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javaannotationsfunctional-interface

What does mean annotation without any params in java?

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 {}
Solution

  • 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.