Annotations in Java(Built-in Annotation) part-1

Introduction to Annotations in Java

In Java 5.0, a new feature called annotations was added to the Java to make it easier to create new kinds of metadata for Java programs.

• Annotations are not mere comments; they can help us in different ways.
• Annotations, decorates programs with additional information, they are not the part of program itself.
• Annotations don't directly affect program semantics but provides general purpose metadata.
• Annotations can affect the treatment of programs by the software tools and libraries.
• Annotations can be used by the compiler to detect errors and suppress warnings.
• Annotations can be used by the software tools to generate the code like XML files.
• Few Annotations can be examined at run-time also. 
• Annotations encourage declarative programming style.
•  Annotations simplify several programming tasks and help a great deal to develop Web Services and Enterprise application development like EJBs.
• Annotations are also used for tasks like Testing Frameworks and Parser Generators etc.

In its basic form, an annotation will look like:

@TestAnnotation

The @ character indicates to the compiler that what follows is an annotation. In the following example, the annotation's name is Override:

@Override

void method() { ... }

There are several Built-in Annotations defined Java but nine are general purpose Annotations.

 Built-in Annotations included in java.lang

• @Override
• @Deprecated
• @SuppressWarnings
• @FunctionalInterface
• @SafeVarargs

Built-in Annotations used by other Annotations internally, included in java.lang.Annotation

• @Target
• @Retention
• @Inherited
• @Documented

@Override

@Override is a standard marker annotation that can be used to annotate method definitions. It means that the method is intended to override (that is replace) a method with the same signature that was defined in some super class. A compiler can check that the super class method actually exists; if not, it can inform the programmer.

Example:

class A
{
public void testMethod()
{
System.out.println("somthong in super class");
}
}
public class B extends A{
@Override
public void testMethod()// compile time error will be reported if method does not 
{                                     //exist in super class of A
System.out.println("something in sub class");
}


}

@Deprecated

@Deprecated, is a marker Annotation and indicates this declaration is obsolete and has been replaced by newer implementation.

@SuppressWarnings

This Annotation indicates that one or more warnings might be issued by the compiler are needed to be surprised. The warning to be suppressed are specified by the name in string format.

@FunctionalInterface

@FunctionalInterface is a marker annotation which was added in JDK 8. This Annotation was designed to be used with interfaces only. It indicates that the annotated interface is a functional interface. A functional interface is an interface containing one and only one abstract method. Functional interfaces are used by Lambda expressions. If the annotated interface is not a functional interface a compile time error will be reported. @FunctionalInterface is purely informational.

@SafeVarargs

This marker annotation can be applied to the methods and the constructors. This annotation specifies that no unsafe acts related to varargs  parameter occur. This annotation must be applied only to the the varargs method constructors those are static or final.

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