Effective Java

// Use of asSubclass to safely cast to a bounded type token
static Annotation getAnnotation(AnnotatedElement element,
String annotationTypeName) {
Class annotationType
= null; // Unbounded type token
try {
annotationType = Class.forName(annotationTypeName);
} catch (Exception ex) {
throw new IllegalArgumentException(ex);
}
return element.getAnnotation(
annotationType.asSubclass(Annotation.class)
);
}
In summary, the normal use of generics, exemplified by the collections APIs,
restricts you to a fixed number of type parameters per container. You can get
around this restriction by placing the type parameter on the key rather than the
container. You can use 
Class
objects as keys for such typesafe heterogeneous
containers. A 
Class
object used in this fashion is called a type token. You can also
use a custom key type. For example, you could have a 
DatabaseRow
type repre-
senting a database row (the container), and a generic type 
Column
as its key.

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157
C H A P T E R
6
Enums and Annotations
J
AVA
supports two special-purpose families of reference types: a kind of class
called an 
enum type,
and a kind of interface called an 
annotation type
. This chap-
ter discusses best practices for using these type families.
Item 34: Use enums instead of 
int
 constants
An 
enumerated type
is a type whose legal values consist of a fixed set of
constants, such as the seasons of the year, the planets in the solar system, or the
suits in a deck of playing cards. Before enum types were added to the language, a
common pattern for representing enumerated types was to declare a group of
named 
int
constants, one for each member of the type: