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.  Since default methods have an implementation, they are not abstract.  If
 an interface declares an abstract method overriding one of the
 public methods of 
java.lang.Object, that also does
 not count toward the interface's abstract method count
 since any implementation of the interface will have an
 implementation from java.lang.Object or elsewhere.
 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.
However, the compiler will treat any interface meeting the
 definition of a functional interface as a functional interface
 regardless of whether or not a FunctionalInterface
 annotation is present on the interface declaration.
- See Java Language Specification:
- 
4.3.2 The Class Object
 9.8 Functional Interfaces
 9.4.3 Interface Method Body
 9.6.4.9 @FunctionalInterface
- Since:
- 1.8