FunctionalInterface

  • @FunctionalInterface annotation indicates that an interface is designed to be a functional interface, meaning it has a single abstract method.

  • Default methods and methods overriding java.lang.Object's public methods don't count towards the abstract method count for a functional interface.

  • Compilers enforce the functional interface requirements when @FunctionalInterface is present, generating an error if the annotated type doesn't meet the criteria.

  • Functional interfaces can be instantiated using lambda expressions, method references, or constructor references.

  • Even without the @FunctionalInterface annotation, the compiler recognizes and treats any interface meeting the functional interface definition as such.

public abstract @interface FunctionalInterface implements Annotation

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 {@linkplain java.lang.reflect.Method#isDefault() 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.

Inherited Method Summary