Advanced generics, Maps and double use of '?'

Question

I have a variable declared like this
private Map<Class<? extends Customer>, CustomerManager<? extends Customer>> customerManagers;
This map maps from some class of customer to the manager that manages customers of this class. To fetch the correct manager I have a method
public <V extends Customer> CustomerManager<V> getCustomerManager(Class<V> customerClass) {
     CustomerManager<V> customerManager = customerManagers.get(customerClass);
     return valueManager;
Unfortunately this method does not compile. The compiler says that I cannot asume that the customer I get crom the customerManagers is of type CustomerManager<V> - rather it is of type CustomerManager<? extends Value>.
Thinking about this for a while I concluded that the problem is caused by the fact that according to my code the customerManagers map could map from classes of one type of customer (FooCustomer) to managers of another and potentially different subtype of customer (BarCustomer) , whereas my method uses the map as if it maps from classes of FooCustomer to managers of FooCustomer. And that is exactly what I want the map to do. So how do I write a map which acts like this
private Map<Class<T extends Customer>, CustomerManager<T extends Customer>> customerManagers;
so that T is exactly the same type? That would make my method correct - however I have not been able to express such a map in my code. Can this be done, or is there some other work-around?
Thanks
Randahl

Answer 1

And that is exactly what I want the map
to do. So how do I write a map which acts like this
private Map<Class<T extends Customer>,
CustomerManager<T extends Customer>> customerManagers;
so that T is exactly the same type? That would make my
method correct - however I have not been able to
express such a map in my code. Can this be done, or is
there some other work-around?A map has exactly the same types for every key-value-pair it contains, so you cannot declare a map that maps different types of keys to different, albeit pre-defined types of values.
But your original declaration of the map (using wildcards) was perfectly ok, you simply need to augment it with the correct set of public acess methods that enforce the relation between the key-types and value-types.
The method getCustomerManager that you wrote does that already, it just needs a cast before the returned value. This will provoke an "unchecked" warning, though, which cannot be avoided. If you add another method
public <V extends Customer> void setCustomerManager(Class<V> cl, CustomerManager<V> mgr)you're on safe grounds!
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