The Java Language Specification, Third Edition

TYPES, VALUES, AND VARIABLES Type Arguments and Wildcards 4.5.1
Note that using Collection as the type of the incoming parameter, c, would not
be nearly as useful; the method could only be used with an actual parameter that had type
Collection, which would be quite rare. In contrast, the use of an unbounded
wildcard allows any kind of collection to be used as a parameter.
Wildcards are useful in situations where only partial knowledge about the
type parameter is required.
DISCUSSION
Example - Wildcard parameterized types as component types of array types.
public Method getMethod(Class[] parameterTypes) { ... }
Wildcards may be given explicit bounds, just like regular type variable declarations.
An upper bound is signified by the syntax:
? extends B
, where B is the bound.
DISCUSSION
Example: Bounded wildcards.
DRAFT
boolean addAll(Collection