FunctionalInterface (Java Platform SE 8 )
Keywords: java.lang.FunctionalInterface class
Date: 2020-03-26

java.lang

## Annotation Type FunctionalInterface

-     @Documented
       @Retention(value=RUNTIME)
       @Target(value=TYPE)
      public @interface 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. 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.

  Since:
    1.8
  See The Java™ Language Specification:
    4.3.2. The Class Object, 9.8 Functional Interfaces, 9.4.3 Interface Method Body

Submit a bug or feature
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
Copyright © 1993, 2020, Oracle and/or its affiliates. All rights reserved. Use is subject to license terms. Also see the documentation redistribution policy.