/*

 * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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 */

package java.lang;

import java.lang.annotation.*;

/**

 * An informative annotation type used to indicate that an interface

 * type declaration is intended to be a <i>functional interface</i> 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 {@code java.lang.Object}, that also does

 * <em>not</em> count toward the interface's abstract method count

 * since any implementation of the interface will have an

 * implementation from {@code java.lang.Object} or elsewhere.

 *

 * <p>Note that instances of functional interfaces can be created with

 * lambda expressions, method references, or constructor references.

 *

 * <p>If a type is annotated with this annotation type, compilers are

 * required to generate an error message unless:

 *

 * <ul>

 * <li> The type is an interface type and not an annotation type, enum, or class.

 * <li> The annotated type satisfies the requirements of a functional interface.

 * </ul>

 *

 * <p>However, the compiler will treat any interface meeting the

 * definition of a functional interface as a functional interface

 * regardless of whether or not a {@code FunctionalInterface}

 * annotation is present on the interface declaration.

 *

 * @jls 4.3.2. The Class Object

 * @jls 9.8 Functional Interfaces

 * @jls 9.4.3 Interface Method Body

 * @since 1.8

 */

@Documented

@Retention(RetentionPolicy.RUNTIME)

@Target(ElementType.TYPE)

public @interface FunctionalInterface {}