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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() ) {

}








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