1 /*
   2  * Copyright (c) 2012, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package java.lang;
  27 
  28 import java.lang.annotation.*;
  29 
  30 /**
  31  * Indicates that an interface type declaration is intended to be a
  32  * <i>functional interface</i> as defined by the Java Language
  33  * Specification.
  34  *
  35  * Conceptually, a functional interface has exactly one abstract
  36  * method.  Since {@linkplain java.lang.reflect.Method#isDefault()
  37  * default methods} have an implementation, they are not abstract.  If
  38  * an interface declares an abstract method overriding one of the
  39  * public methods of {@code java.lang.Object}, that also does
  40  * <em>not</em> count toward the interface's abstract method count
  41  * since any implementation of the interface will have an
  42  * implementation from {@code java.lang.Object} or elsewhere.
  43  *
  44  * <p>Note that instances of functional interfaces can be created with
  45  * lambda expressions, method references, or constructor references.
  46  *
  47  * <p>If a type is annotated with this annotation type, compilers are
  48  * required to generate an error message unless:
  49  *
  50  * <ul>
  51  * <li> The type is an interface type and not an annotation type, enum, or class.
  52  * <li> The annotated type satisfies the requirements of a functional interface.
  53  * </ul>
  54  *
  55  * @jls 4.3.2. The Class Object
  56  * @jls 9.8 Functional Interfaces
  57  * @jls 9.4.3 Interface Method Body
  58  * @since 1.8
  59  */
  60 @Documented
  61 @Retention(RetentionPolicy.RUNTIME)
  62 @Target(ElementType.TYPE)
  63 public @interface FunctionalInterface {}