Datasheet

{

ProgramFlags flag = ProgramFlags.showErrors;

System.out.println(“Flag selected is: “ +

flag.ordinal() +

“ which is “ +

flag.name());

}

The ordinal() method returns the position of the constant in the list. The value of showErrors is 0

because it comes first in the list, and the ordinal values are 0-based. The

name() method can be used to

get the name of the constant, which provides for getting more information about enumerations.

Metadata

Another feature that Sun has decided to include in the JDK 5 release of Java is a metadata facility. This

enables tagging classes with extra information that tools can analyze, and also applying certain blocks

of code to classes automatically. The metadata facility is introduced in the

java.lang.annotation

package. An annotation is the association of a tag to a construct in Java such as a class, known as a target

in annotation terminology. The types of constructs that can be annotated are listed in the

java.lang

.annotation.ElementType

enumeration, and are listed in the following table. Even annotations can

be annotated.

TYPE covers classes, interfaces, and enum declarations.

ElementType Constant

ANNOTATION_TYPE

CONSTRUCTOR

FIELD

LOCAL_VARIABLE

METHOD

PACKAGE

PARAMETER

TYPE

Another concept introduced is the life of an annotation, known as the retention. Certain annotations may

only be useful at the Java source code level, such as an annotation for the

javadoc tool. Others might be

needed while the program is executing. The

RetentionPolicy enumeration lists three type lifetimes

for an annotation. The

SOURCE policy indicates the annotations should be discarded by the compiler, that

is, should only available at the source code level. The

CLASS policy indicates that the annotation should

appear in the class file, but is possibly discarded at runtime. The

RUNTIME policy indicates the annota-

tions should make it through to the executing program, and these can then be viewed using reflection.

Part I: Thinking Like a Java Developer

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