Java Custom Annotations

In Java, you can create custom annotations by defining a new interface that extends the java.lang.annotation.Annotation interface.

Here is an example of how to create a custom annotation in Java:

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

// Define the custom annotation
@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnnotation {
  String value();
}

In this example, we define a custom annotation called MyAnnotation that has a single element called value. The @Retention annotation specifies that the annotation should be retained at runtime, which means that it can be accessed using reflection.

To use the custom annotation, you can annotate class, method, or field declarations with it. For example:

@MyAnnotation("Hello, World!")
public class Main {
  @MyAnnotation("Foo")
  public static void main(String[] args) {
    @MyAnnotation("Bar")
    int x = 10;
  }
}

In this example, we annotate the Main class, the main method, and the x field with the MyAnnotation annotation.

To access the annotation at runtime, you can use the java.lang.reflect.AnnotatedElement interface and its related classes. For example:

import java.lang.reflect.AnnotatedElement;

public class Main {
  public static void main(String[] args) {
    // Get the Main class
    Class<Main> cls = Main.class;

    // Check if the Main class is annotated with the MyAnnotation annotation
    if (cls.isAnnotationPresent(MyAnnotation.class)) {
      // Get the MyAnnotation annotation
      MyAnnotation annotation = cls.getAnnotation(MyAnnotation.class);

      // Get the value of the annotation's value element
      String value = annotation.value();

      // Print the value
      System.out.println(value);  // Output: Hello, World!
    }
  }
}