Java Annotations

the word annotate means to add noes to a book or giving explanations or comments.In relation to java Annotations provide data about a program that is not part of the program itself. They have no direct effect on the operation of the code they annotate.

@Author(
   name = "Sougata Das",
   date = "2/25/2012"
)
class MyClass() { }
let a class looks like

public class Generation3List
    extends Generation2List {

   // Author:Sougata Das
   // Date: 2/18/2012
   // Current revision: 6
   // Last modified: 1/1/2012
   // By: sougata 
   // 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();
}
Once the annotation type has been defined, you can use annotations of that type, with the values filled in, like this:
@ClassPreamble (
   author = "Sougata Das",
   date = "3/17/2002",
   currentRevision = 6,
   lastModified = "4/12/2004",
   lastModifiedBy = "Sougata",
   // Note array notation
   reviewers = {"Alice", "Bob", "Cindy"}
)
public class Generation3List extends Generation2List {

// class code goes here

}
To make the information in @ClassPreamble appear in Javadoc-generated documentation, you must annotate the @ClassPreamble definition itself with the @Documented annotation:
// import this to use @Documented
import java.lang.annotation.*;

@Documented
@interface ClassPreamble {

   // Annotation element definitions
   
}
Annotations Used by the Compiler
@Deprecated, @Override, and @SuppressWarnings.
@Deprecated—the @Deprecated annotation indicates that the marked element is deprecated and should no longer be used. The compiler generates a warning whenever a program uses a method, class, or field with the @Deprecated annotation. When an element is deprecated, it should also be documented using the Javadoc @deprecated tag, as shown in the following example. The use of the "@" symbol in both Javadoc comments and in annotations is not coincidental — they are related conceptually. Also, note that the Javadoc tag starts with a lowercase "d" and the annotation starts with an uppercase "D".
// Javadoc comment follows
    /**
     * @deprecated
     * explanation of why it
     * was deprecated
     */
    @Deprecated
    static void deprecatedMethod() { }
}
@Override—the @Override annotation informs the compiler that the element is meant to override an element declared in a superclass (overriding methods will be discussed in the the lesson titled "Interfaces and Inheritance").
// mark method as a superclass method
   // that has been overridden
   @Override 
   int overriddenMethod() { }
@SuppressWarnings—the @SuppressWarnings annotation tells the compiler to suppress specific warnings that it would otherwise generate. In the example below, a deprecated method is used and the compiler would normally generate a warning. In this case, however, the annotation causes the warning to be suppressed.
// use a deprecated method and tell 
   // compiler not to generate a warning
   @SuppressWarnings("deprecation")
    void useDeprecatedMethod() {
        // deprecation warning
        // - suppressed
        objectOne.deprecatedMethod();
    }