Generics - Java collection within a collection

Question

I'm new to generics, so not sure where I'm going wrong...

I have classes, called Cat, Dog and Rabbit, which implement the interface Animal.

The following code will compile

Set<? extends Animal> animalSet;
Set<Dog> dogSet = new HashSet<Dog>();
animalSet = dogSet;

But the following code will not

Map<String, Set<? extends Animal>> animalMap;
Map<String, Set<Dog>> dogMap = new HashMap<String, Set<Dog>>();
animalMap = dogMap; // this line will not compile

The compiler says the types are incompatible. Where am I going wrong?

UPDATE

Thanks for everyone's help

I've changed the first line of code by adding another wildcard The following code will compile

Map<String, ? extends Set<? extends Animal>> animalMap;
Map<String, Set<Dog>> dogMap = new HashMap<String, Set<Dog>>();
animalMap = dogMap;

See also the solution given by Cyrille Ka below - use putAll() to transfer values from dogMap to animalMap, instead of assigning dogMap to animalMap.

Answer 1

Basically, when you write:

Map<String, Set<? extends Animal>> animalMap;

You declare that any value of the map is a set that can contain any object whose type is a subclass of Animal. Then it would be perfectly reasonable for a client code to write:

animalMap.put("miaow", aCatSet);

where acatSet is a Set<Cat>.

But dogMap can not accept a Set<Cat> as value, it can only accept Set<Dog>. Therefore there is a possibility of type incompatibility and that's why this construction is forbidden.

Edit: As for how to fix that, it depends on what you want to do. If you have a dogMap somewhere and want to put its content into animalMap then you can simply copy the content like this:

Map<String, Set<? extends Animal>> animalMap = new HashMap<String, Set<? extends Animal>>();
Map<String, Set<Dog>> dogMap = new HashMap<String, Set<Dog>>();

/// fill dogMap

animalMap.putAll(dogMap);

Answer 2

Because Java is strongly typed: ? extends Animal is not Dog, it could be Cat, or another subclass.

For example, in this case, if you define Set<? extends Animal> animalSet, I can legally do animalSet.add(new Cat()), right?. But you want to initialize this animalSet with a Set<Dog>, which would not allow animalSet.add(new Cat()) anymore. This is inconsisent, and this is why it is not allowed.