What does mean annotation without any params in java?

Question

I' m trying to understand @Functional interface from JDK and can't catch why did they create this annotation without a body?

I don't understand what is difference between using @FunctionalInterface in class description and not using? what does this annotation without params provide?

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface FunctionalInterface {}

All functional interfaces are recommended to have an informative @FunctionalInterface annotation. This not only clearly communicates the purpose of this interface, but also allows a compiler to generate an error if the annotated interface does not satisfy the conditions. See https://www.baeldung.com/java-8-functional-interfaces — fantaghirocco, Jun 11 '19 at 10:07
`@FunctionalInterface` annotation is used to ensure an interface can’t have more than one abstract method. The use of this annotation is optional. — Sudhir Ojha, Jun 11 '19 at 10:08
It is purely a _marker_ interface, to signal that some annotated interface is intended to be used/usable as lambda, that it contains just _one_ function. Containing one function would mean calling `getDeclaredMethods` for counting, which is an unnecessary overhead. — Joop Eggen, Jun 11 '19 at 10:09

score 3 · Accepted Answer · answered Jun 11 '19 at 10:10

If you add it, someone won't be able to add more methods without breaking your functional interface and will get a compile time error:

Invalid '@FunctionalInterface' annotation; MyInterface is not a functional interface

If a type is annotated with this annotation type, compilers are required to generate an error message unless:

The type is an interface type and not an annotation type, enum, or class.

The annotated type satisfies the requirements of a functional interface.

What does mean annotation without any params in java?

1 Answers1