1 /* 2 * Copyright (c) 2012, 2013, 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 * 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 * @since 1.8 64 */ 65 @Documented 66 @Retention(RetentionPolicy.RUNTIME) 67 @Target(ElementType.TYPE) 68 public @interface FunctionalInterface {} 69