Section 20.8 Wildcards in Methods That Accept Type Parameters
• Class Number is the superclass of both Integer and Double.
• Number method doubleValue (p. 862) obtains the Number’s underlying primitive value as a double value.
• Wildcard type arguments enable you to specify method parameters, return values, variables, and so on, that act as supertypes of parameterized types. A wildcard-type argument is denoted by ? (p. 862), which represents an “unknown type.” A wildcard can also have an upper bound.
• Because a wildcard (?) is not a type-parameter name, you cannot use it as a type name throughout a method’s body.
• If a wildcard is specified without an upper bound, then only the methods of type Object can be invoked on values of the wildcard ...
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