70 lines
2.9 KiB
Java
70 lines
2.9 KiB
Java
/*
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* Copyright (c) 2012, 2019, Oracle and/or its affiliates. All rights reserved.
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Oracle designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Oracle in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*/
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package java.lang;
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import java.lang.annotation.*;
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/**
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* An informative annotation type used to indicate that an interface
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* type declaration is intended to be a <i>functional interface</i> as
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* defined by the Java Language Specification.
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*
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* Conceptually, a functional interface has exactly one abstract
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* method. Since {@linkplain java.lang.reflect.Method#isDefault()
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* default methods} have an implementation, they are not abstract. If
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* an interface declares an abstract method overriding one of the
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* public methods of {@code java.lang.Object}, that also does
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* <em>not</em> count toward the interface's abstract method count
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* since any implementation of the interface will have an
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* implementation from {@code java.lang.Object} or elsewhere.
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*
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* <p>Note that instances of functional interfaces can be created with
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* lambda expressions, method references, or constructor references.
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*
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* <p>If a type is annotated with this annotation type, compilers are
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* required to generate an error message unless:
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*
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* <ul>
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* <li> The type is an interface type and not an annotation type, enum, or class.
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* <li> The annotated type satisfies the requirements of a functional interface.
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* </ul>
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*
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* <p>However, the compiler will treat any interface meeting the
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* definition of a functional interface as a functional interface
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* regardless of whether or not a {@code FunctionalInterface}
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* annotation is present on the interface declaration.
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*
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* @jls 4.3.2 The Class Object
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* @jls 9.8 Functional Interfaces
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* @jls 9.4.3 Interface Method Body
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* @jls 9.6.4.9 @FunctionalInterface
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* @since 1.8
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*/
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@Documented
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@Retention(RetentionPolicy.RUNTIME)
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@Target(ElementType.TYPE)
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public @interface FunctionalInterface {}
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