In the Java collection map <Key,?> What is a "?" To refer to?

Question

In Java collections, I saw something like this: Map<Key,?> . I do not know how this works, can someone help me with this or give an example?

Answer 1

The question mark (?) Represents an unknown type.

In your example, Map<Key, ?> This means that it will correspond to a map containing values ​​of any type. It does not mean that you can create a Map<Key, ?> And insert values ​​of any type into it.

Quote from the documentation :

In general code, a question mark (?), Called a wildcard, represents an unknown type. You can use a wildcard in various situations: as the type of a parameter, field, or local variable; sometimes as a return type (although better programming practice is more specific). A wildcard is never used as a type argument for a generic method call, an instance of a generic class, or a supertype.

For example, suppose you want to create a function that will print the values ​​of any map, regardless of the types of values:

 static void printMapValues(Map<String, ?> myMap) { for (Object value : myMap.values()) { System.out.print(value + " "); } } 

Then call this function by passing the argument Map<String, Integer> as an argument:

 Map<String, Integer> myIntMap = new HashMap<>(); myIntMap.put("a", 1); myIntMap.put("b", 2); printMapValues(myIntMap); 

And you will get:

 1 2 

The wildcard allows you to call the same function by passing Map<String, String> or any other type of value as an argument:

 Map<String, String> myStrMap = new HashMap<>(); myStrMap.put("a", "one"); myStrMap.put("b", "two"); printMapValues(myStrMap); 

Result:

 one two 

This wildcard is called unlimited because it does not provide type information. There are several scenarios in which you can use an unlimited template:

• If you call methods other than those defined in the Object class.
• When you use methods that are independent of the type parameter, such as Map.size() or List.clear() .

The wildcard can be unlimited, bounded above or bounded below:

• List<?> Is an example of an unlimited template . It presents a list of elements of an unknown type.

• List<? extends Number> List<? extends Number> is an example of an upper bounded pattern . It corresponds to a List type Number , as well as its subtypes, such as Integer or Double .

• List<? super Integer> List<? super Integer> is an example of a restricted wildcard . It corresponds to a List type Integer , as well as its supertypes Number and Object .

Answer 2

Unknown wildcard

? can be any data type

List<?> Means a list typed by an unknown type. It can be a list, list, list, etc.

Now going to your example Map<Key,?> Means Value , which should be inserted into this map, can be any data type.