Generic map not compiling with nested structure

Question

I want to implement a map wrapper that use genericity to enforce a type relation between the key and the value. The following code does not compile :

Map<Class<? extends Serializable>, List<List<? extends Serializable>>> collection = new HashMap();

private <T extends Serializable> void add(Class<T> type, List<List<T>> item) {
    collection.put(type, item);
}

whereas this compile :

Map<Class<? extends Serializable>, List<? extends Serializable>> collection = new HashMap();

private <T extends Serializable> void add(Class<T> type, List<T> item) {
    collection.put(type, item);
}

why is there a difference between a 2nd and a 3rd level generic type ?

There are several other questions that are also essentially the same -- the proposed dupe is the one among those I found whose answers I think are the best. — John Bollinger, Sep 18 '16 at 20:20

score 2 · Accepted Answer · answered Sep 18 '16 at 20:13

Generics are invariant, the second type parameter of your map is List<List<? extends Serializable>>, that means that the second argument of put must match this exactly.

i.e. You can only add a List to the map if it has the exact parameter List<? extends Serializable>, and T is not exactly ? extends Serializable.

You can fix this by having another ? extends in your map and method declaration:

Map<Class<? extends Serializable>, List<? extends List<? extends Serializable>>> collection = new HashMap<>();

private <T extends Serializable> void add(Class<T> type, List<? extends List<T>> item) {
    collection.put(type, item);
}

Generic map not compiling with nested structure

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