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.
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.
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>
Map<String, ?> is a short form of Map<String,? extends Object> and doesn't mean that anything can be added as value. It says that the Map-object can have any generic value type extending Object.
This means that the Map object can be a HashMap<String, String> or a HashMap<String, Integer> as well. Because the compiler can't check which value types will be accepted, he won't let you call methods with the value type as a parameter.
Note:
Map<String, ? super String> will have the opposite effect: You can always use a String as parameter, but the return-type is unclear.Try this:
public Map<String, Object> getData();
[EDIT] This is really wrong... I understood.
My first answer was:
That's java : String is not an object.
Try with
obj.getData().put("key",new String("value"));
But String extends Object... while I thought String was a primitive. I learned something ^^
