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

Why it does not work in java, but it does

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

Just to make it clear that the change to the nested HashMap below shows a compiler error, while the above does not; with map (not hashmap)

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

This is due to the fact that generics in Java are invariant, i.e. even if class B is A, a Collection<B> not Collection<A> .

And this is for a good reason. If your example was 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!
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In the second case, myMap is a map whose keys are of type String , and values ​​are of type Map<String, Boolean> . HashMap<String, Boolean> 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>>();
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I think due to the difference between Map<String, Boolean> and HashMap<String,Boolean> . Indeed, generics here are specifications that should be the same on both sides. (or at least my opinion).

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