8005298: Add FunctionalInterface type to the core libraries

Reviewed-by: mduigou

src/share/classes/java/lang/FunctionalInterface.java

+/*
+ * Copyright (c) 2012, 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
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+package java.lang;
+
+import java.lang.annotation.*;
+
+/**
+ * Indicates 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>
+ *
+ * @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 {}