1/*
2 * Copyright (c) 2012, 2015, 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
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24 */
25
26package java.lang;
27
28import java.lang.annotation.*;
29
30/**
31 * An informative annotation type used to indicate that an interface
32 * type declaration is intended to be a <i>functional interface</i> as
33 * defined by the Java Language 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 * <p>However, the compiler will treat any interface meeting the
56 * definition of a functional interface as a functional interface
57 * regardless of whether or not a {@code FunctionalInterface}
58 * annotation is present on the interface declaration.
59 *
60 * @jls 4.3.2. The Class Object
61 * @jls 9.8 Functional Interfaces
62 * @jls 9.4.3 Interface Method Body
63 * @jls 9.6.4.9 @FunctionalInterface
64 * @since 1.8
65 */
66@Documented
67@Retention(RetentionPolicy.RUNTIME)
68@Target(ElementType.TYPE)
69public @interface FunctionalInterface {}
70