Question

I would like to do this (minimal repro):

String key = "foo";
Object value = new Bar();

if (target instanceof Map<?,?>) {
    Map<?,?> map = (Map<?,?>)target;
    map.put(key, value);
    ...

But I am told this:

The method put(capture#25-of ?, capture#26-of ?) in the type
Map is not applicable for the
arguments (String, Object)

It seems like String and Object should both be okay here. My question has two parts: (1) why? And (2) how can I make something like this work?

Answer 1

The problem is that collections that use unbounded wildcards don’t allow elements to be added to them. If they did, you could cast the collection to have more specific type parameters, and all of a sudden the type-safety that generics are supposed to offer is gone:

Map<?,?> map = (Map<?,?>)target;
map.put(key, value);  // Not actually allowed
Map<String, String> evilMap = (Map<String, String>)map;
String barAsString = evilMap.get(key); // But it's actually a Bar!