Java Annotation Example

Alien.java

package com.example.demo;

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

@Target({ElementType.TYPE, ElementType.TYPE_USE})
@Retention(RetentionPolicy.RUNTIME)
public @interface Alien {
    @Required int @Required [] even() default {1, 3, 5};
    
    // C-Style array element declaration (or extended dimensions according to aspectj)
    @Required int odd() @Required [] default {2, 4, 6};
}

Coupled.java

package com.example.demo;

import org.checkerframework.checker.index.qual.Positive;

import java.lang.annotation.*;

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Coupled(@Coupled.CouplingMethod("virtual"))
@Target({ElementType.ANNOTATION_TYPE, ElementType.TYPE, ElementType.TYPE_USE})
@Inherited
@Repeatable(Couples.class)
public @interface Coupled {
    CouplingMethod value() default @CouplingMethod("default");
    @Positive int[] arguments() default {1, 2, 3};

    @interface CouplingMethod {
        String value();
    }
}

Couples.java

package com.example.demo;

import java.lang.annotation.*;

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE, ElementType.ANNOTATION_TYPE})
@Inherited
public @interface Couples {
    Coupled[] value();
}

Main.java

package com.example.demo;

import org.jetbrains.annotations.Contract;
import org.jetbrains.annotations.NotNull;

@Alien
public class Main {
    public static void main(String[] args) throws NoSuchMethodException {
        var method1 = Main.class.getMethod("getRequiredString");
        var method2 = Main.class.getMethod("getRequiredString2");

        /*
        The output is:
            1
            2
            1
            3
         */
        System.out.println(method1.getAnnotations().length);
        System.out.println(method2.getAnnotations().length);
        System.out.println(method1.getAnnotatedReturnType().getAnnotations().length);
        System.out.println(method2.getAnnotatedReturnType().getAnnotations().length);
    }

    @Contract(value = " -> new", pure = true)
    public static @Required int @Required @NotNull [] createIntArray() {
        return new int[]{1, 2, 3};
    }

    @Contract(value = " -> new", pure = true)
    public static @Required @NotNull int createIntArrayAlternate() @Required [] {
        return new int[]{1, 2, 3};
    }

    @Preliminary
    public String getRequiredString() {
        return "okay";
    }

    public @Preliminary @Required @Coupled String getRequiredString2() {
        return "wow";
    }
}

Preliminary.java

package com.example.demo;

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

@Target({ElementType.TYPE, ElementType.TYPE_USE, ElementType.METHOD, ElementType.PARAMETER})
@Retention(RUNTIME)
public @interface Preliminary {
    String value() default "[default]";
}

Required.java

package com.example.demo;

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

@Target({ElementType.TYPE, ElementType.TYPE_USE, ElementType.METHOD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
public @interface Required {

}