Restrictions on wildcards in generics:

The get and put principle is one of the restrictions on wildcards. Additional to the above principle there are also a few restrictions on wildcards: Don’t use wilcards when creating instances, specifying generic method calls and extending superclasses:

List<?> integers = new LinkedList<?>();         // Won't compile!List<?> integers = Lists.<?>factory();          // Won't compile!class AnyList implements List<?> {}             // Won't compile!