Problem using generic map with wildcard

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 return type of

Map<String, ?>

is the same as

Map<String, ? extends Object>

The means that the concrete type returned could be a Map<String, AnyClass>. You can't put a String into an AnyClass, hence the error.

A good general principle is to not use wildcards in method return types.

Answer 2

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>