The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
The unbounded wildcard type is specified using the wildcard character (?), for example, List<?>. This is called a list of unknown type. There are two scenarios where an unbounded wildcard is a useful approach:
Consider the following method, printList:
public static void printList(List<Object> list) { for (Object elem : list) System.out.println(elem + " "); System.out.println(); }
The goal of printList is to print a list of any type, but it fails to achieve that goal — it prints only a list of Object instances; it cannot print List<Integer>, List<String>, List<Double>, and so on, because they are not subtypes of List<Object>. To write a generic printList method, use List<?>:
public static void printList(List<?> list) { for (Object elem: list) System.out.print(elem + " "); System.out.println(); }
Because for any concrete type A, List<A> is a subtype of List<?>, you can use printList to print a list of any type:
List<Integer> li = Arrays.asList(1, 2, 3); List<String> ls = Arrays.asList("one", "two", "three"); printList(li); printList(ls);
It's important to note that List<Object> and List<?> are not the same. You can insert an Object, or any subtype of Object, into a List<Object>. But you can only insert null into a List<?>. The Guidelines for Wildcard Use section has more information on how to determine what kind of wildcard, if any, should be used in a given situation.