Java Annotations

 Annotations provide data about a class that is not part of the programming logic itself. Annotations are defined via the @interface annotation before the class name.

·         @Retention you define if the annotation should be retained at runtime or not.

RetentionPolicy.SOURCE

RetentionPolicy.CLASS

RetentionPolicy.RUNTIME

· The @Target annotation lets you define where this annotation can be used, e.g. the class, fields, methods, etc.

ElementType.ANNOTATION_TYPE

ElementType.CONSTRUCTOR

ElementType.FIELD

ElementType.LOCAL_VARIABLE

ElementType.METHOD

ElementType.PACKAGE

ElementType.PARAMETER

ElementType.TYPE

· The @Inherited annotation signals that a custom annotation used in a class should be inherited by subclasses inheriting from that class.

·The @Documented annotation is used to signal to the JavaDoc tool that your custom annotation should be visible in the JavaDoc for classes using your custom annotation

Java provides three built-in annotations:

· @Override – When we want to override a method of Super class, we should use this annotation to inform compiler that we are overriding a method. So when super class method is removed or changed, compiler will show error message. Learn why we should always use java override annotation while overriding a method.

·@Deprecated – when we want the compiler to know that a method is deprecated, we should use this annotation. Java recommends that in javadoc, we should provide information for why this method is deprecated and what is the alternative to use.

·@SuppressWarnings – This is just to tell compiler to ignore specific warnings they produce, for example using raw types in java generics. Its retention policy is SOURCE and it gets discarded by compiler.