Question

I have a method that returns a map defined as:

public Map<String, ?> getData();

The actual implementation of this method is not clear to me, but, when I try to do:

obj.getData().put("key","value")

I get following compile time error message:

The method put(String, capture#9-of ?)
in the type Map
is not applicable for the arguments
(String, String)

What is the problem? Is String not of type anything?

Thanks in advance.

Answer 1

The wildcard means “the value type parameter could be anything” – it doesn’t mean “you can use this as if it were anything you want it to be”. In other words, a Map<String, UUID> is valid as a Map<String, ?> – but you wouldn’t want to be able to put a String value into it.

If you want a map which can definitely accept string values, you want:

Map<String, ? super String>