Problem using generic map with wildcard

问题:

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.

回答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>

回答2:

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.

回答3:

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:

• You can call methods with the value type as a return value, because everything must extend Object (? extends Object)
• A Map<String, ? super String> will have the opposite effect: You can always use a String as parameter, but the return-type is unclear.

回答4:

Try this:

public Map<String, Object> getData();

回答5:

[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 ^^