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Map<String, Map<String, Boolean>> myMap = new HashMap<String,HashMap<String,Boolean>>();

Answer 1

A:

I think that's because of the difference between Map<String, Boolean> and HashMap<String,Boolean>. Indeed, the generics are here a specification, which must be the same on both sides. (or at least that's my opinion).

Riduidel 2010-09-03 14:44:59

Answer 2

+3 A:

In the second case myMap is a map which keys are of type String and values are of type Map<String, Boolean>. HashMap<String, Boolean> is not a Map<String, Boolean> it implements it. Therefore, this will compile:

Map<String, ? extends Map<String, Boolean>> myOtherMap = 
    new HashMap<String,HashMap<String,Boolean>>();

Boris Pavlović 2010-09-03 14:46:27

A HashMap<String, Boolean> is a Map<String, Boolean>.

Ricky Clarkson 2010-09-03 17:00:28

Answer 3

+6 A:

This is because generics in Java are invariant, i.e. even if class B is an A, a Collection<B> is not a Collection<A>.

And this is for a good reason. If your example were legal, this would be possible:

Map<String, HashMap<String, Boolean>> myHashMap = new HashMap<String,HashMap<String,Boolean>>();
Map<String, Map<String, Boolean>> myMap = myHashMap;
myMap.put("oops", new TreeMap<String, Boolean>());
HashMap<String, Boolean> aHashMap = myMap.get("oops"); // oops - ClassCastException!

Péter Török 2010-09-03 14:46:43

But, given the declaration of myMap, what would I care what kind of Maps are put in there as long as they implement the Map interface?

extraneon 2010-09-03 14:52:19

@extraneon: In that case, see Boris's answer.

R. Bemrose 2010-09-03 14:56:00

@extraneon, see my updated example.

Péter Török 2010-09-03 14:57:41

Java generics can have use-side variance, but not declaration-side variance. (The latter makes sense for languages where it is idiomatic for interfaces to be either "tell don't ask" or are immutable.)

Tom Hawtin - tackline 2010-09-03 16:41:05

I understand the why from a technical point of view, it made the implementation of generics easier. But from a functional point of view it's not all that logical. If I say (which I can't do) that my map contains map values, than I also commit to not getting a specific type of map out of there. But I can't say this map contains maps, but it must contain a specific type of map. And this is, to me, a compromise to make implementation of generics feasible, and not a feature :)

extraneon 2010-09-04 09:08:56

@extraneon, this specific example with maps is a bit contorted; see [this earlier answer of mine](http://stackoverflow.com/questions/2292308/generics-method-signatures-assignments/2292339#2292339) for a simpler and (hopefully) more revealing example.

Péter Török 2010-09-04 11:50:34

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Map<String, Map<String, Boolean>> myMap = new HashMap<String,HashMap<String,Boolean>>();

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