Java Fundamentals - Annotations

Annotations

• Act as a metadata
• Preceded by @

• @Override

Check that the method signature override an interface method otherwise the compiler will send an error.

• @Deprecated

The compiler indicate that this method is deprecated

• @SuppressWarnings("deprecation")

Avoid that the compiler indicate the use of a deprecated method

• @Retention

To indicate the availability according to the RetentionPolicy Values

• Java Source - Exist only in source file / Discarted by compiler
• Class - (Default value) Compile into the class file / Discarted by runtime
• Runtime - Loaded into runtime / Accesible with reflection

• @Target

Define the elements where the annotation can be used:

Target(ElementType.CONSTRUCTOR)

Target(  { ElementType.CONSTRUCTOR,  ElementType.METHOD, ElementType.TYPE })

Create own annotations

• Declare the attibutes as methods
• Setting is similar to fields

Declare annotation

@Target(ElementType.TYPE)

@Retention( RetentionPolicy.RUNTIME )

public @interface MyAnnotation{

    boolean useThisFeature();

Default value

//In this case we are establishing a default value.

    boolean useThisFeature() default true;

}

Use annotation

@MyAnnotation( useThisFeature = false)

   or 

@MyAnnotation// Using the default value

public class MyClass (){

}

Valid Annotation Element Types

• Primitive Types
• String
• Enum
• Annotation
• Class<?>
• Array of all the above 

Access to annotation

//This get if the class contains an annotation of type : MyAnnotation

MyAnnotation ma = myclassWithAnnotation.getAnnotation( MyAnnotation.class );

if( ma == null){

}

else if( ma.useThisFeature() ) {

}