Java generics - Class constructor argument issue

Question

I've used Java for quite some time now, but some things are still not very clear to me, especially when it comes to generics...

Here's the thing: I have this Search class that I'm using (see here for details), that is constructed like this:

public Search(Class<?> searchClass) {...}

Further more, I have a parametrized generic wrapper around this, as follows:

public class HibernateSearch<E> extends Search implements Serializable {
    public HibernateSearch(Class<E> entityClass) {
        super(entityClass);
    }
    // ... omitted for brevity
}

Now, what I need is the following: I'd like to create parametrized class, that contains this class as its field, e.g.

public class BaseSelectorComposer<T> extends SelectorComposer<Window> {

    private HibernateSearch<T> searchObject;
    ...
    @Override
    public void doAfterCompose(Window comp) throws Exception {
        super.doAfterCompose(comp);
        this.searchObject =
             new HibernateSearchObject<T>( now what...??? );
        ...
    }
    ...
}

I think that the problem I'm facing is obvious from the given example.
Can someone advise what can be done here, or some alternative ?

score 5 · Accepted Answer · answered Jun 06 '13 at 13:15

The easiest way would be to pass the responsibility on to whoever actually instanciates and uses instances of BaseSelectorComposer, i.e.:

public class BaseSelectorComposer<T> extends SelectorComposer<Window> {

    private HibernateSearch<T> searchObject;
    private final Class<T> theType;

    public BaseSelectorComposer(Class<T> token) {
        theType = token;
    }

    ...
    @Override
    public void doAfterCompose(Window comp) throws Exception {
        super.doAfterCompose(comp);
        this.searchObject = new HibernateSearchObject<T>(theType);
        ...
    }
    ...
}

If your BaseSelectorComposer is an abstract subclass, which is only ever used like

class IntSelectorComposer extends BaseSelectorComposer<Integer> {
    ...
}

i.e., as base class for classes, which "bind" the type parameters, there are ways to obtain the type information via reflection (though this is incredibly ugly, since this is not really well-supported in the API IMHO).

Ivaylo Slavov · Answer 2 · 2013-06-06T14:25:14.923

In general, this is a hard-to-workaround scenario. Coming from .NET background, I have personally found very few cases where I could work this out, since Java does not support static generic type resolution (like for instance in .NET where you can use typeof(T)).

To work this around I recommend that you find a way to either get the class externally, or an instance of that class. A possible solution would be to change the doAfterCompose method in one of the following ways:

@Override
public void doAfterCompose(Window comp, Class<?> entityClass) throws Exception {
    super.doAfterCompose(comp);
    this.searchObject =
         new HibernateSearchObject<T>(entityClass);
    ...
}

-or-

@Override
public void doAfterCompose(Window comp, SomeEntity entity) throws Exception {
    super.doAfterCompose(comp);
    this.searchObject =
         new HibernateSearchObject<T>(entity.getClass());
    ...
}

If you cannot modify the method signature (I see it is an override), try to use a member of the class BaseSelectorComposer<T> that will hold the desired Class<?> instance.

public class BaseSelectorComposer<T> extends SelectorComposer<Window> {
    private Class<?> entityClass;

    public BaseSelectorComposer<T>(Class<?> entityClasss) {
        this.entityClass = entityClass;
    }

   ....
    @Override
    public void doAfterCompose(Window comp) throws Exception {
        super.doAfterCompose(comp);
        this.searchObject =
            new HibernateSearchObject<T>(this.entityClass);
       ...
    }
}

Java generics - Class constructor argument issue

2 Answers2