Wildcard Parameterized Types

Java Tutorials - Herong's Tutorial Examples - Version 7.03, by Dr. Herong Yang

Java Tutorials - Herong's Tutorial Examples

∟Generic Classes and Parameterized Types

∟Wildcard Parameterized Types

This section describes wildcard parameterized types where a argument type contains the wildcard character '?' and represents a group of types instead of a single type.

What Is a Wildcard Parameterized Type? A wildcard parameterized type is a parameterized type of a generic class or interface, where the argument type represents a group of types, instead of a single type.

The objective of using a wildcard parameterized type is to create a new parameterized type that is wider than a normal parameterized type, so that you can use it a supertype of the normal parameterized type. Here is some examples of wildcard parameterized type:

   java.util.List<?> a;
   java.util.List<? extends Number> b;
   java.util.List<? extends Integer> c;
   java.util.List<? super Integer> d;

Wildcard parameterized types can be divided into 3 groups:

Unbounded Wildcard Parameterized Type with a syntax of <?> creates a supertype of all other parameterized types of the same generic class. For example, java.util.List<?> is a supertype of java.util.List<Number>.
Upper Bounded Wildcard Parameterized Type with a syntax of <? extends T> creates a supertype of parameterized types with argument types narrower than T of the same generic class. For example, java.util.List<? extends Number> is a supertype of java.util.List<Integer>.
Lower Bounded Wildcard Parameterized Type with a syntax of <? super T> creates a supertype of parameterized types with argument types wider than T of the same generic class. For example, java.util.List<? super Integer> is a supertype of java.util.List<Number>.

The diagram below shows you some examples of supertype-subtype relationships of wildcard parameterized types and parameterized types:
Java Wildcard Parameterized Type Subtyping