instead of a list of String as I expected. Someone can explain me why Java says: "Type mismatch: cannot convert from List of Element to List of String"?? PS: I have defined:public static final String FIND_ALLNAMES = "Element.findAllNames";

Answer 1

When you write em.createNamedQuery("NameOfNamedQuery", MyClass.class), you say you want to fetch MyClass instances (in your query MyClass==Element). The problem is now, that your query SELECT e.name FROM Element e returns Strings instead of Element instances (as the e.name fields are of type Strings).

In order to correct the problem you either correct the call to em.createNamedQuery(Element.FIND_ALLNAMES, String.class) (the preferred solution, as it returns a TypedQuery<String> instance), or you remove the second parameter: em.createNamedQuery(Element.FIND_ALLNAMES) (in this case you are not type-safe anymore, as it returns a Query instance).