Annotations to represent the metadata in java

Introduction :

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.

Java Annotations

Java Annotation is a tag which is used to represents the metadata(data about data) i.e. attached with class, interface, methods or fields to indicate some additional information which can be used by java compiler and Java Virtual Machine (JVM).

Annotations are used to give additional information, so it is an alternative way for XML and java marker interfaces.

Uses of Annotations :

Annotations have a number of uses, among them: java/j2ee classes bangalore

1. Information for the compiler — Annotations can be used by the compiler to detect errors or suppress warnings.
2. Compile-time and deployment-time processing — Software tools can process annotation information to generate code, XML files, and so forth.
3. Runtime processing — Some annotations are available to be examined at runtime.

Where Annotations Can Be Used :

Annotations can be applied to declarations: declarations of classes, fields, methods, and other program elements. When used on a declaration, each annotation often appears, by convention, on its own line.

Built-In Java Annotations :

There are several built-in annotations in java. Some annotations are applied to java code and some to other annotations. Advanced java training in bangalore These are -

• Override
• SuppressWarnings
• Deprecated

@Override Annotation :

@Override annotation assures that the subclass method is overriding the parent class method. If it is not so, compile time error occurs.

Sometimes, we does the silly mistake such as spelling mistakes etc. So, it is better to mark @Override annotation that provides assurity that method is overridden.

@SuppressWarnings :

@SuppressWarnings annotationis used to suppress warnings issued by the compiler.

@Deprecated :

@Deprecated annoation marks that this method is deprecated so compiler prints warning. It informs user that it may be removed in the future versions. So, it is better not to use such methods.

Java Custom Annotation :

Java Custom annotations or Java User-defined annotations are easy to create and use. The @interface element is used to declare an annotation. Java course in bangalore

@interface MyAnnotation{}

Here, MyAnnotation is the custom annotation name.

Points to remember for java custom annotation :

There are few points that should be remembered :

1. Method should not have any throws clauses

2. Method should return one of the following: primitive data types, String, Class, enum or array of these data types.

3. Method should not have any parameter.

4. We should attach @ just before interface keyword to define annotation.

5. It may assign a default value to the method.

Types of Annotation :

There are three types of annotations.

1. Marker Annotation

2. Single-Value Annotation

3. Multi-Value Annotation

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1. Marker Annotation :

An annotation that has no method, is called marker annotation. For example:

@interface MyAnnotation{} The @Override and @Deprecated are marker annotations

2. Single-Value Annotation :

An annotation that has one method, is called single-value annotation. For example:

@interface MyAnnotation{

int value();

}

3. Multi-Value Annotation :

An annotation that has more than one method, is called Multi-Value annotation. For example:

@interface MyAnnotation{

int value1();

String value2();

String value3();

}

Built-in Annotations used in custom annotations in java :

• Target
• Retention
• Inherited
• Documented

@Target :

@Target tag is used to specify at which type, the annotation is used.

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@Retention :

@Retention annotation is used to specify to what level annotation will be available.

@Inherited ;

By default, annotations are not inherited to subclasses. The @Inherited annotation marks the annotation to be inherited to subclasses.

@Documented ;

The @Documented Marks the annotation for inclusion in the documentation.

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