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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 {}
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  • 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 baeldung.com/java-8-functional-interfaces Commented Jun 11, 2019 at 10:07
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    @FunctionalInterface annotation is used to ensure an interface can’t have more than one abstract method. The use of this annotation is optional. Commented Jun 11, 2019 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. Commented Jun 11, 2019 at 10:09

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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.
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