content: /*
   * 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 sun.invoke.util.Wrapper;
  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;
  import sun.invoke.util.BytecodeDescriptor;
  import static java.lang.invoke.MethodHandleStatics.*;
  
  /**
   * 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;
  
      // The remaining fields are caches of various sorts:
      private @Stable MethodTypeForm form; // erased form, plus cached data about primitives
      private @Stable MethodType wrapAlt;  // alternative wrapped/unwrapped version
      // Android-removed: Cache of higher order adapters.
      // We're not dynamically generating any adapters at this point.
      // private @Stable Invokers invokers;   // cache of handy higher-order adapters
      private @Stable String methodDescriptor;  // cache for toMethodDescriptorString
  
      /**
       * Check the given parameters for validity and store them into the final fields.
       */
      private MethodType(Class<?> rtype, Class<?>[] ptypes, boolean trusted) {
          checkRtype(rtype);
          checkPtypes(ptypes);
          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;
      }
  
      /*trusted*/ MethodTypeForm form() { return form; }
      // Android-changed: Make rtype()/ptypes() public @hide for implementation use.
      // /*trusted*/ Class<?> rtype() { return rtype; }
      // /*trusted*/ Class<?>[] ptypes() { return ptypes; }
      /*trusted*/ /** @hide */ public Class<?> rtype() { return rtype; }
      /*trusted*/ /** @hide */ public Class<?>[] ptypes() { return ptypes; }
  
      // Android-removed: Implementation methods unused on Android.
      // void setForm(MethodTypeForm f) { form = f; }
  
      /** 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);
      }
      private static void checkPtype(Class<?> ptype) {
          Objects.requireNonNull(ptype);
          if (ptype == void.class)
              throw newIllegalArgumentException("parameter type cannot be void");
      }
      /** Return number of extra slots (count of long/double args). */
      private static int checkPtypes(Class<?>[] ptypes) {
          int slots = 0;
          for (Class<?> ptype : ptypes) {
              checkPtype(ptype);
              if (ptype == double.class || ptype == long.class) {
                  slots++;
              }
          }
          checkSlotCount(ptypes.length + slots);
          return slots;
      }
      static void checkSlotCount(int count) {
          assert((MAX_JVM_ARITY & (MAX_JVM_ARITY+1)) == 0);
          // MAX_JVM_ARITY must be power of 2 minus 1 for following code trick to work:
          if ((count & MAX_JVM_ARITY) != count)
              throw newIllegalArgumentException("bad parameter count "+count);
      }
      private static IndexOutOfBoundsException newIndexOutOfBoundsException(Object num) {
          if (num instanceof Integer)  num = "bad index: "+num;
          return new IndexOutOfBoundsException(num.toString());
      }
  
      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(Class, 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) {
          // sanity check the size before the toArray call, since size might be huge
          checkSlotCount(ptypes.size());
          return ptypes.toArray(NO_PTYPES);
      }
  
      /**
       * Finds or creates a method type with the given components.
       * Convenience method for {@link #methodType(Class, 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(Class, 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(Class, 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(Class, 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) {
          MethodType mt = internTable.get(new MethodType(ptypes, rtype));
          if (mt != null)
              return mt;
          if (ptypes.length == 0) {
              ptypes = NO_PTYPES; trusted = true;
          }
          mt = new MethodType(rtype, ptypes, trusted);
          // promote the object to the Real Thing, and reprobe
          mt.form = MethodTypeForm.findForm(mt);
          return internTable.add(mt);
      }
      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(Class, 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) {
          MethodType mt;
          checkSlotCount(objectArgCount);
          int ivarargs = (!finalArray ? 0 : 1);
          int ootIndex = objectArgCount*2 + ivarargs;
          if (ootIndex < objectOnlyTypes.length) {
              mt = objectOnlyTypes[ootIndex];
              if (mt != null)  return mt;
          }
          Class<?>[] ptypes = new Class<?>[objectArgCount + ivarargs];
          Arrays.fill(ptypes, Object.class);
          if (ivarargs != 0)  ptypes[objectArgCount] = Object[].class;
          mt = makeImpl(Object.class, ptypes, true);
          if (ootIndex < objectOnlyTypes.length) {
              objectOnlyTypes[ootIndex] = mt;     // cache it here also!
          }
          return mt;
      }
  
      /**
       * Finds or creates a method type whose components are all {@code Object}.
       * Convenience method for {@link #methodType(Class, 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(Class, 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) {
          if (parameterType(num) == nptype)  return this;
          checkPtype(nptype);
          Class<?>[] nptypes = ptypes.clone();
          nptypes[num] = nptype;
          return makeImpl(rtype, nptypes, true);
      }
  
      /**
       * Finds or creates a method type with additional parameter types.
       * Convenience method for {@link #methodType(Class, 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) {
          int len = ptypes.length;
          if (num < 0 || num > len)
              throw newIndexOutOfBoundsException(num);
          int ins = checkPtypes(ptypesToInsert);
          checkSlotCount(parameterSlotCount() + ptypesToInsert.length + ins);
          int ilen = ptypesToInsert.length;
          if (ilen == 0)  return this;
          Class<?>[] nptypes = Arrays.copyOfRange(ptypes, 0, len+ilen);
          System.arraycopy(nptypes, num, nptypes, num+ilen, len-num);
          System.arraycopy(ptypesToInsert, 0, nptypes, num, ilen);
          return makeImpl(rtype, nptypes, true);
      }
  
      /**
       * Finds or creates a method type with additional parameter types.
       * Convenience method for {@link #methodType(Class, 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(Class, 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(Class, 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(Class, 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) {
          if (start == end)
              return insertParameterTypes(start, ptypesToInsert);
          int len = ptypes.length;
          if (!(0 <= start && start <= end && end <= len))
              throw newIndexOutOfBoundsException("start="+start+" end="+end);
          int ilen = ptypesToInsert.length;
          if (ilen == 0)
              return dropParameterTypes(start, end);
          return dropParameterTypes(start, end).insertParameterTypes(start, ptypesToInsert);
      }
  
      /** Replace the last arrayLength parameter types with the component type of arrayType.
       * @param arrayType any array type
       * @param arrayLength the number of parameter types to change
       * @return the resulting type
       */
      /*non-public*/ MethodType asSpreaderType(Class<?> arrayType, int arrayLength) {
          assert(parameterCount() >= arrayLength);
          int spreadPos = ptypes.length - arrayLength;
          if (arrayLength == 0)  return this;  // nothing to change
          if (arrayType == Object[].class) {
              if (isGeneric())  return this;  // nothing to change
              if (spreadPos == 0) {
                  // no leading arguments to preserve; go generic
                  MethodType res = genericMethodType(arrayLength);
                  if (rtype != Object.class) {
                      res = res.changeReturnType(rtype);
                  }
                  return res;
              }
          }
          Class<?> elemType = arrayType.getComponentType();
          assert(elemType != null);
          for (int i = spreadPos; i < ptypes.length; i++) {
              if (ptypes[i] != elemType) {
                  Class<?>[] fixedPtypes = ptypes.clone();
                  Arrays.fill(fixedPtypes, i, ptypes.length, elemType);
                  return methodType(rtype, fixedPtypes);
              }
          }
          return this;  // arguments check out; no change
      }
  
      /** Return the leading parameter type, which must exist and be a reference.
       *  @return the leading parameter type, after error checks
       */
      /*non-public*/ Class<?> leadingReferenceParameter() {
          Class<?> ptype;
          if (ptypes.length == 0 ||
              (ptype = ptypes[0]).isPrimitive())
              throw newIllegalArgumentException("no leading reference parameter");
          return ptype;
      }
  
      /** Delete the last parameter type and replace it with arrayLength copies of the component type of arrayType.
       * @param arrayType any array type
       * @param arrayLength the number of parameter types to insert
       * @return the resulting type
       */
      /*non-public*/ MethodType asCollectorType(Class<?> arrayType, int arrayLength) {
          assert(parameterCount() >= 1);
          assert(lastParameterType().isAssignableFrom(arrayType));
          MethodType res;
          if (arrayType == Object[].class) {
              res = genericMethodType(arrayLength);
              if (rtype != Object.class) {
                  res = res.changeReturnType(rtype);
              }
          } else {
              Class<?> elemType = arrayType.getComponentType();
              assert(elemType != null);
              res = methodType(rtype, Collections.nCopies(arrayLength, elemType));
          }
          if (ptypes.length == 1) {
              return res;
          } else {
              return res.insertParameterTypes(0, parameterList().subList(0, ptypes.length-1));
          }
      }
  
      /**
       * Finds or creates a method type with some parameter types omitted.
       * Convenience method for {@link #methodType(Class, 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) {
          int len = ptypes.length;
          if (!(0 <= start && start <= end && end <= len))
              throw newIndexOutOfBoundsException("start="+start+" end="+end);
          if (start == end)  return this;
          Class<?>[] nptypes;
          if (start == 0) {
              if (end == len) {
                  // drop all parameters
                  nptypes = NO_PTYPES;
              } else {
                  // drop initial parameter(s)
                  nptypes = Arrays.copyOfRange(ptypes, end, len);
              }
          } else {
              if (end == len) {
                  // drop trailing parameter(s)
                  nptypes = Arrays.copyOfRange(ptypes, 0, start);
              } else {
                  int tail = len - end;
                  nptypes = Arrays.copyOfRange(ptypes, 0, start + tail);
                  System.arraycopy(ptypes, end, nptypes, start, tail);
              }
          }
          return makeImpl(rtype, nptypes, true);
      }
  
      /**
       * Finds or creates a method type with a different return type.
       * Convenience method for {@link #methodType(Class, 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) {
          if (returnType() == nrtype)  return this;
          return makeImpl(nrtype, ptypes, true);
      }
  
      /**
       * 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() {
          return form.hasPrimitives();
      }
  
      /**
       * 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(Class, 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() {
          return form.erasedType();
      }
  
      // BEGIN Android-removed: Implementation methods unused on Android.
      /*
      /**
       * 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() {
          return form.basicType();
      }
  
      /**
       * @return a version of the original type with MethodHandle prepended as the first argument
       *
      /*non-public* MethodType invokerType() {
          return insertParameterTypes(0, MethodHandle.class);
      }
      */
      // END Android-removed: Implementation methods unused on Android.
  
      /**
       * 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());
      }
  
      /*non-public*/ boolean isGeneric() {
          return this == erase() && !hasPrimitives();
      }
  
      /**
       * Converts all primitive types to their corresponding wrapper types.
       * Convenience method for {@link #methodType(Class, 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(Class, 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) {
          assert(pt.hasPrimitives());
          MethodType wt = pt.wrapAlt;
          if (wt == null) {
              // fill in lazily
              wt = MethodTypeForm.canonicalize(pt, MethodTypeForm.WRAP, MethodTypeForm.WRAP);
              assert(wt != null);
              pt.wrapAlt = wt;
          }
          return wt;
      }
  
      private static MethodType unwrapWithNoPrims(MethodType wt) {
          assert(!wt.hasPrimitives());
          MethodType uwt = wt.wrapAlt;
          if (uwt == null) {
              // fill in lazily
              uwt = MethodTypeForm.canonicalize(wt, MethodTypeForm.UNWRAP, MethodTypeForm.UNWRAP);
              if (uwt == null)
                  uwt = wt;    // type has no wrappers or prims at all
              wt.wrapAlt = uwt;
          }
          return uwt;
      }
  
      /**
       * 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.clone()));
      }
  
      /*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 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();
      }
  
      // BEGIN Android-removed: Implementation methods unused on Android.
      /*
      /** True if the old return type can always be viewed (w/o casting) under new return type,
       *  and the new parameters can be viewed (w/o casting) under the old parameter types.
       *
      /*non-public*
      boolean isViewableAs(MethodType newType, boolean keepInterfaces) {
          if (!VerifyType.isNullConversion(returnType(), newType.returnType(), keepInterfaces))
              return false;
          return parametersAreViewableAs(newType, keepInterfaces);
      }
      /** True if the new parameters can be viewed (w/o casting) under the old parameter types. *
      /*non-public*
      boolean parametersAreViewableAs(MethodType newType, boolean keepInterfaces) {
          if (form == newType.form && form.erasedType == this)
              return true;  // my reference parameters are all Object
          if (ptypes == newType.ptypes)
              return true;
          int argc = parameterCount();
          if (argc != newType.parameterCount())
              return false;
          for (int i = 0; i < argc; i++) {
              if (!VerifyType.isNullConversion(newType.parameterType(i), parameterType(i), keepInterfaces))
                  return false;
          }
          return true;
      }
      */
      // END Android-removed: Implementation methods unused on Android.
  
      /*non-public*/
      boolean isConvertibleTo(MethodType newType) {
          MethodTypeForm oldForm = this.form();
          MethodTypeForm newForm = newType.form();
          if (oldForm == newForm)
              // same parameter count, same primitive/object mix
              return true;
          if (!canConvert(returnType(), newType.returnType()))
              return false;
          Class<?>[] srcTypes = newType.ptypes;
          Class<?>[] dstTypes = ptypes;
          if (srcTypes == dstTypes)
              return true;
          int argc;
          if ((argc = srcTypes.length) != dstTypes.length)
              return false;
          if (argc <= 1) {
              if (argc == 1 && !canConvert(srcTypes[0], dstTypes[0]))
                  return false;
              return true;
          }
          if ((oldForm.primitiveParameterCount() == 0 && oldForm.erasedType == this) ||
              (newForm.primitiveParameterCount() == 0 && newForm.erasedType == newType)) {
              // Somewhat complicated test to avoid a loop of 2 or more trips.
              // If either type has only Object parameters, we know we can convert.
              assert(canConvertParameters(srcTypes, dstTypes));
              return true;
          }
          return canConvertParameters(srcTypes, dstTypes);
      }
  
      /** Returns true if MHs.explicitCastArguments produces the same result as MH.asType.
       *  If the type conversion is impossible for either, the result should be false.
       */
      /*non-public*/
      boolean explicitCastEquivalentToAsType(MethodType newType) {
          if (this == newType)  return true;
          if (!explicitCastEquivalentToAsType(rtype, newType.rtype)) {
              return false;
          }
          Class<?>[] srcTypes = newType.ptypes;
          Class<?>[] dstTypes = ptypes;
          if (dstTypes == srcTypes) {
              return true;
          }
          assert(dstTypes.length == srcTypes.length);
          for (int i = 0; i < dstTypes.length; i++) {
              if (!explicitCastEquivalentToAsType(srcTypes[i], dstTypes[i])) {
                  return false;
              }
          }
          return true;
      }
  
      /** Reports true if the src can be converted to the dst, by both asType and MHs.eCE,
       *  and with the same effect.
       *  MHs.eCA has the following "upgrades" to MH.asType:
       *  1. interfaces are unchecked (that is, treated as if aliased to Object)
       *     Therefore, {@code Object->CharSequence} is possible in both cases but has different semantics
       *  2a. the full matrix of primitive-to-primitive conversions is supported
       *      Narrowing like {@code long->byte} and basic-typing like {@code boolean->int}
       *      are not supported by asType, but anything supported by asType is equivalent
       *      with MHs.eCE.
       *  2b. conversion of void->primitive means explicit cast has to insert zero/false/null.
       *  3a. unboxing conversions can be followed by the full matrix of primitive conversions
       *  3b. unboxing of null is permitted (creates a zero primitive value)
       * Other than interfaces, reference-to-reference conversions are the same.
       * Boxing primitives to references is the same for both operators.
       */
      private static boolean explicitCastEquivalentToAsType(Class<?> src, Class<?> dst) {
          if (src == dst || dst == Object.class || dst == void.class)  return true;
          if (src.isPrimitive()) {
              // Could be a prim/prim conversion, where casting is a strict superset.
              // Or a boxing conversion, which is always to an exact wrapper class.
              return canConvert(src, dst);
          } else if (dst.isPrimitive()) {
              // Unboxing behavior is different between MHs.eCA & MH.asType (see 3b).
              return false;
          } else {
              // R->R always works, but we have to avoid a check-cast to an interface.
              return !dst.isInterface() || dst.isAssignableFrom(src);
          }
      }
  
      private boolean canConvertParameters(Class<?>[] srcTypes, Class<?>[] dstTypes) {
          for (int i = 0; i < srcTypes.length; i++) {
              if (!canConvert(srcTypes[i], dstTypes[i])) {
                  return false;
              }
          }
          return true;
      }
  
      /*non-public*/
      static boolean canConvert(Class<?> src, Class<?> dst) {
          // short-circuit a few cases:
          if (src == dst || src == Object.class || dst == Object.class)  return true;
          // the remainder of this logic is documented in MethodHandle.asType
          if (src.isPrimitive()) {
              // can force void to an explicit null, a la reflect.Method.invoke
              // can also force void to a primitive zero, by analogy
              if (src == void.class)  return true;  //or !dst.isPrimitive()?
              Wrapper sw = Wrapper.forPrimitiveType(src);
              if (dst.isPrimitive()) {
                  // P->P must widen
                  return Wrapper.forPrimitiveType(dst).isConvertibleFrom(sw);
              } else {
                  // P->R must box and widen
                  return dst.isAssignableFrom(sw.wrapperType());
              }
          } else if (dst.isPrimitive()) {
              // any value can be dropped
              if (dst == void.class)  return true;
              Wrapper dw = Wrapper.forPrimitiveType(dst);
              // R->P must be able to unbox (from a dynamically chosen type) and widen
              // For example:
              //   Byte/Number/Comparable/Object -> dw:Byte -> byte.
              //   Character/Comparable/Object -> dw:Character -> char
              //   Boolean/Comparable/Object -> dw:Boolean -> boolean
              // This means that dw must be cast-compatible with src.
              if (src.isAssignableFrom(dw.wrapperType())) {
                  return true;
              }
              // The above does not work if the source reference is strongly typed
              // to a wrapper whose primitive must be widened.  For example:
              //   Byte -> unbox:byte -> short/int/long/float/double
              //   Character -> unbox:char -> int/long/float/double
              if (Wrapper.isWrapperType(src) &&
                  dw.isConvertibleFrom(Wrapper.forWrapperType(src))) {
                  // can unbox from src and then widen to dst
                  return true;
              }
              // We have already covered cases which arise due to runtime unboxing
              // of a reference type which covers several wrapper types:
              //   Object -> cast:Integer -> unbox:int -> long/float/double
              //   Serializable -> cast:Byte -> unbox:byte -> byte/short/int/long/float/double
              // An marginal case is Number -> dw:Character -> char, which would be OK if there were a
              // subclass of Number which wraps a value that can convert to char.
              // Since there is none, we don't need an extra check here to cover char or boolean.
              return false;
          } else {
              // R->R always works, since null is always valid dynamically
              return true;
          }
      }
  
      /// Queries which have to do with the bytecode architecture
  
      /** Reports the number of JVM stack slots required to invoke a method
       * of this type.  Note that (for historical reasons) the JVM requires
       * a second stack slot to pass long and double arguments.
       * So this method returns {@link #parameterCount() parameterCount} plus the
       * number of long and double parameters (if any).
       * <p>
       * This method is included for the benefit of applications that must
       * generate bytecodes that process method handles and invokedynamic.
       * @return the number of JVM stack slots for this type's parameters
       */
      /*non-public*/ int parameterSlotCount() {
          return form.parameterSlotCount();
      }
  
      // BEGIN Android-removed: Cache of higher order adapters.
      /*
      /*non-public* Invokers invokers() {
          Invokers inv = invokers;
          if (inv != null)  return inv;
          invokers = inv = new Invokers(this);
          return inv;
      }
      */
      // END Android-removed: Cache of higher order adapters.
  
      // BEGIN Android-removed: Implementation methods unused on Android.
      /*
      /** Reports the number of JVM stack slots which carry all parameters including and after
       * the given position, which must be in the range of 0 to
       * {@code parameterCount} inclusive.  Successive parameters are
       * more shallowly stacked, and parameters are indexed in the bytecodes
       * according to their trailing edge.  Thus, to obtain the depth
       * in the outgoing call stack of parameter {@code N}, obtain
       * the {@code parameterSlotDepth} of its trailing edge
       * at position {@code N+1}.
       * <p>
       * Parameters of type {@code long} and {@code double} occupy
       * two stack slots (for historical reasons) and all others occupy one.
       * Therefore, the number returned is the number of arguments
       * <em>including</em> and <em>after</em> the given parameter,
       * <em>plus</em> the number of long or double arguments
       * at or after after the argument for the given parameter.
       * <p>
       * This method is included for the benefit of applications that must
       * generate bytecodes that process method handles and invokedynamic.
       * @param num an index (zero-based, inclusive) within the parameter types
       * @return the index of the (shallowest) JVM stack slot transmitting the
       *         given parameter
       * @throws IllegalArgumentException if {@code num} is negative or greater than {@code parameterCount()}
       *
      /*non-public* int parameterSlotDepth(int num) {
          if (num < 0 || num > ptypes.length)
              parameterType(num);  // force a range check
          return form.parameterToArgSlot(num-1);
      }
  
      /** Reports the number of JVM stack slots required to receive a return value
       * from a method of this type.
       * If the {@link #returnType() return type} is void, it will be zero,
       * else if the return type is long or double, it will be two, else one.
       * <p>
       * This method is included for the benefit of applications that must
       * generate bytecodes that process method handles and invokedynamic.
       * @return the number of JVM stack slots (0, 1, or 2) for this type's return value
       * Will be removed for PFD.
       *
      /*non-public* int returnSlotCount() {
          return form.returnSlotCount();
      }
      */
      // END Android-removed: Implementation methods unused on Android.
  
      /**
       * Finds or creates an instance of a method type, given the spelling of its bytecode descriptor.
       * Convenience method for {@link #methodType(Class, Class[]) methodType}.
       * Any class or interface name embedded in the descriptor string
       * will be resolved by calling {@link ClassLoader#loadClass(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, TypeNotPresentException
      {
          if (!descriptor.startsWith("(") ||  // also generates NPE if needed
              descriptor.indexOf(')') < 0 ||
              descriptor.indexOf('.') >= 0)
              throw newIllegalArgumentException("not a method descriptor: "+descriptor);
          List<Class<?>> types = BytecodeDescriptor.parseMethod(descriptor, loader);
          Class<?> rtype = types.remove(types.size() - 1);
          checkSlotCount(types.size());
          Class<?>[] ptypes = listToArray(types);
          return makeImpl(rtype, ptypes, true);
      }
  
      /**
       * 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(String, ClassLoader) fromMethodDescriptorString},
       * because the latter requires a suitable class loader argument.
       * @return the bytecode type descriptor representation
       */
      public String toMethodDescriptorString() {
          String desc = methodDescriptor;
          if (desc == null) {
              desc = BytecodeDescriptor.unparse(this);
              methodDescriptor = desc;
          }
          return desc;
      }
  
      /*non-public*/ static String toFieldDescriptorString(Class<?> cls) {
          return BytecodeDescriptor.unparse(cls);
      }
  
      /// 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;
              }
  
          }
      }
  
  }
  