Java Reference
In-Depth Information
This declares an annotation called
MyAnno
. Notice the
@
that precedes the keyword
in-
terface
. This tells the compiler that an annotation type is being declared. Next, notice
the two members
str( )
and
val( )
. All annotations consist solely of method declarations.
However, you don't provide bodies for these methods. Instead, Java implements these
methods. Moreover, the methods act much like fields.
All annotation types automatically extend the
Annotation
interface. Thus,
Annotation
is a super-interface of all annotations. It is declared within the
java.lang.annotation
pack-
age.
Originally, annotations were used to annotate only declarations. In this usage, any type
of declaration can have an annotation associated with it. For example, classes, methods,
fields, parameters, and
enum
constants can be annotated. Even an annotation can be annot-
ated. In such cases, the annotation precedes the rest of the declaration. Beginning with JDK
8, you can also annotate a
type use
, such as a cast or a method return type.
When you apply an annotation, you give values to its members. For example, here is an
example of
MyAnno
being applied to a method:
This annotation is linked with the method
myMeth( )
. Look closely at the annotation syn-
tax. The name of the annotation, preceded by an @, is followed by a parenthesized list of
member initializations. To give a member a value, that member's name is assigned a value.
Therefore, in the example, the string "Annotation Example" is assigned to the
str
member
of
MyAnno
. Notice that no parentheses follow
str
in this assignment. When an annotation
member is given a value, only its name is used. Thus, annotation members look like fields
in this context.
Annotations that don't have parameters are called
marker annotations
. These are speci-
fied without passing any arguments and without using parentheses. Their sole purpose is to
mark an item with some attribute.
Java defines many built-in annotations. Most are specialized, but nine are general pur-
pose. Four are imported from
java.lang.annotation
: @
Retention
, @
Documented
, @
Tar-
