In generic code, the question mark (
? ), called the wildcard , represents an unknown type. The wildcard can be used in a variety of situations: as the type of a parameter, field, or local variable; sometimes as a return type (though it is better programming practice to be more specific). The wildcard is never used as a type argument for a generic method invocation, a generic class instance creation, or a supertype.
The following sections discuss wildcards in more detail, including upper bounded wildcards, lower bounded wildcards, and wildcard capture.