/*
 * Copyright (c) 2018, 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.constant;

import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodType;
import java.lang.invoke.VarHandle;
import java.util.Optional;

/**
 * Represents a type which is <em>constable</em>.  A constable type is one whose
 * values are constants that can be represented in the constant pool of a Java
 * classfile as described in JVMS 4.4, and whose instances can describe themselves
 * nominally as a {@link ConstantDesc}.
 *
 * <p>Some constable types have a native representation in the constant pool:
 * {@link String}, {@link Integer}, {@link Long}, {@link Float},
 * {@link Double}, {@link Class}, {@link MethodType}, and {@link MethodHandle}.
 * The types {@link String}, {@link Integer}, {@link Long}, {@link Float},
 * and {@link Double} serve as their own nominal descriptors; {@link Class},
 * {@link MethodType}, and {@link MethodHandle} have corresponding nominal
 * descriptors {@link ClassDesc}, {@link MethodTypeDesc}, and {@link MethodHandleDesc}.
 *
 * <p>Other reference types can be constable if their instances can describe
 * themselves in nominal form as a {@link ConstantDesc}. Examples in the Java SE
 * Platform API are types that support Java language features such as {@link Enum},
 * and runtime support classes such as {@link VarHandle}.  These are typically
 * described with a {@link DynamicConstantDesc}, which describes dynamically
 * generated constants (JVMS 4.4.10).
 *
 * <p>The nominal form of an instance of a constable type is obtained via
 * {@link #describeConstable()}. A {@linkplain Constable} need
 * not be able to (or may choose not to) describe all its instances in the form of
 * a {@link ConstantDesc}; this method returns an {@link Optional} that can be
 * empty to indicate that a nominal descriptor could not be created for an instance.
 * (For example, {@link MethodHandle} will produce nominal descriptors for direct
 * method handles, but not necessarily those produced by method handle
 * combinators.)
 * @jvms 4.4 The Constant Pool
 * @jvms 4.4.10 The CONSTANT_InvokeDynamic_info Structure
 *
 * @since 12
 */
public interface Constable {
    /**
     * Returns an {@link Optional} containing the nominal descriptor for this
     * instance, if one can be constructed, or an empty {@link Optional}
     * if one cannot be constructed.
     *
     * @return An {@link Optional} containing the resulting nominal descriptor,
     * or an empty {@link Optional} if one cannot be constructed.
     */
    Optional<? extends ConstantDesc> describeConstable();
}