Java Reference
In-Depth Information
Note Java has always supported ad hoc annotation mechanisms. For example, the
java.lang.Cloneable interface identifies classes whose instances can be shal-
lowlyclonedvia Object 's clone() method,the transient reservedwordmarks
fieldsthataretobeignoredduringserialization(discussedin Chapter8 ) ,andthe @de-
precated javadoc tag documents methods that are no longer supported. Java 6
formalized the need for annotations by introducing the annotations language feature.
Discovering Annotations
An annotation isaninstanceofanannotationtypeandassociatesmetadatawithanap-
plicationelement.Itisexpressedinsourcecodebyprefixingthetypenamewiththe @
symbol. For example, @Readonly is an annotation and Readonly is its type.
Note Youcanuseannotationstoassociatemetadatawithconstructors,fields,local
variables, methods, packages, parameters, and types (annotation, class, enum, and in-
Thecompilersupportsthe Override , Deprecated , SuppressWarnings ,and
SafeVarargs annotationtypes.Thesetypesarelocatedinthe java.lang package.
@Override annotationsareusefulforexpressingthatasubclassmethodoverrides
a method in the superclass, and does not overload that method instead. The following
example reveals this annotation being used to prefix the overriding method:
public void draw(int color)
// drawing code
@Deprecated annotationsareusefulforindicatingthatthemarkedapplicationele-
mentis deprecated (phasedout)andshouldnolongerbeused.Thecompilerwarnsyou
when a deprecated application element is accessed by ondeprecated code.
Incontrast,the @deprecated javadoc tagandassociatedtextwarnsyouagainst
using the deprecated item, and tells you what to use instead. The following example
demonstrates that @Deprecated and @deprecated can be used together:
* Allocates a <code>Date</code> object and initializes it
so that
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