Question

I am trying to determine if it is possible to use Generics in the following situation. It can best be described by my code. (This code is just an example; I took out most of the code not relevant to the problem I’m having.)

public class FooBar {

    public <T extends MyModel> Map<Class<T>, List<T>> convertToModelList(
            Map<String, Class<T>> infoMap) {

        // do stuff...
    }
}

public class MyClient {

public void doSomething() {
    Map<String, Class<? extends MyModel>> oldMap = new HashMap<String, Class<? extends MyModel>>();
    oldMap.put ("car", Car.class);
    oldMap.put("truck", Truck.class);

    FooBar f = new FooBar();
    Map<Class<? extends MyModel>, List<? extends MyModel>> newMap = f
            .convertToModelList(oldMap);
}
}

public class Car extends MyModel {

}

public class Truck extends MyModel {

}

public class MyModel { 

}

The compiler is saying that I can’t call convertToModelList (in MyClient) because a Map<String, Class<? extends MyModel>> is not equivalent to Map<String, Class<T>>. I somewhat understand why this is occurring, but is there a way around this?

EDIT:

To be more specific, the problem I’d like to solve is using Generics in the convertToModelList() method above. If I can’t use generics here, then whatever I return from that method has to be cast on the client. For example, if I change FooBar to this:

public class FooBar {

    public Map<Class<? extends MyModel>, List<? extends MyModel>> convertToModelList(
            Map<String, Class<? extends MyModel>> infoMap) {

        // do stuff...
    }
}

If I pass in a

Map<String, Class<Truck>> 

to convertToModelList, it will return a

Map<Class<Truck>, List<Truck>>

, however the client won’t know it’s a Truck – all it will know is that it’s of type MyModel – using generics lets me avoid the cast to Truck in the MyClient code:

public class MyClient {

public void doSomething() {
    Map<String, Class<? extends MyModel>> oldMap = new HashMap<String, Class<? extends MyModel>>();
    oldMap.put ("car", Car.class);
    oldMap.put("truck", Truck.class);

    FooBar f = new FooBar();
    Map<Class<? extends MyModel>, List<? extends MyModel>> newMap = f
            .convertToModelList(oldMap);

            // I'm trying to avoid this cast
            List<Truck> trucks = (List<Truck>)newMap.get(Truck.class);
}
}

Answer 1

Although the common superclass of Car and Truck is MyModel, the common superclass of List<Car> and List<Truck> is Object. The map should be of type Map<Class<? extends MyModel>, Object>>. You better wrap the map with some invariant checks and casting instead of getting info directly from the map.

This may be helpful too: Heterogeneous container to store genericly typed objects in Java