Annotation

a form of metadata, provide data about a program that is not part of the program itself
- Information for the compiler — Annotations can be used by the compiler to detect errors or suppress warnings.
- Compile-time and deployment-time processing — Software tools can process annotation information to generate code, XML files, and so forth.
- Runtime processing — Some annotations are available to be examined at runtime.

Examples

@Entity

@Override
void mySuperMethod() { ... }

@Author(
  name = "Benjamin Franklin",
  date = "3/27/2003"
)
class MyClass() { ... }

@SuppressWarnings(value = "unchecked")
void myMethod() { ... }

@SuppressWarnings("unchecked")
void myMethod() { ... }

@Author(name = "Jane Doe")
@EBook
class MyClass { ... }

Predefined Java Annotations : Override, SuppressWarnings
- https://docs.oracle.com/javase/tutorial/java/annotations/predefined.html
Custom Annotation types : EBook, Author
Java SE 8 Annotations

@Author(name = "Jane Doe")
@Author(name = "John Smith")
class MyClass { ... }

Type Annotations
- Class instance creation expression:

new @Interned MyObject();

Type cast:

myString = (@NonNull String) str;

implements clause:

class UnmodifiableList<T> implements @Readonly List<@Readonly T> { ... }

Thrown exception declaration:

void monitorTemperature() throws @Critical TemperatureException { ... }

Custom annotation

Annotation types are a form of interface

package okdevtv;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Name {

  public String myName();

}

package okdevtv.java;

import okdevtv.Name;

public class Z {

  @Name(myName = "Kenu")
  public void something() {
    System.out.println("Do something");
  }
}

package okdevtv.java;

import okdevtv.Name;

import java.lang.annotation.Annotation;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

public class NameTest {
  public static void main(String[] args) throws NoSuchMethodException, SecurityException {
    final Method method = Z.class.getMethod("something");
    if (method.isAnnotationPresent(Name.class)) {
      final Annotation annotation = method.getAnnotation(Name.class);
      final Name name = (Name) annotation;
      System.out.println(name.myName()); // Prints Kenu
    }
    Z z = new Z();
    try {
      method.invoke(z);
    } catch (IllegalAccessException e) {
      e.printStackTrace();
    } catch (InvocationTargetException e) {
      e.printStackTrace();
    }
    z.something();
  }
}