JAVA ANNOTATIONS: @FunctionalInterface

An informative annotation type used to indicate that an interface type declaration is intended to be a functional interface as defined by the Java Language Specification. Conceptually, a functional interface has exactly one abstract method.

Note that instances of functional interfaces can be created with lambda expressions, method references, or constructor references.

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.