Putting into a Map<String, ?>

Question

So I have a Map that has some values in it being passed into a method:

public String doThis(Map<String, ?> context){
.....
}

And I'm trying to insert an addition attribute to this Map

String abc="123";
context.put("newAttr",abc);

But I am getting this error:

The method put(String, capture#8-of ?) in the type Map is not applicable for the arguments (String, String)

Is there anyway to perform this put without "cloning" the Map?

Solution 1

If you want to put values of type X into a generic Map you need to declare the Map as Map<String, ? super X>. In your example X is String, so:

public String doThis(Map<String, ? super String> context){
.....
}

Map<String, ? super X> means: a map with keys of type String and values of a type which is X or a super-type of X. All such maps are ready to accept String instances as keys and X instances as values.

Solution 2

Remember PECS (Producer Extends, Consumer Super). You have a consumer (putting in), therefore it cannot be extends.

Comments

[ruakh]

+1, but your last paragraph has a typo: you mean super rather than extends. :-)