/*

  * Copyright (c) 2008, 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

  * or visit www.oracle.com if you need additional information or have any

  * questions.

  */

 package java.lang.invoke;

 import java.lang.ref.WeakReference;

 import java.lang.ref.Reference;

 import java.lang.ref.ReferenceQueue;

 import java.util.Arrays;

 import java.util.Collections;

 import java.util.List;

 import java.util.Objects;

 import java.util.concurrent.ConcurrentMap;

 import java.util.concurrent.ConcurrentHashMap;

 /**

  * A method type represents the arguments and return type accepted and

  * returned by a method handle, or the arguments and return type passed

  * and expected  by a method handle caller.  Method types must be properly

  * matched between a method handle and all its callers,

  * and the JVM's operations enforce this matching at, specifically

  * during calls to {@link MethodHandle#invokeExact MethodHandle.invokeExact}

  * and {@link MethodHandle#invoke MethodHandle.invoke}, and during execution

  * of {@code invokedynamic} instructions.

  * <p>

  * The structure is a return type accompanied by any number of parameter types.

  * The types (primitive, {@code void}, and reference) are represented by {@link Class} objects.

  * (For ease of exposition, we treat {@code void} as if it were a type.

  * In fact, it denotes the absence of a return type.)

  * <p>

  * All instances of {@code MethodType} are immutable.

  * Two instances are completely interchangeable if they compare equal.

  * Equality depends on pairwise correspondence of the return and parameter types and on nothing else.

  * <p>

  * This type can be created only by factory methods.

  * All factory methods may cache values, though caching is not guaranteed.

  * Some factory methods are static, while others are virtual methods which

  * modify precursor method types, e.g., by changing a selected parameter.

  * <p>

  * Factory methods which operate on groups of parameter types

  * are systematically presented in two versions, so that both Java arrays and

  * Java lists can be used to work with groups of parameter types.

  * The query methods {@code parameterArray} and {@code parameterList}

  * also provide a choice between arrays and lists.

  * <p>

  * {@code MethodType} objects are sometimes derived from bytecode instructions

  * such as {@code invokedynamic}, specifically from the type descriptor strings associated

  * with the instructions in a class file's constant pool.

  * <p>

  * Like classes and strings, method types can also be represented directly

  * in a class file's constant pool as constants.

  * A method type may be loaded by an {@code ldc} instruction which refers

  * to a suitable {@code CONSTANT_MethodType} constant pool entry.

  * The entry refers to a {@code CONSTANT_Utf8} spelling for the descriptor string.

  * (For full details on method type constants,

  * see sections 4.4.8 and 5.4.3.5 of the Java Virtual Machine Specification.)

  * <p>

  * When the JVM materializes a {@code MethodType} from a descriptor string,

  * all classes named in the descriptor must be accessible, and will be loaded.

  * (But the classes need not be initialized, as is the case with a {@code CONSTANT_Class}.)

  * This loading may occur at any time before the {@code MethodType} object is first derived.

  * @author John Rose, JSR 292 EG

  */

 public final

 class MethodType implements java.io.Serializable {

     private static final long serialVersionUID = 292L;  // {rtype, {ptype...}}

     // The rtype and ptypes fields define the structural identity of the method type:

     private final Class<?>   rtype;

     private final Class<?>[] ptypes;

     /**

      * Check the given parameters for validity and store them into the final fields.

      */

     private MethodType(Class<?> rtype, Class<?>[] ptypes, boolean trusted) {

         this.rtype = rtype;

         // defensively copy the array passed in by the user

         this.ptypes = trusted ? ptypes : Arrays.copyOf(ptypes, ptypes.length);

     }

     /**

      * Construct a temporary unchecked instance of MethodType for use only as a key to the intern table.

      * Does not check the given parameters for validity, and must be discarded after it is used as a searching key.

      * The parameters are reversed for this constructor, so that is is not accidentally used.

      */

     private MethodType(Class<?>[] ptypes, Class<?> rtype) {

         this.rtype = rtype;

         this.ptypes = ptypes;

     }

     /** This number, mandated by the JVM spec as 255,

      *  is the maximum number of <em>slots</em>

      *  that any Java method can receive in its argument list.

      *  It limits both JVM signatures and method type objects.

      *  The longest possible invocation will look like

      *  {@code staticMethod(arg1, arg2, ..., arg255)} or

      *  {@code x.virtualMethod(arg1, arg2, ..., arg254)}.

      */

     /*non-public*/ static final int MAX_JVM_ARITY = 255;  // this is mandated by the JVM spec.

     /** This number is the maximum arity of a method handle, 254.

      *  It is derived from the absolute JVM-imposed arity by subtracting one,

      *  which is the slot occupied by the method handle itself at the

      *  beginning of the argument list used to invoke the method handle.

      *  The longest possible invocation will look like

      *  {@code mh.invoke(arg1, arg2, ..., arg254)}.

      */

     // Issue:  Should we allow MH.invokeWithArguments to go to the full 255?

     /*non-public*/ static final int MAX_MH_ARITY = MAX_JVM_ARITY-1;  // deduct one for mh receiver

     /** This number is the maximum arity of a method handle invoker, 253.

      *  It is derived from the absolute JVM-imposed arity by subtracting two,

      *  which are the slots occupied by invoke method handle, and the

      *  target method handle, which are both at the beginning of the argument

      *  list used to invoke the target method handle.

      *  The longest possible invocation will look like

      *  {@code invokermh.invoke(targetmh, arg1, arg2, ..., arg253)}.

      */

     /*non-public*/ static final int MAX_MH_INVOKER_ARITY = MAX_MH_ARITY-1;  // deduct one more for invoker

     private static void checkRtype(Class<?> rtype) {

         Objects.requireNonNull(rtype);

     }

     static final ConcurrentWeakInternSet<MethodType> internTable = new ConcurrentWeakInternSet<>();

     static final Class<?>[] NO_PTYPES = {};

     /**

      * Finds or creates an instance of the given method type.

      * @param rtype  the return type

      * @param ptypes the parameter types

      * @return a method type with the given components

      * @throws NullPointerException if {@code rtype} or {@code ptypes} or any element of {@code ptypes} is null

      * @throws IllegalArgumentException if any element of {@code ptypes} is {@code void.class}

      */

     public static

     MethodType methodType(Class<?> rtype, Class<?>[] ptypes) {

         return makeImpl(rtype, ptypes, false);

     }

     /**

      * Finds or creates a method type with the given components.

      * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

      * @param rtype  the return type

      * @param ptypes the parameter types

      * @return a method type with the given components

      * @throws NullPointerException if {@code rtype} or {@code ptypes} or any element of {@code ptypes} is null

      * @throws IllegalArgumentException if any element of {@code ptypes} is {@code void.class}

      */

     public static

     MethodType methodType(Class<?> rtype, List<Class<?>> ptypes) {

         boolean notrust = false;  // random List impl. could return evil ptypes array

         return makeImpl(rtype, listToArray(ptypes), notrust);

     }

     private static Class<?>[] listToArray(List<Class<?>> ptypes) {

         return ptypes.toArray(NO_PTYPES);

     }

     /**

      * Finds or creates a method type with the given components.

      * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

      * The leading parameter type is prepended to the remaining array.

      * @param rtype  the return type

      * @param ptype0 the first parameter type

      * @param ptypes the remaining parameter types

      * @return a method type with the given components

      * @throws NullPointerException if {@code rtype} or {@code ptype0} or {@code ptypes} or any element of {@code ptypes} is null

      * @throws IllegalArgumentException if {@code ptype0} or {@code ptypes} or any element of {@code ptypes} is {@code void.class}

      */

     public static

     MethodType methodType(Class<?> rtype, Class<?> ptype0, Class<?>... ptypes) {

         Class<?>[] ptypes1 = new Class<?>[1+ptypes.length];

         ptypes1[0] = ptype0;

         System.arraycopy(ptypes, 0, ptypes1, 1, ptypes.length);

         return makeImpl(rtype, ptypes1, true);

     }

     /**

      * Finds or creates a method type with the given components.

      * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

      * The resulting method has no parameter types.

      * @param rtype  the return type

      * @return a method type with the given return value

      * @throws NullPointerException if {@code rtype} is null

      */

     public static

     MethodType methodType(Class<?> rtype) {

         return makeImpl(rtype, NO_PTYPES, true);

     }

     /**

      * Finds or creates a method type with the given components.

      * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

      * The resulting method has the single given parameter type.

      * @param rtype  the return type

      * @param ptype0 the parameter type

      * @return a method type with the given return value and parameter type

      * @throws NullPointerException if {@code rtype} or {@code ptype0} is null

      * @throws IllegalArgumentException if {@code ptype0} is {@code void.class}

      */

     public static

     MethodType methodType(Class<?> rtype, Class<?> ptype0) {

         return makeImpl(rtype, new Class<?>[]{ ptype0 }, true);

     }

     /**

      * Finds or creates a method type with the given components.

      * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

      * The resulting method has the same parameter types as {@code ptypes},

      * and the specified return type.

      * @param rtype  the return type

      * @param ptypes the method type which supplies the parameter types

      * @return a method type with the given components

      * @throws NullPointerException if {@code rtype} or {@code ptypes} is null

      */

     public static

     MethodType methodType(Class<?> rtype, MethodType ptypes) {

         return makeImpl(rtype, ptypes.ptypes, true);

     }

     /**

      * Sole factory method to find or create an interned method type.

      * @param rtype desired return type

      * @param ptypes desired parameter types

      * @param trusted whether the ptypes can be used without cloning

      * @return the unique method type of the desired structure

      */

     /*trusted*/ static

     MethodType makeImpl(Class<?> rtype, Class<?>[] ptypes, boolean trusted) {

         throw new IllegalStateException();

     }

     private static final MethodType[] objectOnlyTypes = new MethodType[20];

     /**

      * Finds or creates a method type whose components are {@code Object} with an optional trailing {@code Object[]} array.

      * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

      * All parameters and the return type will be {@code Object},

      * except the final array parameter if any, which will be {@code Object[]}.

      * @param objectArgCount number of parameters (excluding the final array parameter if any)

      * @param finalArray whether there will be a trailing array parameter, of type {@code Object[]}

      * @return a generally applicable method type, for all calls of the given fixed argument count and a collected array of further arguments

      * @throws IllegalArgumentException if {@code objectArgCount} is negative or greater than 255 (or 254, if {@code finalArray} is true)

      * @see #genericMethodType(int)

      */

     public static

     MethodType genericMethodType(int objectArgCount, boolean finalArray) {

         throw new IllegalStateException();

     }

     /**

      * Finds or creates a method type whose components are all {@code Object}.

      * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

      * All parameters and the return type will be Object.

      * @param objectArgCount number of parameters

      * @return a generally applicable method type, for all calls of the given argument count

      * @throws IllegalArgumentException if {@code objectArgCount} is negative or greater than 255

      * @see #genericMethodType(int, boolean)

      */

     public static

     MethodType genericMethodType(int objectArgCount) {

         return genericMethodType(objectArgCount, false);

     }

     /**

      * Finds or creates a method type with a single different parameter type.

      * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

      * @param num    the index (zero-based) of the parameter type to change

      * @param nptype a new parameter type to replace the old one with

      * @return the same type, except with the selected parameter changed

      * @throws IndexOutOfBoundsException if {@code num} is not a valid index into {@code parameterArray()}

      * @throws IllegalArgumentException if {@code nptype} is {@code void.class}

      * @throws NullPointerException if {@code nptype} is null

      */

     public MethodType changeParameterType(int num, Class<?> nptype) {

         throw new IllegalStateException();

     }

     /**

      * Finds or creates a method type with additional parameter types.

      * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

      * @param num    the position (zero-based) of the inserted parameter type(s)

      * @param ptypesToInsert zero or more new parameter types to insert into the parameter list

      * @return the same type, except with the selected parameter(s) inserted

      * @throws IndexOutOfBoundsException if {@code num} is negative or greater than {@code parameterCount()}

      * @throws IllegalArgumentException if any element of {@code ptypesToInsert} is {@code void.class}

      *                                  or if the resulting method type would have more than 255 parameter slots

      * @throws NullPointerException if {@code ptypesToInsert} or any of its elements is null

      */

     public MethodType insertParameterTypes(int num, Class<?>... ptypesToInsert) {

         throw new IllegalStateException();

     }

     /**

      * Finds or creates a method type with additional parameter types.

      * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

      * @param ptypesToInsert zero or more new parameter types to insert after the end of the parameter list

      * @return the same type, except with the selected parameter(s) appended

      * @throws IllegalArgumentException if any element of {@code ptypesToInsert} is {@code void.class}

      *                                  or if the resulting method type would have more than 255 parameter slots

      * @throws NullPointerException if {@code ptypesToInsert} or any of its elements is null

      */

     public MethodType appendParameterTypes(Class<?>... ptypesToInsert) {

         return insertParameterTypes(parameterCount(), ptypesToInsert);

     }

     /**

      * Finds or creates a method type with additional parameter types.

      * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

      * @param num    the position (zero-based) of the inserted parameter type(s)

      * @param ptypesToInsert zero or more new parameter types to insert into the parameter list

      * @return the same type, except with the selected parameter(s) inserted

      * @throws IndexOutOfBoundsException if {@code num} is negative or greater than {@code parameterCount()}

      * @throws IllegalArgumentException if any element of {@code ptypesToInsert} is {@code void.class}

      *                                  or if the resulting method type would have more than 255 parameter slots

      * @throws NullPointerException if {@code ptypesToInsert} or any of its elements is null

      */

     public MethodType insertParameterTypes(int num, List<Class<?>> ptypesToInsert) {

         return insertParameterTypes(num, listToArray(ptypesToInsert));

     }

     /**

      * Finds or creates a method type with additional parameter types.

      * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

      * @param ptypesToInsert zero or more new parameter types to insert after the end of the parameter list

      * @return the same type, except with the selected parameter(s) appended

      * @throws IllegalArgumentException if any element of {@code ptypesToInsert} is {@code void.class}

      *                                  or if the resulting method type would have more than 255 parameter slots

      * @throws NullPointerException if {@code ptypesToInsert} or any of its elements is null

      */

     public MethodType appendParameterTypes(List<Class<?>> ptypesToInsert) {

         return insertParameterTypes(parameterCount(), ptypesToInsert);

     }

      /**

      * Finds or creates a method type with modified parameter types.

      * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

      * @param start  the position (zero-based) of the first replaced parameter type(s)

      * @param end    the position (zero-based) after the last replaced parameter type(s)

      * @param ptypesToInsert zero or more new parameter types to insert into the parameter list

      * @return the same type, except with the selected parameter(s) replaced

      * @throws IndexOutOfBoundsException if {@code start} is negative or greater than {@code parameterCount()}

      *                                  or if {@code end} is negative or greater than {@code parameterCount()}

      *                                  or if {@code start} is greater than {@code end}

      * @throws IllegalArgumentException if any element of {@code ptypesToInsert} is {@code void.class}

      *                                  or if the resulting method type would have more than 255 parameter slots

      * @throws NullPointerException if {@code ptypesToInsert} or any of its elements is null

      */

     /*non-public*/ MethodType replaceParameterTypes(int start, int end, Class<?>... ptypesToInsert) {

         throw new IllegalStateException();

     }

     /**

      * Finds or creates a method type with some parameter types omitted.

      * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

      * @param start  the index (zero-based) of the first parameter type to remove

      * @param end    the index (greater than {@code start}) of the first parameter type after not to remove

      * @return the same type, except with the selected parameter(s) removed

      * @throws IndexOutOfBoundsException if {@code start} is negative or greater than {@code parameterCount()}

      *                                  or if {@code end} is negative or greater than {@code parameterCount()}

      *                                  or if {@code start} is greater than {@code end}

      */

     public MethodType dropParameterTypes(int start, int end) {

         throw new IllegalStateException();

     }

     /**

      * Finds or creates a method type with a different return type.

      * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

      * @param nrtype a return parameter type to replace the old one with

      * @return the same type, except with the return type change

      * @throws NullPointerException if {@code nrtype} is null

      */

     public MethodType changeReturnType(Class<?> nrtype) {

         throw new IllegalStateException();

     }

     /**

      * Reports if this type contains a primitive argument or return value.

      * The return type {@code void} counts as a primitive.

      * @return true if any of the types are primitives

      */

     public boolean hasPrimitives() {

         throw new IllegalStateException();

     }

     /**

      * Reports if this type contains a wrapper argument or return value.

      * Wrappers are types which box primitive values, such as {@link Integer}.

      * The reference type {@code java.lang.Void} counts as a wrapper,

      * if it occurs as a return type.

      * @return true if any of the types are wrappers

      */

     public boolean hasWrappers() {

         return unwrap() != this;

     }

     /**

      * Erases all reference types to {@code Object}.

      * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

      * All primitive types (including {@code void}) will remain unchanged.

      * @return a version of the original type with all reference types replaced

      */

     public MethodType erase() {

         throw new IllegalStateException();

     }

     /**

      * Erases all reference types to {@code Object}, and all subword types to {@code int}.

      * This is the reduced type polymorphism used by private methods

      * such as {@link MethodHandle#invokeBasic invokeBasic}.

      * @return a version of the original type with all reference and subword types replaced

      */

     /*non-public*/ MethodType basicType() {

         throw new IllegalStateException();

     }

     /**

      * @return a version of the original type with MethodHandle prepended as the first argument

      */

     /*non-public*/ MethodType invokerType() {

         throw new IllegalStateException();

     }

     /**

      * Converts all types, both reference and primitive, to {@code Object}.

      * Convenience method for {@link #genericMethodType(int) genericMethodType}.

      * The expression {@code type.wrap().erase()} produces the same value

      * as {@code type.generic()}.

      * @return a version of the original type with all types replaced

      */

     public MethodType generic() {

         return genericMethodType(parameterCount());

     }

     /**

      * Converts all primitive types to their corresponding wrapper types.

      * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

      * All reference types (including wrapper types) will remain unchanged.

      * A {@code void} return type is changed to the type {@code java.lang.Void}.

      * The expression {@code type.wrap().erase()} produces the same value

      * as {@code type.generic()}.

      * @return a version of the original type with all primitive types replaced

      */

     public MethodType wrap() {

         return hasPrimitives() ? wrapWithPrims(this) : this;

     }

     /**

      * Converts all wrapper types to their corresponding primitive types.

      * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

      * All primitive types (including {@code void}) will remain unchanged.

      * A return type of {@code java.lang.Void} is changed to {@code void}.

      * @return a version of the original type with all wrapper types replaced

      */

     public MethodType unwrap() {

         MethodType noprims = !hasPrimitives() ? this : wrapWithPrims(this);

         return unwrapWithNoPrims(noprims);

     }

     private static MethodType wrapWithPrims(MethodType pt) {

         throw new IllegalStateException();

     }

     private static MethodType unwrapWithNoPrims(MethodType wt) {

         throw new IllegalStateException();

     }

     /**

      * Returns the parameter type at the specified index, within this method type.

      * @param num the index (zero-based) of the desired parameter type

      * @return the selected parameter type

      * @throws IndexOutOfBoundsException if {@code num} is not a valid index into {@code parameterArray()}

      */

     public Class<?> parameterType(int num) {

         return ptypes[num];

     }

     /**

      * Returns the number of parameter types in this method type.

      * @return the number of parameter types

      */

     public int parameterCount() {

         return ptypes.length;

     }

     /**

      * Returns the return type of this method type.

      * @return the return type

      */

     public Class<?> returnType() {

         return rtype;

     }

     /**

      * Presents the parameter types as a list (a convenience method).

      * The list will be immutable.

      * @return the parameter types (as an immutable list)

      */

     public List<Class<?>> parameterList() {

         return Collections.unmodifiableList(Arrays.asList(ptypes));

     }

     /*non-public*/ Class<?> lastParameterType() {

         int len = ptypes.length;

         return len == 0 ? void.class : ptypes[len-1];

     }

     /**

      * Presents the parameter types as an array (a convenience method).

      * Changes to the array will not result in changes to the type.

      * @return the parameter types (as a fresh copy if necessary)

      */

     public Class<?>[] parameterArray() {

         return ptypes.clone();

     }

     /**

      * Compares the specified object with this type for equality.

      * That is, it returns <tt>true</tt> if and only if the specified object

      * is also a method type with exactly the same parameters and return type.

      * @param x object to compare

      * @see Object#equals(Object)

      */

     @Override

     public boolean equals(Object x) {

         return this == x || x instanceof MethodType && equals((MethodType)x);

     }

     private boolean equals(MethodType that) {

         return this.rtype == that.rtype

             && Arrays.equals(this.ptypes, that.ptypes);

     }

     /**

      * Returns the hash code value for this method type.

      * It is defined to be the same as the hashcode of a List

      * whose elements are the return type followed by the

      * parameter types.

      * @return the hash code value for this method type

      * @see Object#hashCode()

      * @see #equals(Object)

      * @see List#hashCode()

      */

     @Override

     public int hashCode() {

       int hashCode = 31 + rtype.hashCode();

       for (Class<?> ptype : ptypes)

           hashCode = 31*hashCode + ptype.hashCode();

       return hashCode;

     }

     /**

      * Returns a string representation of the method type,

      * of the form {@code "(PT0,PT1...)RT"}.

      * The string representation of a method type is a

      * parenthesis enclosed, comma separated list of type names,

      * followed immediately by the return type.

      * <p>

      * Each type is represented by its

      * {@link java.lang.Class#getSimpleName simple name}.

      */

     @Override

     public String toString() {

         StringBuilder sb = new StringBuilder();

         sb.append("(");

         for (int i = 0; i < ptypes.length; i++) {

             if (i > 0)  sb.append(",");

             sb.append(ptypes[i].getSimpleName());

         }

         sb.append(")");

         sb.append(rtype.getSimpleName());

         return sb.toString();

     }

     /**

      * Finds or creates an instance of a method type, given the spelling of its bytecode descriptor.

      * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

      * Any class or interface name embedded in the descriptor string

      * will be resolved by calling {@link ClassLoader#loadClass(java.lang.String)}

      * on the given loader (or if it is null, on the system class loader).

      * <p>

      * Note that it is possible to encounter method types which cannot be

      * constructed by this method, because their component types are

      * not all reachable from a common class loader.

      * <p>

      * This method is included for the benefit of applications that must

      * generate bytecodes that process method handles and {@code invokedynamic}.

      * @param descriptor a bytecode-level type descriptor string "(T...)T"

      * @param loader the class loader in which to look up the types

      * @return a method type matching the bytecode-level type descriptor

      * @throws NullPointerException if the string is null

      * @throws IllegalArgumentException if the string is not well-formed

      * @throws TypeNotPresentException if a named type cannot be found

      */

     public static MethodType fromMethodDescriptorString(String descriptor, ClassLoader loader)

         throws IllegalArgumentException

     {

         throw new IllegalStateException();

     }

     /**

      * Produces a bytecode descriptor representation of the method type.

      * <p>

      * Note that this is not a strict inverse of {@link #fromMethodDescriptorString fromMethodDescriptorString}.

      * Two distinct classes which share a common name but have different class loaders

      * will appear identical when viewed within descriptor strings.

      * <p>

      * This method is included for the benefit of applications that must

      * generate bytecodes that process method handles and {@code invokedynamic}.

      * {@link #fromMethodDescriptorString(java.lang.String, java.lang.ClassLoader) fromMethodDescriptorString},

      * because the latter requires a suitable class loader argument.

      * @return the bytecode type descriptor representation

      */

     public String toMethodDescriptorString() {

         throw new IllegalStateException();

     }

     /// Serialization.

     /**

      * There are no serializable fields for {@code MethodType}.

      */

     private static final java.io.ObjectStreamField[] serialPersistentFields = { };

 //    /**

 //     * Save the {@code MethodType} instance to a stream.

 //     *

 //     * @serialData

 //     * For portability, the serialized format does not refer to named fields.

 //     * Instead, the return type and parameter type arrays are written directly

 //     * from the {@code writeObject} method, using two calls to {@code s.writeObject}

 //     * as follows:

 //     * <blockquote><pre>{@code

 //s.writeObject(this.returnType());

 //s.writeObject(this.parameterArray());

 //     * }</pre></blockquote>

 //     * <p>

 //     * The deserialized field values are checked as if they were

 //     * provided to the factory method {@link #methodType(Class,Class[]) methodType}.

 //     * For example, null values, or {@code void} parameter types,

 //     * will lead to exceptions during deserialization.

 //     * @param s the stream to write the object to

 //     * @throws java.io.IOException if there is a problem writing the object

 //     */

 //    private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException {

 //        s.defaultWriteObject();  // requires serialPersistentFields to be an empty array

 //        s.writeObject(returnType());

 //        s.writeObject(parameterArray());

 //    }

 //

 //    /**

 //     * Reconstitute the {@code MethodType} instance from a stream (that is,

 //     * deserialize it).

 //     * This instance is a scratch object with bogus final fields.

 //     * It provides the parameters to the factory method called by

 //     * {@link #readResolve readResolve}.

 //     * After that call it is discarded.

 //     * @param s the stream to read the object from

 //     * @throws java.io.IOException if there is a problem reading the object

 //     * @throws ClassNotFoundException if one of the component classes cannot be resolved

 //     * @see #MethodType()

 //     * @see #readResolve

 //     * @see #writeObject

 //     */

 //    private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException {

 //        s.defaultReadObject();  // requires serialPersistentFields to be an empty array

 //

 //        Class<?>   returnType     = (Class<?>)   s.readObject();

 //        Class<?>[] parameterArray = (Class<?>[]) s.readObject();

 //

 //        // Probably this object will never escape, but let's check

 //        // the field values now, just to be sure.

 //        checkRtype(returnType);

 //        checkPtypes(parameterArray);

 //

 //        parameterArray = parameterArray.clone();  // make sure it is unshared

 //        MethodType_init(returnType, parameterArray);

 //    }

     /**

      * For serialization only.

      * Sets the final fields to null, pending {@code Unsafe.putObject}.

      */

     private MethodType() {

         this.rtype = null;

         this.ptypes = null;

     }

 //    private void MethodType_init(Class<?> rtype, Class<?>[] ptypes) {

 //        // In order to communicate these values to readResolve, we must

 //        // store them into the implementation-specific final fields.

 //        checkRtype(rtype);

 //        checkPtypes(ptypes);

 //        UNSAFE.putObject(this, rtypeOffset, rtype);

 //        UNSAFE.putObject(this, ptypesOffset, ptypes);

 //    }

     // Support for resetting final fields while deserializing

 //    private static final long rtypeOffset, ptypesOffset;

 //    static {

 //        try {

 //            rtypeOffset = UNSAFE.objectFieldOffset

 //                (MethodType.class.getDeclaredField("rtype"));

 //            ptypesOffset = UNSAFE.objectFieldOffset

 //                (MethodType.class.getDeclaredField("ptypes"));

 //        } catch (Exception ex) {

 //            throw new Error(ex);

 //        }

 //    }

     /**

      * Resolves and initializes a {@code MethodType} object

      * after serialization.

      * @return the fully initialized {@code MethodType} object

      */

     private Object readResolve() {

         // Do not use a trusted path for deserialization:

         //return makeImpl(rtype, ptypes, true);

         // Verify all operands, and make sure ptypes is unshared:

         return methodType(rtype, ptypes);

     }

     /**

      * Simple implementation of weak concurrent intern set.

      *

      * @param <T> interned type

      */

     private static class ConcurrentWeakInternSet<T> {

         private final ConcurrentMap<WeakEntry<T>, WeakEntry<T>> map;

         private final ReferenceQueue<T> stale;

         public ConcurrentWeakInternSet() {

             this.map = new ConcurrentHashMap<>();

             this.stale = new ReferenceQueue<>();

         }

         /**

          * Get the existing interned element.

          * This method returns null if no element is interned.

          *

          * @param elem element to look up

          * @return the interned element

          */

         public T get(T elem) {

             if (elem == null) throw new NullPointerException();

             expungeStaleElements();

             WeakEntry<T> value = map.get(new WeakEntry<>(elem));

             if (value != null) {

                 T res = value.get();

                 if (res != null) {

                     return res;

                 }

             }

             return null;

         }

         /**

          * Interns the element.

          * Always returns non-null element, matching the one in the intern set.

          * Under the race against another add(), it can return <i>different</i>

          * element, if another thread beats us to interning it.

          *

          * @param elem element to add

          * @return element that was actually added

          */

         public T add(T elem) {

             if (elem == null) throw new NullPointerException();

             // Playing double race here, and so spinloop is required.

             // First race is with two concurrent updaters.

             // Second race is with GC purging weak ref under our feet.

             // Hopefully, we almost always end up with a single pass.

             T interned;

             WeakEntry<T> e = new WeakEntry<>(elem, stale);

             do {

                 expungeStaleElements();

                 WeakEntry<T> exist = map.putIfAbsent(e, e);

                 interned = (exist == null) ? elem : exist.get();

             } while (interned == null);

             return interned;

         }

         private void expungeStaleElements() {

             Reference<? extends T> reference;

             while ((reference = stale.poll()) != null) {

                 map.remove(reference);

             }

         }

         private static class WeakEntry<T> extends WeakReference<T> {

             public final int hashcode;

             public WeakEntry(T key, ReferenceQueue<T> queue) {

                 super(key, queue);

                 hashcode = key.hashCode();

             }

             public WeakEntry(T key) {

                 super(key);

                 hashcode = key.hashCode();

             }

             @Override

             public boolean equals(Object obj) {

                 if (obj instanceof WeakEntry) {

                     Object that = ((WeakEntry) obj).get();

                     Object mine = get();

                     return (that == null || mine == null) ? (this == obj) : mine.equals(that);

                 }

                 return false;

             }

             @Override

             public int hashCode() {

                 return hashcode;

             }

         }

     }

 }