JAVA 泛型FAQ

Wildcard Guidelines:
An "in" variable is defined with an upper bounded wildcard, using the extends keyword.
An "out" variable is defined with a lower bounded wildcard, using the super keyword.
In the case where the "in" variable can be accessed using methods defined in the Object class, use an unbounded wildcard.
In the case where the code needs to access the variable as both an "in" and an "out" variable, do not use a wildcard.

http://docs.oracle.com/javase/tutorial/java/generics/wildcardGuidelines.html

JAVA 泛型FAQ
http://www.angelikalanger.com/GenericsFAQ/FAQSections/ParameterizedTypes.html