Difference between revisions of "Java Generics"

Revision as of 08:05, 14 March 2011

Generics are a means of expressing type constraints on the behavior of a class or method in terms of unknown types, such as "whatever the types of parameters x and y of this method are, they must be the same type," "you must provide a parameter of the same type to both of these methods," or "the return value of foo() is the same type as the parameter of bar()."

Wildcards

Wildcards — ? — are a means of expressing a type constraint in terms of an unknown type.

The wildcard type List<?> is different from both the raw type List and the concrete type List<Object>. To say a variable x has type List<?> means that there exists some type T for which x is of type List<T>, that x is homogeneous even though we don't know what particular type its elements have. It's not that the contents can be anything, it's that we don't know what the type constraints on the contents are — but we know that there is a constraint. On the other hand, the raw type List is heterogeneous; we are not able to place any type constraints on its elements, and the concrete type List<Object> means that we explicitly know that it can contain any object.