Annotation in java

November 26, 2022

Element Types	Where the annotation can be applied
TYPE	class, interface
FIELD	fields
METHOD	methods
CONSTRUCTOR	constructors
ANNOTATION_TYPE	annotation type
LOCAL_VARIABLE	local variables
PARAMETER	parameter

Annotation

Annotations, a form of metadata, provide data about a program that is not part of the program itself. Annotations have no direct effect on the operation of the code they annotate. Or simply we can say , adding additional information about a program is done with annotations.

There are generally 5 types of annotations, as follows:

Marker Annotations
Single value Annotations
Full Annotations
Type Annotations
Repeating Annotations

What is the use of Annotation ?

Information for the compiler — The compiler can use annotations to identify errors or turn off warnings.
Deployment and compilation— Annotation data can be processed in real-time by software tools to produce code, XML files, and other output.
Processing done at runtime — Some annotations can be looked at right away.

Key Features of Annotation

Annotations begin with an “@.”
Annotations do not affect how a compiled program behaves.
Instance variables, constructors, methods, classes, and other program elements can all be annotated to add metadata (information).
As they can alter how a program is handled by the compiler, annotations are not simply comments.
An alternative to XML and Java marker interfaces, annotations are essentially used to provide extra information.

Polymorphism in Java

Encapsulation in Java

Abstraction in Java

Multithreading in Java

Built-In Java Annotations

Java comes with a number of built-in annotations. A few annotations are used with Java code, while others are used with other annotations.

Java source code that uses built-in annotations

@Override
@SuppressWarnings
@Deprecated

Java built-in Annotations used in other annotations

@Target
@Retention
@Inherited
@Documented

Declaring of an Annotation

Many annotations in code take the place of comments.

Consider a scenario where a software group customarily begins each class with comments outlining crucial information:

Example code :

public class Generation3List extends Generation2List {

   // Author: John Doe
   // Date: 3/17/2002
   // Current revision: 6
   // Last modified: 4/12/2004
   // By: Jane Doe
   // Reviewers: Alice, Bill, Cindy

   // class code goes here

}

To add this same metadata with an annotation, you must first define the annotation type.

The syntax for doing this is:

@interface ClassPreamble {
   String author();
   String date();
   int currentRevision() default 1;
   String lastModified() default "N/A";
   String lastModifiedBy() default "N/A";
   // Note use of array
   String[] reviewers();
}

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