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  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
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  25 
  26 package java.lang.invoke;
  27 
  28 
  29 import java.util.*;
  30 
  31 import static java.lang.invoke.MethodHandleStatics.*;
  32 
  33 /**
  34  * A method handle is a typed, directly executable reference to an underlying method,
  35  * constructor, field, or similar low-level operation, with optional
  36  * transformations of arguments or return values.
  37  * These transformations are quite general, and include such patterns as
  38  * {@linkplain #asType conversion},
  39  * {@linkplain #bindTo insertion},
  40  * {@linkplain java.lang.invoke.MethodHandles#dropArguments deletion},
  41  * and {@linkplain java.lang.invoke.MethodHandles#filterArguments substitution}.
  42  *
  43  * <h1>Method handle contents</h1>
  44  * Method handles are dynamically and strongly typed according to their parameter and return types.
  45  * They are not distinguished by the name or the defining class of their underlying methods.
  46  * A method handle must be invoked using a symbolic type descriptor which matches
  47  * the method handle's own {@linkplain #type type descriptor}.
  48  * <p>
  49  * Every method handle reports its type descriptor via the {@link #type type} accessor.
  50  * This type descriptor is a {@link java.lang.invoke.MethodType MethodType} object,
  51  * whose structure is a series of classes, one of which is
  52  * the return type of the method (or {@code void.class} if none).
  53  * <p>
  54  * A method handle's type controls the types of invocations it accepts,
  55  * and the kinds of transformations that apply to it.
  56  * <p>
  57  * A method handle contains a pair of special invoker methods
  58  * called {@link #invokeExact invokeExact} and {@link #invoke invoke}.
  59  * Both invoker methods provide direct access to the method handle's
  60  * underlying method, constructor, field, or other operation,
  61  * as modified by transformations of arguments and return values.
  62  * Both invokers accept calls which exactly match the method handle's own type.
  63  * The plain, inexact invoker also accepts a range of other call types.
  64  * <p>
  65  * Method handles are immutable and have no visible state.
  66  * Of course, they can be bound to underlying methods or data which exhibit state.
  67  * With respect to the Java Memory Model, any method handle will behave
  68  * as if all of its (internal) fields are final variables.  This means that any method
  69  * handle made visible to the application will always be fully formed.
  70  * This is true even if the method handle is published through a shared
  71  * variable in a data race.
  72  * <p>
  73  * Method handles cannot be subclassed by the user.
  74  * Implementations may (or may not) create internal subclasses of {@code MethodHandle}
  75  * which may be visible via the {@link java.lang.Object#getClass Object.getClass}
  76  * operation.  The programmer should not draw conclusions about a method handle
  77  * from its specific class, as the method handle class hierarchy (if any)
  78  * may change from time to time or across implementations from different vendors.
  79  *
  80  * <h1>Method handle compilation</h1>
  81  * A Java method call expression naming {@code invokeExact} or {@code invoke}
  82  * can invoke a method handle from Java source code.
  83  * From the viewpoint of source code, these methods can take any arguments
  84  * and their result can be cast to any return type.
  85  * Formally this is accomplished by giving the invoker methods
  86  * {@code Object} return types and variable arity {@code Object} arguments,
  87  * but they have an additional quality called <em>signature polymorphism</em>
  88  * which connects this freedom of invocation directly to the JVM execution stack.
  89  * <p>
  90  * As is usual with virtual methods, source-level calls to {@code invokeExact}
  91  * and {@code invoke} compile to an {@code invokevirtual} instruction.
  92  * More unusually, the compiler must record the actual argument types,
  93  * and may not perform method invocation conversions on the arguments.
  94  * Instead, it must push them on the stack according to their own unconverted types.
  95  * The method handle object itself is pushed on the stack before the arguments.
  96  * The compiler then calls the method handle with a symbolic type descriptor which
  97  * describes the argument and return types.
  98  * <p>
  99  * To issue a complete symbolic type descriptor, the compiler must also determine
 100  * the return type.  This is based on a cast on the method invocation expression,
 101  * if there is one, or else {@code Object} if the invocation is an expression
 102  * or else {@code void} if the invocation is a statement.
 103  * The cast may be to a primitive type (but not {@code void}).
 104  * <p>
 105  * As a corner case, an uncasted {@code null} argument is given
 106  * a symbolic type descriptor of {@code java.lang.Void}.
 107  * The ambiguity with the type {@code Void} is harmless, since there are no references of type
 108  * {@code Void} except the null reference.
 109  *
 110  * <h1>Method handle invocation</h1>
 111  * The first time a {@code invokevirtual} instruction is executed
 112  * it is linked, by symbolically resolving the names in the instruction
 113  * and verifying that the method call is statically legal.
 114  * This is true of calls to {@code invokeExact} and {@code invoke}.
 115  * In this case, the symbolic type descriptor emitted by the compiler is checked for
 116  * correct syntax and names it contains are resolved.
 117  * Thus, an {@code invokevirtual} instruction which invokes
 118  * a method handle will always link, as long
 119  * as the symbolic type descriptor is syntactically well-formed
 120  * and the types exist.
 121  * <p>
 122  * When the {@code invokevirtual} is executed after linking,
 123  * the receiving method handle's type is first checked by the JVM
 124  * to ensure that it matches the symbolic type descriptor.
 125  * If the type match fails, it means that the method which the
 126  * caller is invoking is not present on the individual
 127  * method handle being invoked.
 128  * <p>
 129  * In the case of {@code invokeExact}, the type descriptor of the invocation
 130  * (after resolving symbolic type names) must exactly match the method type
 131  * of the receiving method handle.
 132  * In the case of plain, inexact {@code invoke}, the resolved type descriptor
 133  * must be a valid argument to the receiver's {@link #asType asType} method.
 134  * Thus, plain {@code invoke} is more permissive than {@code invokeExact}.
 135  * <p>
 136  * After type matching, a call to {@code invokeExact} directly
 137  * and immediately invoke the method handle's underlying method
 138  * (or other behavior, as the case may be).
 139  * <p>
 140  * A call to plain {@code invoke} works the same as a call to
 141  * {@code invokeExact}, if the symbolic type descriptor specified by the caller
 142  * exactly matches the method handle's own type.
 143  * If there is a type mismatch, {@code invoke} attempts
 144  * to adjust the type of the receiving method handle,
 145  * as if by a call to {@link #asType asType},
 146  * to obtain an exactly invokable method handle {@code M2}.
 147  * This allows a more powerful negotiation of method type
 148  * between caller and callee.
 149  * <p>
 150  * (<em>Note:</em> The adjusted method handle {@code M2} is not directly observable,
 151  * and implementations are therefore not required to materialize it.)
 152  *
 153  * <h1>Invocation checking</h1>
 154  * In typical programs, method handle type matching will usually succeed.
 155  * But if a match fails, the JVM will throw a {@link WrongMethodTypeException},
 156  * either directly (in the case of {@code invokeExact}) or indirectly as if
 157  * by a failed call to {@code asType} (in the case of {@code invoke}).
 158  * <p>
 159  * Thus, a method type mismatch which might show up as a linkage error
 160  * in a statically typed program can show up as
 161  * a dynamic {@code WrongMethodTypeException}
 162  * in a program which uses method handles.
 163  * <p>
 164  * Because method types contain "live" {@code Class} objects,
 165  * method type matching takes into account both types names and class loaders.
 166  * Thus, even if a method handle {@code M} is created in one
 167  * class loader {@code L1} and used in another {@code L2},
 168  * method handle calls are type-safe, because the caller's symbolic type
 169  * descriptor, as resolved in {@code L2},
 170  * is matched against the original callee method's symbolic type descriptor,
 171  * as resolved in {@code L1}.
 172  * The resolution in {@code L1} happens when {@code M} is created
 173  * and its type is assigned, while the resolution in {@code L2} happens
 174  * when the {@code invokevirtual} instruction is linked.
 175  * <p>
 176  * Apart from the checking of type descriptors,
 177  * a method handle's capability to call its underlying method is unrestricted.
 178  * If a method handle is formed on a non-public method by a class
 179  * that has access to that method, the resulting handle can be used
 180  * in any place by any caller who receives a reference to it.
 181  * <p>
 182  * Unlike with the Core Reflection API, where access is checked every time
 183  * a reflective method is invoked,
 184  * method handle access checking is performed
 185  * <a href="MethodHandles.Lookup.html#access">when the method handle is created</a>.
 186  * In the case of {@code ldc} (see below), access checking is performed as part of linking
 187  * the constant pool entry underlying the constant method handle.
 188  * <p>
 189  * Thus, handles to non-public methods, or to methods in non-public classes,
 190  * should generally be kept secret.
 191  * They should not be passed to untrusted code unless their use from
 192  * the untrusted code would be harmless.
 193  *
 194  * <h1>Method handle creation</h1>
 195  * Java code can create a method handle that directly accesses
 196  * any method, constructor, or field that is accessible to that code.
 197  * This is done via a reflective, capability-based API called
 198  * {@link java.lang.invoke.MethodHandles.Lookup MethodHandles.Lookup}
 199  * For example, a static method handle can be obtained
 200  * from {@link java.lang.invoke.MethodHandles.Lookup#findStatic Lookup.findStatic}.
 201  * There are also conversion methods from Core Reflection API objects,
 202  * such as {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
 203  * <p>
 204  * Like classes and strings, method handles that correspond to accessible
 205  * fields, methods, and constructors can also be represented directly
 206  * in a class file's constant pool as constants to be loaded by {@code ldc} bytecodes.
 207  * A new type of constant pool entry, {@code CONSTANT_MethodHandle},
 208  * refers directly to an associated {@code CONSTANT_Methodref},
 209  * {@code CONSTANT_InterfaceMethodref}, or {@code CONSTANT_Fieldref}
 210  * constant pool entry.
 211  * (For full details on method handle constants,
 212  * see sections 4.4.8 and 5.4.3.5 of the Java Virtual Machine Specification.)
 213  * <p>
 214  * Method handles produced by lookups or constant loads from methods or
 215  * constructors with the variable arity modifier bit ({@code 0x0080})
 216  * have a corresponding variable arity, as if they were defined with
 217  * the help of {@link #asVarargsCollector asVarargsCollector}.
 218  * <p>
 219  * A method reference may refer either to a static or non-static method.
 220  * In the non-static case, the method handle type includes an explicit
 221  * receiver argument, prepended before any other arguments.
 222  * In the method handle's type, the initial receiver argument is typed
 223  * according to the class under which the method was initially requested.
 224  * (E.g., if a non-static method handle is obtained via {@code ldc},
 225  * the type of the receiver is the class named in the constant pool entry.)
 226  * <p>
 227  * Method handle constants are subject to the same link-time access checks
 228  * their corresponding bytecode instructions, and the {@code ldc} instruction
 229  * will throw corresponding linkage errors if the bytecode behaviors would
 230  * throw such errors.
 231  * <p>
 232  * As a corollary of this, access to protected members is restricted
 233  * to receivers only of the accessing class, or one of its subclasses,
 234  * and the accessing class must in turn be a subclass (or package sibling)
 235  * of the protected member's defining class.
 236  * If a method reference refers to a protected non-static method or field
 237  * of a class outside the current package, the receiver argument will
 238  * be narrowed to the type of the accessing class.
 239  * <p>
 240  * When a method handle to a virtual method is invoked, the method is
 241  * always looked up in the receiver (that is, the first argument).
 242  * <p>
 243  * A non-virtual method handle to a specific virtual method implementation
 244  * can also be created.  These do not perform virtual lookup based on
 245  * receiver type.  Such a method handle simulates the effect of
 246  * an {@code invokespecial} instruction to the same method.
 247  *
 248  * <h1>Usage examples</h1>
 249  * Here are some examples of usage:
 250  * <blockquote><pre>{@code
 251 Object x, y; String s; int i;
 252 MethodType mt; MethodHandle mh;
 253 MethodHandles.Lookup lookup = MethodHandles.lookup();
 254 // mt is (char,char)String
 255 mt = MethodType.methodType(String.class, char.class, char.class);
 256 mh = lookup.findVirtual(String.class, "replace", mt);
 257 s = (String) mh.invokeExact("daddy",'d','n');
 258 // invokeExact(Ljava/lang/String;CC)Ljava/lang/String;
 259 assertEquals(s, "nanny");
 260 // weakly typed invocation (using MHs.invoke)
 261 s = (String) mh.invokeWithArguments("sappy", 'p', 'v');
 262 assertEquals(s, "savvy");
 263 // mt is (Object[])List
 264 mt = MethodType.methodType(java.util.List.class, Object[].class);
 265 mh = lookup.findStatic(java.util.Arrays.class, "asList", mt);
 266 assert(mh.isVarargsCollector());
 267 x = mh.invoke("one", "two");
 268 // invoke(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/Object;
 269 assertEquals(x, java.util.Arrays.asList("one","two"));
 270 // mt is (Object,Object,Object)Object
 271 mt = MethodType.genericMethodType(3);
 272 mh = mh.asType(mt);
 273 x = mh.invokeExact((Object)1, (Object)2, (Object)3);
 274 // invokeExact(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
 275 assertEquals(x, java.util.Arrays.asList(1,2,3));
 276 // mt is ()int
 277 mt = MethodType.methodType(int.class);
 278 mh = lookup.findVirtual(java.util.List.class, "size", mt);
 279 i = (int) mh.invokeExact(java.util.Arrays.asList(1,2,3));
 280 // invokeExact(Ljava/util/List;)I
 281 assert(i == 3);
 282 mt = MethodType.methodType(void.class, String.class);
 283 mh = lookup.findVirtual(java.io.PrintStream.class, "println", mt);
 284 mh.invokeExact(System.out, "Hello, world.");
 285 // invokeExact(Ljava/io/PrintStream;Ljava/lang/String;)V
 286  * }</pre></blockquote>
 287  * Each of the above calls to {@code invokeExact} or plain {@code invoke}
 288  * generates a single invokevirtual instruction with
 289  * the symbolic type descriptor indicated in the following comment.
 290  * In these examples, the helper method {@code assertEquals} is assumed to
 291  * be a method which calls {@link java.util.Objects#equals(Object,Object) Objects.equals}
 292  * on its arguments, and asserts that the result is true.
 293  *
 294  * <h1>Exceptions</h1>
 295  * The methods {@code invokeExact} and {@code invoke} are declared
 296  * to throw {@link java.lang.Throwable Throwable},
 297  * which is to say that there is no static restriction on what a method handle
 298  * can throw.  Since the JVM does not distinguish between checked
 299  * and unchecked exceptions (other than by their class, of course),
 300  * there is no particular effect on bytecode shape from ascribing
 301  * checked exceptions to method handle invocations.  But in Java source
 302  * code, methods which perform method handle calls must either explicitly
 303  * throw {@code Throwable}, or else must catch all
 304  * throwables locally, rethrowing only those which are legal in the context,
 305  * and wrapping ones which are illegal.
 306  *
 307  * <h1><a name="sigpoly"></a>Signature polymorphism</h1>
 308  * The unusual compilation and linkage behavior of
 309  * {@code invokeExact} and plain {@code invoke}
 310  * is referenced by the term <em>signature polymorphism</em>.
 311  * As defined in the Java Language Specification,
 312  * a signature polymorphic method is one which can operate with
 313  * any of a wide range of call signatures and return types.
 314  * <p>
 315  * In source code, a call to a signature polymorphic method will
 316  * compile, regardless of the requested symbolic type descriptor.
 317  * As usual, the Java compiler emits an {@code invokevirtual}
 318  * instruction with the given symbolic type descriptor against the named method.
 319  * The unusual part is that the symbolic type descriptor is derived from
 320  * the actual argument and return types, not from the method declaration.
 321  * <p>
 322  * When the JVM processes bytecode containing signature polymorphic calls,
 323  * it will successfully link any such call, regardless of its symbolic type descriptor.
 324  * (In order to retain type safety, the JVM will guard such calls with suitable
 325  * dynamic type checks, as described elsewhere.)
 326  * <p>
 327  * Bytecode generators, including the compiler back end, are required to emit
 328  * untransformed symbolic type descriptors for these methods.
 329  * Tools which determine symbolic linkage are required to accept such
 330  * untransformed descriptors, without reporting linkage errors.
 331  *
 332  * <h1>Interoperation between method handles and the Core Reflection API</h1>
 333  * Using factory methods in the {@link java.lang.invoke.MethodHandles.Lookup Lookup} API,
 334  * any class member represented by a Core Reflection API object
 335  * can be converted to a behaviorally equivalent method handle.
 336  * For example, a reflective {@link java.lang.reflect.Method Method} can
 337  * be converted to a method handle using
 338  * {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
 339  * The resulting method handles generally provide more direct and efficient
 340  * access to the underlying class members.
 341  * <p>
 342  * As a special case,
 343  * when the Core Reflection API is used to view the signature polymorphic
 344  * methods {@code invokeExact} or plain {@code invoke} in this class,
 345  * they appear as ordinary non-polymorphic methods.
 346  * Their reflective appearance, as viewed by
 347  * {@link java.lang.Class#getDeclaredMethod Class.getDeclaredMethod},
 348  * is unaffected by their special status in this API.
 349  * For example, {@link java.lang.reflect.Method#getModifiers Method.getModifiers}
 350  * will report exactly those modifier bits required for any similarly
 351  * declared method, including in this case {@code native} and {@code varargs} bits.
 352  * <p>
 353  * As with any reflected method, these methods (when reflected) may be
 354  * invoked via {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.
 355  * However, such reflective calls do not result in method handle invocations.
 356  * Such a call, if passed the required argument
 357  * (a single one, of type {@code Object[]}), will ignore the argument and
 358  * will throw an {@code UnsupportedOperationException}.
 359  * <p>
 360  * Since {@code invokevirtual} instructions can natively
 361  * invoke method handles under any symbolic type descriptor, this reflective view conflicts
 362  * with the normal presentation of these methods via bytecodes.
 363  * Thus, these two native methods, when reflectively viewed by
 364  * {@code Class.getDeclaredMethod}, may be regarded as placeholders only.
 365  * <p>
 366  * In order to obtain an invoker method for a particular type descriptor,
 367  * use {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker},
 368  * or {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}.
 369  * The {@link java.lang.invoke.MethodHandles.Lookup#findVirtual Lookup.findVirtual}
 370  * API is also able to return a method handle
 371  * to call {@code invokeExact} or plain {@code invoke},
 372  * for any specified type descriptor .
 373  *
 374  * <h1>Interoperation between method handles and Java generics</h1>
 375  * A method handle can be obtained on a method, constructor, or field
 376  * which is declared with Java generic types.
 377  * As with the Core Reflection API, the type of the method handle
 378  * will constructed from the erasure of the source-level type.
 379  * When a method handle is invoked, the types of its arguments
 380  * or the return value cast type may be generic types or type instances.
 381  * If this occurs, the compiler will replace those
 382  * types by their erasures when it constructs the symbolic type descriptor
 383  * for the {@code invokevirtual} instruction.
 384  * <p>
 385  * Method handles do not represent
 386  * their function-like types in terms of Java parameterized (generic) types,
 387  * because there are three mismatches between function-like types and parameterized
 388  * Java types.
 389  * <ul>
 390  * <li>Method types range over all possible arities,
 391  * from no arguments to up to the  <a href="MethodHandle.html#maxarity">maximum number</a> of allowed arguments.
 392  * Generics are not variadic, and so cannot represent this.</li>
 393  * <li>Method types can specify arguments of primitive types,
 394  * which Java generic types cannot range over.</li>
 395  * <li>Higher order functions over method handles (combinators) are
 396  * often generic across a wide range of function types, including
 397  * those of multiple arities.  It is impossible to represent such
 398  * genericity with a Java type parameter.</li>
 399  * </ul>
 400  *
 401  * <h1><a name="maxarity"></a>Arity limits</h1>
 402  * The JVM imposes on all methods and constructors of any kind an absolute
 403  * limit of 255 stacked arguments.  This limit can appear more restrictive
 404  * in certain cases:
 405  * <ul>
 406  * <li>A {@code long} or {@code double} argument counts (for purposes of arity limits) as two argument slots.
 407  * <li>A non-static method consumes an extra argument for the object on which the method is called.
 408  * <li>A constructor consumes an extra argument for the object which is being constructed.
 409  * <li>Since a method handle&rsquo;s {@code invoke} method (or other signature-polymorphic method) is non-virtual,
 410  *     it consumes an extra argument for the method handle itself, in addition to any non-virtual receiver object.
 411  * </ul>
 412  * These limits imply that certain method handles cannot be created, solely because of the JVM limit on stacked arguments.
 413  * For example, if a static JVM method accepts exactly 255 arguments, a method handle cannot be created for it.
 414  * Attempts to create method handles with impossible method types lead to an {@link IllegalArgumentException}.
 415  * In particular, a method handle&rsquo;s type must not have an arity of the exact maximum 255.
 416  *
 417  * @see MethodType
 418  * @see MethodHandles
 419  * @author John Rose, JSR 292 EG
 420  */
 421 public abstract class MethodHandle {
 422     static { MethodHandleImpl.initStatics(); }
 423 
 424     /**
 425      * Internal marker interface which distinguishes (to the Java compiler)
 426      * those methods which are <a href="MethodHandle.html#sigpoly">signature polymorphic</a>.
 427      */
 428     @java.lang.annotation.Target({java.lang.annotation.ElementType.METHOD})
 429     @java.lang.annotation.Retention(java.lang.annotation.RetentionPolicy.RUNTIME)
 430     @interface PolymorphicSignature { }
 431 
 432     private final MethodType type;
 433     /*private*/ final LambdaForm form;
 434     // form is not private so that invokers can easily fetch it
 435     /*private*/ MethodHandle asTypeCache;
 436     // asTypeCache is not private so that invokers can easily fetch it
 437     /*non-public*/ byte customizationCount;
 438     // customizationCount should be accessible from invokers
 439 
 440     /**
 441      * Reports the type of this method handle.
 442      * Every invocation of this method handle via {@code invokeExact} must exactly match this type.
 443      * @return the method handle type
 444      */
 445     public MethodType type() {
 446         return type;
 447     }
 448 
 449     /**
 450      * Package-private constructor for the method handle implementation hierarchy.
 451      * Method handle inheritance will be contained completely within
 452      * the {@code java.lang.invoke} package.
 453      */
 454     // @param type type (permanently assigned) of the new method handle
 455     /*non-public*/ MethodHandle(MethodType type, LambdaForm form) {
 456         type.getClass();  // explicit NPE
 457         form.getClass();  // explicit NPE
 458         this.type = type;
 459         this.form = form;
 460 
 461         form.prepare();  // TO DO:  Try to delay this step until just before invocation.
 462     }
 463 
 464     /**
 465      * Invokes the method handle, allowing any caller type descriptor, but requiring an exact type match.
 466      * The symbolic type descriptor at the call site of {@code invokeExact} must
 467      * exactly match this method handle's {@link #type type}.
 468      * No conversions are allowed on arguments or return values.
 469      * <p>
 470      * When this method is observed via the Core Reflection API,
 471      * it will appear as a single native method, taking an object array and returning an object.
 472      * If this native method is invoked directly via
 473      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
 474      * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
 475      * it will throw an {@code UnsupportedOperationException}.
 476      * @param args the signature-polymorphic parameter list, statically represented using varargs
 477      * @return the signature-polymorphic result, statically represented using {@code Object}
 478      * @throws WrongMethodTypeException if the target's type is not identical with the caller's symbolic type descriptor
 479      * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
 480      */
 481     public final native @PolymorphicSignature Object invokeExact(Object... args) throws Throwable;
 482 
 483     /**
 484      * Invokes the method handle, allowing any caller type descriptor,
 485      * and optionally performing conversions on arguments and return values.
 486      * <p>
 487      * If the call site's symbolic type descriptor exactly matches this method handle's {@link #type type},
 488      * the call proceeds as if by {@link #invokeExact invokeExact}.
 489      * <p>
 490      * Otherwise, the call proceeds as if this method handle were first
 491      * adjusted by calling {@link #asType asType} to adjust this method handle
 492      * to the required type, and then the call proceeds as if by
 493      * {@link #invokeExact invokeExact} on the adjusted method handle.
 494      * <p>
 495      * There is no guarantee that the {@code asType} call is actually made.
 496      * If the JVM can predict the results of making the call, it may perform
 497      * adaptations directly on the caller's arguments,
 498      * and call the target method handle according to its own exact type.
 499      * <p>
 500      * The resolved type descriptor at the call site of {@code invoke} must
 501      * be a valid argument to the receivers {@code asType} method.
 502      * In particular, the caller must specify the same argument arity
 503      * as the callee's type,
 504      * if the callee is not a {@linkplain #asVarargsCollector variable arity collector}.
 505      * <p>
 506      * When this method is observed via the Core Reflection API,
 507      * it will appear as a single native method, taking an object array and returning an object.
 508      * If this native method is invoked directly via
 509      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
 510      * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
 511      * it will throw an {@code UnsupportedOperationException}.
 512      * @param args the signature-polymorphic parameter list, statically represented using varargs
 513      * @return the signature-polymorphic result, statically represented using {@code Object}
 514      * @throws WrongMethodTypeException if the target's type cannot be adjusted to the caller's symbolic type descriptor
 515      * @throws ClassCastException if the target's type can be adjusted to the caller, but a reference cast fails
 516      * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
 517      */
 518     public final native @PolymorphicSignature Object invoke(Object... args) throws Throwable;
 519 
 520     /**
 521      * Private method for trusted invocation of a method handle respecting simplified signatures.
 522      * Type mismatches will not throw {@code WrongMethodTypeException}, but could crash the JVM.
 523      * <p>
 524      * The caller signature is restricted to the following basic types:
 525      * Object, int, long, float, double, and void return.
 526      * <p>
 527      * The caller is responsible for maintaining type correctness by ensuring
 528      * that the each outgoing argument value is a member of the range of the corresponding
 529      * callee argument type.
 530      * (The caller should therefore issue appropriate casts and integer narrowing
 531      * operations on outgoing argument values.)
 532      * The caller can assume that the incoming result value is part of the range
 533      * of the callee's return type.
 534      * @param args the signature-polymorphic parameter list, statically represented using varargs
 535      * @return the signature-polymorphic result, statically represented using {@code Object}
 536      */
 537     /*non-public*/ final native @PolymorphicSignature Object invokeBasic(Object... args) throws Throwable;
 538 
 539     /**
 540      * Private method for trusted invocation of a MemberName of kind {@code REF_invokeVirtual}.
 541      * The caller signature is restricted to basic types as with {@code invokeBasic}.
 542      * The trailing (not leading) argument must be a MemberName.
 543      * @param args the signature-polymorphic parameter list, statically represented using varargs
 544      * @return the signature-polymorphic result, statically represented using {@code Object}
 545      */
 546     /*non-public*/ static native @PolymorphicSignature Object linkToVirtual(Object... args) throws Throwable;
 547 
 548     /**
 549      * Private method for trusted invocation of a MemberName of kind {@code REF_invokeStatic}.
 550      * The caller signature is restricted to basic types as with {@code invokeBasic}.
 551      * The trailing (not leading) argument must be a MemberName.
 552      * @param args the signature-polymorphic parameter list, statically represented using varargs
 553      * @return the signature-polymorphic result, statically represented using {@code Object}
 554      */
 555     /*non-public*/ static native @PolymorphicSignature Object linkToStatic(Object... args) throws Throwable;
 556 
 557     /**
 558      * Private method for trusted invocation of a MemberName of kind {@code REF_invokeSpecial}.
 559      * The caller signature is restricted to basic types as with {@code invokeBasic}.
 560      * The trailing (not leading) argument must be a MemberName.
 561      * @param args the signature-polymorphic parameter list, statically represented using varargs
 562      * @return the signature-polymorphic result, statically represented using {@code Object}
 563      */
 564     /*non-public*/ static native @PolymorphicSignature Object linkToSpecial(Object... args) throws Throwable;
 565 
 566     /**
 567      * Private method for trusted invocation of a MemberName of kind {@code REF_invokeInterface}.
 568      * The caller signature is restricted to basic types as with {@code invokeBasic}.
 569      * The trailing (not leading) argument must be a MemberName.
 570      * @param args the signature-polymorphic parameter list, statically represented using varargs
 571      * @return the signature-polymorphic result, statically represented using {@code Object}
 572      */
 573     /*non-public*/ static native @PolymorphicSignature Object linkToInterface(Object... args) throws Throwable;
 574 
 575     /**
 576      * Performs a variable arity invocation, passing the arguments in the given list
 577      * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
 578      * which mentions only the type {@code Object}, and whose arity is the length
 579      * of the argument list.
 580      * <p>
 581      * Specifically, execution proceeds as if by the following steps,
 582      * although the methods are not guaranteed to be called if the JVM
 583      * can predict their effects.
 584      * <ul>
 585      * <li>Determine the length of the argument array as {@code N}.
 586      *     For a null reference, {@code N=0}. </li>
 587      * <li>Determine the general type {@code TN} of {@code N} arguments as
 588      *     as {@code TN=MethodType.genericMethodType(N)}.</li>
 589      * <li>Force the original target method handle {@code MH0} to the
 590      *     required type, as {@code MH1 = MH0.asType(TN)}. </li>
 591      * <li>Spread the array into {@code N} separate arguments {@code A0, ...}. </li>
 592      * <li>Invoke the type-adjusted method handle on the unpacked arguments:
 593      *     MH1.invokeExact(A0, ...). </li>
 594      * <li>Take the return value as an {@code Object} reference. </li>
 595      * </ul>
 596      * <p>
 597      * Because of the action of the {@code asType} step, the following argument
 598      * conversions are applied as necessary:
 599      * <ul>
 600      * <li>reference casting
 601      * <li>unboxing
 602      * <li>widening primitive conversions
 603      * </ul>
 604      * <p>
 605      * The result returned by the call is boxed if it is a primitive,
 606      * or forced to null if the return type is void.
 607      * <p>
 608      * This call is equivalent to the following code:
 609      * <blockquote><pre>{@code
 610      * MethodHandle invoker = MethodHandles.spreadInvoker(this.type(), 0);
 611      * Object result = invoker.invokeExact(this, arguments);
 612      * }</pre></blockquote>
 613      * <p>
 614      * Unlike the signature polymorphic methods {@code invokeExact} and {@code invoke},
 615      * {@code invokeWithArguments} can be accessed normally via the Core Reflection API and JNI.
 616      * It can therefore be used as a bridge between native or reflective code and method handles.
 617      *
 618      * @param arguments the arguments to pass to the target
 619      * @return the result returned by the target
 620      * @throws ClassCastException if an argument cannot be converted by reference casting
 621      * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
 622      * @throws Throwable anything thrown by the target method invocation
 623      * @see MethodHandles#spreadInvoker
 624      */
 625     public Object invokeWithArguments(Object... arguments) throws Throwable {
 626         MethodType invocationType = MethodType.genericMethodType(arguments == null ? 0 : arguments.length);
 627         return invocationType.invokers().spreadInvoker(0).invokeExact(asType(invocationType), arguments);
 628     }
 629 
 630     /**
 631      * Performs a variable arity invocation, passing the arguments in the given array
 632      * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
 633      * which mentions only the type {@code Object}, and whose arity is the length
 634      * of the argument array.
 635      * <p>
 636      * This method is also equivalent to the following code:
 637      * <blockquote><pre>{@code
 638      *   invokeWithArguments(arguments.toArray()
 639      * }</pre></blockquote>
 640      *
 641      * @param arguments the arguments to pass to the target
 642      * @return the result returned by the target
 643      * @throws NullPointerException if {@code arguments} is a null reference
 644      * @throws ClassCastException if an argument cannot be converted by reference casting
 645      * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
 646      * @throws Throwable anything thrown by the target method invocation
 647      */
 648     public Object invokeWithArguments(java.util.List<?> arguments) throws Throwable {
 649         return invokeWithArguments(arguments.toArray());
 650     }
 651 
 652     /**
 653      * Produces an adapter method handle which adapts the type of the
 654      * current method handle to a new type.
 655      * The resulting method handle is guaranteed to report a type
 656      * which is equal to the desired new type.
 657      * <p>
 658      * If the original type and new type are equal, returns {@code this}.
 659      * <p>
 660      * The new method handle, when invoked, will perform the following
 661      * steps:
 662      * <ul>
 663      * <li>Convert the incoming argument list to match the original
 664      *     method handle's argument list.
 665      * <li>Invoke the original method handle on the converted argument list.
 666      * <li>Convert any result returned by the original method handle
 667      *     to the return type of new method handle.
 668      * </ul>
 669      * <p>
 670      * This method provides the crucial behavioral difference between
 671      * {@link #invokeExact invokeExact} and plain, inexact {@link #invoke invoke}.
 672      * The two methods
 673      * perform the same steps when the caller's type descriptor exactly matches
 674      * the callee's, but when the types differ, plain {@link #invoke invoke}
 675      * also calls {@code asType} (or some internal equivalent) in order
 676      * to match up the caller's and callee's types.
 677      * <p>
 678      * If the current method is a variable arity method handle
 679      * argument list conversion may involve the conversion and collection
 680      * of several arguments into an array, as
 681      * {@linkplain #asVarargsCollector described elsewhere}.
 682      * In every other case, all conversions are applied <em>pairwise</em>,
 683      * which means that each argument or return value is converted to
 684      * exactly one argument or return value (or no return value).
 685      * The applied conversions are defined by consulting the
 686      * the corresponding component types of the old and new
 687      * method handle types.
 688      * <p>
 689      * Let <em>T0</em> and <em>T1</em> be corresponding new and old parameter types,
 690      * or old and new return types.  Specifically, for some valid index {@code i}, let
 691      * <em>T0</em>{@code =newType.parameterType(i)} and <em>T1</em>{@code =this.type().parameterType(i)}.
 692      * Or else, going the other way for return values, let
 693      * <em>T0</em>{@code =this.type().returnType()} and <em>T1</em>{@code =newType.returnType()}.
 694      * If the types are the same, the new method handle makes no change
 695      * to the corresponding argument or return value (if any).
 696      * Otherwise, one of the following conversions is applied
 697      * if possible:
 698      * <ul>
 699      * <li>If <em>T0</em> and <em>T1</em> are references, then a cast to <em>T1</em> is applied.
 700      *     (The types do not need to be related in any particular way.
 701      *     This is because a dynamic value of null can convert to any reference type.)
 702      * <li>If <em>T0</em> and <em>T1</em> are primitives, then a Java method invocation
 703      *     conversion (JLS 5.3) is applied, if one exists.
 704      *     (Specifically, <em>T0</em> must convert to <em>T1</em> by a widening primitive conversion.)
 705      * <li>If <em>T0</em> is a primitive and <em>T1</em> a reference,
 706      *     a Java casting conversion (JLS 5.5) is applied if one exists.
 707      *     (Specifically, the value is boxed from <em>T0</em> to its wrapper class,
 708      *     which is then widened as needed to <em>T1</em>.)
 709      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
 710      *     conversion will be applied at runtime, possibly followed
 711      *     by a Java method invocation conversion (JLS 5.3)
 712      *     on the primitive value.  (These are the primitive widening conversions.)
 713      *     <em>T0</em> must be a wrapper class or a supertype of one.
 714      *     (In the case where <em>T0</em> is Object, these are the conversions
 715      *     allowed by {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.)
 716      *     The unboxing conversion must have a possibility of success, which means that
 717      *     if <em>T0</em> is not itself a wrapper class, there must exist at least one
 718      *     wrapper class <em>TW</em> which is a subtype of <em>T0</em> and whose unboxed
 719      *     primitive value can be widened to <em>T1</em>.
 720      * <li>If the return type <em>T1</em> is marked as void, any returned value is discarded
 721      * <li>If the return type <em>T0</em> is void and <em>T1</em> a reference, a null value is introduced.
 722      * <li>If the return type <em>T0</em> is void and <em>T1</em> a primitive,
 723      *     a zero value is introduced.
 724      * </ul>
 725      * (<em>Note:</em> Both <em>T0</em> and <em>T1</em> may be regarded as static types,
 726      * because neither corresponds specifically to the <em>dynamic type</em> of any
 727      * actual argument or return value.)
 728      * <p>
 729      * The method handle conversion cannot be made if any one of the required
 730      * pairwise conversions cannot be made.
 731      * <p>
 732      * At runtime, the conversions applied to reference arguments
 733      * or return values may require additional runtime checks which can fail.
 734      * An unboxing operation may fail because the original reference is null,
 735      * causing a {@link java.lang.NullPointerException NullPointerException}.
 736      * An unboxing operation or a reference cast may also fail on a reference
 737      * to an object of the wrong type,
 738      * causing a {@link java.lang.ClassCastException ClassCastException}.
 739      * Although an unboxing operation may accept several kinds of wrappers,
 740      * if none are available, a {@code ClassCastException} will be thrown.
 741      *
 742      * @param newType the expected type of the new method handle
 743      * @return a method handle which delegates to {@code this} after performing
 744      *           any necessary argument conversions, and arranges for any
 745      *           necessary return value conversions
 746      * @throws NullPointerException if {@code newType} is a null reference
 747      * @throws WrongMethodTypeException if the conversion cannot be made
 748      * @see MethodHandles#explicitCastArguments
 749      */
 750     public MethodHandle asType(MethodType newType) {
 751         // Fast path alternative to a heavyweight {@code asType} call.
 752         // Return 'this' if the conversion will be a no-op.
 753         if (newType == type) {
 754             return this;
 755         }
 756         // Return 'this.asTypeCache' if the conversion is already memoized.
 757         MethodHandle atc = asTypeCached(newType);
 758         if (atc != null) {
 759             return atc;
 760         }
 761         return asTypeUncached(newType);
 762     }
 763 
 764     private MethodHandle asTypeCached(MethodType newType) {
 765         MethodHandle atc = asTypeCache;
 766         if (atc != null && newType == atc.type) {
 767             return atc;
 768         }
 769         return null;
 770     }
 771 
 772     /** Override this to change asType behavior. */
 773     /*non-public*/ MethodHandle asTypeUncached(MethodType newType) {
 774         if (!type.isConvertibleTo(newType))
 775             throw new WrongMethodTypeException("cannot convert "+this+" to "+newType);
 776         return asTypeCache = MethodHandleImpl.makePairwiseConvert(this, newType, true);
 777     }
 778 
 779     /**
 780      * Makes an <em>array-spreading</em> method handle, which accepts a trailing array argument
 781      * and spreads its elements as positional arguments.
 782      * The new method handle adapts, as its <i>target</i>,
 783      * the current method handle.  The type of the adapter will be
 784      * the same as the type of the target, except that the final
 785      * {@code arrayLength} parameters of the target's type are replaced
 786      * by a single array parameter of type {@code arrayType}.
 787      * <p>
 788      * If the array element type differs from any of the corresponding
 789      * argument types on the original target,
 790      * the original target is adapted to take the array elements directly,
 791      * as if by a call to {@link #asType asType}.
 792      * <p>
 793      * When called, the adapter replaces a trailing array argument
 794      * by the array's elements, each as its own argument to the target.
 795      * (The order of the arguments is preserved.)
 796      * They are converted pairwise by casting and/or unboxing
 797      * to the types of the trailing parameters of the target.
 798      * Finally the target is called.
 799      * What the target eventually returns is returned unchanged by the adapter.
 800      * <p>
 801      * Before calling the target, the adapter verifies that the array
 802      * contains exactly enough elements to provide a correct argument count
 803      * to the target method handle.
 804      * (The array may also be null when zero elements are required.)
 805      * <p>
 806      * If, when the adapter is called, the supplied array argument does
 807      * not have the correct number of elements, the adapter will throw
 808      * an {@link IllegalArgumentException} instead of invoking the target.
 809      * <p>
 810      * Here are some simple examples of array-spreading method handles:
 811      * <blockquote><pre>{@code
 812 MethodHandle equals = publicLookup()
 813   .findVirtual(String.class, "equals", methodType(boolean.class, Object.class));
 814 assert( (boolean) equals.invokeExact("me", (Object)"me"));
 815 assert(!(boolean) equals.invokeExact("me", (Object)"thee"));
 816 // spread both arguments from a 2-array:
 817 MethodHandle eq2 = equals.asSpreader(Object[].class, 2);
 818 assert( (boolean) eq2.invokeExact(new Object[]{ "me", "me" }));
 819 assert(!(boolean) eq2.invokeExact(new Object[]{ "me", "thee" }));
 820 // try to spread from anything but a 2-array:
 821 for (int n = 0; n <= 10; n++) {
 822   Object[] badArityArgs = (n == 2 ? null : new Object[n]);
 823   try { assert((boolean) eq2.invokeExact(badArityArgs) && false); }
 824   catch (IllegalArgumentException ex) { } // OK
 825 }
 826 // spread both arguments from a String array:
 827 MethodHandle eq2s = equals.asSpreader(String[].class, 2);
 828 assert( (boolean) eq2s.invokeExact(new String[]{ "me", "me" }));
 829 assert(!(boolean) eq2s.invokeExact(new String[]{ "me", "thee" }));
 830 // spread second arguments from a 1-array:
 831 MethodHandle eq1 = equals.asSpreader(Object[].class, 1);
 832 assert( (boolean) eq1.invokeExact("me", new Object[]{ "me" }));
 833 assert(!(boolean) eq1.invokeExact("me", new Object[]{ "thee" }));
 834 // spread no arguments from a 0-array or null:
 835 MethodHandle eq0 = equals.asSpreader(Object[].class, 0);
 836 assert( (boolean) eq0.invokeExact("me", (Object)"me", new Object[0]));
 837 assert(!(boolean) eq0.invokeExact("me", (Object)"thee", (Object[])null));
 838 // asSpreader and asCollector are approximate inverses:
 839 for (int n = 0; n <= 2; n++) {
 840     for (Class<?> a : new Class<?>[]{Object[].class, String[].class, CharSequence[].class}) {
 841         MethodHandle equals2 = equals.asSpreader(a, n).asCollector(a, n);
 842         assert( (boolean) equals2.invokeWithArguments("me", "me"));
 843         assert(!(boolean) equals2.invokeWithArguments("me", "thee"));
 844     }
 845 }
 846 MethodHandle caToString = publicLookup()
 847   .findStatic(Arrays.class, "toString", methodType(String.class, char[].class));
 848 assertEquals("[A, B, C]", (String) caToString.invokeExact("ABC".toCharArray()));
 849 MethodHandle caString3 = caToString.asCollector(char[].class, 3);
 850 assertEquals("[A, B, C]", (String) caString3.invokeExact('A', 'B', 'C'));
 851 MethodHandle caToString2 = caString3.asSpreader(char[].class, 2);
 852 assertEquals("[A, B, C]", (String) caToString2.invokeExact('A', "BC".toCharArray()));
 853      * }</pre></blockquote>
 854      * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments
 855      * @param arrayLength the number of arguments to spread from an incoming array argument
 856      * @return a new method handle which spreads its final array argument,
 857      *         before calling the original method handle
 858      * @throws NullPointerException if {@code arrayType} is a null reference
 859      * @throws IllegalArgumentException if {@code arrayType} is not an array type,
 860      *         or if target does not have at least
 861      *         {@code arrayLength} parameter types,
 862      *         or if {@code arrayLength} is negative,
 863      *         or if the resulting method handle's type would have
 864      *         <a href="MethodHandle.html#maxarity">too many parameters</a>
 865      * @throws WrongMethodTypeException if the implied {@code asType} call fails
 866      * @see #asCollector
 867      */
 868     public MethodHandle asSpreader(Class<?> arrayType, int arrayLength) {
 869         MethodType postSpreadType = asSpreaderChecks(arrayType, arrayLength);
 870         int arity = type().parameterCount();
 871         int spreadArgPos = arity - arrayLength;
 872         MethodHandle afterSpread = this.asType(postSpreadType);
 873         BoundMethodHandle mh = afterSpread.rebind();
 874         LambdaForm lform = mh.editor().spreadArgumentsForm(1 + spreadArgPos, arrayType, arrayLength);
 875         MethodType preSpreadType = postSpreadType.replaceParameterTypes(spreadArgPos, arity, arrayType);
 876         return mh.copyWith(preSpreadType, lform);
 877     }
 878 
 879     /**
 880      * See if {@code asSpreader} can be validly called with the given arguments.
 881      * Return the type of the method handle call after spreading but before conversions.
 882      */
 883     private MethodType asSpreaderChecks(Class<?> arrayType, int arrayLength) {
 884         spreadArrayChecks(arrayType, arrayLength);
 885         int nargs = type().parameterCount();
 886         if (nargs < arrayLength || arrayLength < 0)
 887             throw newIllegalArgumentException("bad spread array length");
 888         Class<?> arrayElement = arrayType.getComponentType();
 889         MethodType mtype = type();
 890         boolean match = true, fail = false;
 891         for (int i = nargs - arrayLength; i < nargs; i++) {
 892             Class<?> ptype = mtype.parameterType(i);
 893             if (ptype != arrayElement) {
 894                 match = false;
 895                 if (!MethodType.canConvert(arrayElement, ptype)) {
 896                     fail = true;
 897                     break;
 898                 }
 899             }
 900         }
 901         if (match)  return mtype;
 902         MethodType needType = mtype.asSpreaderType(arrayType, arrayLength);
 903         if (!fail)  return needType;
 904         // elicit an error:
 905         this.asType(needType);
 906         throw newInternalError("should not return", null);
 907     }
 908 
 909     private void spreadArrayChecks(Class<?> arrayType, int arrayLength) {
 910         Class<?> arrayElement = arrayType.getComponentType();
 911         if (arrayElement == null)
 912             throw newIllegalArgumentException("not an array type", arrayType);
 913         if ((arrayLength & 0x7F) != arrayLength) {
 914             if ((arrayLength & 0xFF) != arrayLength)
 915                 throw newIllegalArgumentException("array length is not legal", arrayLength);
 916             assert(arrayLength >= 128);
 917             if (arrayElement == long.class ||
 918                 arrayElement == double.class)
 919                 throw newIllegalArgumentException("array length is not legal for long[] or double[]", arrayLength);
 920         }
 921     }
 922 
 923     /**
 924      * Makes an <em>array-collecting</em> method handle, which accepts a given number of trailing
 925      * positional arguments and collects them into an array argument.
 926      * The new method handle adapts, as its <i>target</i>,
 927      * the current method handle.  The type of the adapter will be
 928      * the same as the type of the target, except that a single trailing
 929      * parameter (usually of type {@code arrayType}) is replaced by
 930      * {@code arrayLength} parameters whose type is element type of {@code arrayType}.
 931      * <p>
 932      * If the array type differs from the final argument type on the original target,
 933      * the original target is adapted to take the array type directly,
 934      * as if by a call to {@link #asType asType}.
 935      * <p>
 936      * When called, the adapter replaces its trailing {@code arrayLength}
 937      * arguments by a single new array of type {@code arrayType}, whose elements
 938      * comprise (in order) the replaced arguments.
 939      * Finally the target is called.
 940      * What the target eventually returns is returned unchanged by the adapter.
 941      * <p>
 942      * (The array may also be a shared constant when {@code arrayLength} is zero.)
 943      * <p>
 944      * (<em>Note:</em> The {@code arrayType} is often identical to the last
 945      * parameter type of the original target.
 946      * It is an explicit argument for symmetry with {@code asSpreader}, and also
 947      * to allow the target to use a simple {@code Object} as its last parameter type.)
 948      * <p>
 949      * In order to create a collecting adapter which is not restricted to a particular
 950      * number of collected arguments, use {@link #asVarargsCollector asVarargsCollector} instead.
 951      * <p>
 952      * Here are some examples of array-collecting method handles:
 953      * <blockquote><pre>{@code
 954 MethodHandle deepToString = publicLookup()
 955   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
 956 assertEquals("[won]",   (String) deepToString.invokeExact(new Object[]{"won"}));
 957 MethodHandle ts1 = deepToString.asCollector(Object[].class, 1);
 958 assertEquals(methodType(String.class, Object.class), ts1.type());
 959 //assertEquals("[won]", (String) ts1.invokeExact(         new Object[]{"won"})); //FAIL
 960 assertEquals("[[won]]", (String) ts1.invokeExact((Object) new Object[]{"won"}));
 961 // arrayType can be a subtype of Object[]
 962 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
 963 assertEquals(methodType(String.class, String.class, String.class), ts2.type());
 964 assertEquals("[two, too]", (String) ts2.invokeExact("two", "too"));
 965 MethodHandle ts0 = deepToString.asCollector(Object[].class, 0);
 966 assertEquals("[]", (String) ts0.invokeExact());
 967 // collectors can be nested, Lisp-style
 968 MethodHandle ts22 = deepToString.asCollector(Object[].class, 3).asCollector(String[].class, 2);
 969 assertEquals("[A, B, [C, D]]", ((String) ts22.invokeExact((Object)'A', (Object)"B", "C", "D")));
 970 // arrayType can be any primitive array type
 971 MethodHandle bytesToString = publicLookup()
 972   .findStatic(Arrays.class, "toString", methodType(String.class, byte[].class))
 973   .asCollector(byte[].class, 3);
 974 assertEquals("[1, 2, 3]", (String) bytesToString.invokeExact((byte)1, (byte)2, (byte)3));
 975 MethodHandle longsToString = publicLookup()
 976   .findStatic(Arrays.class, "toString", methodType(String.class, long[].class))
 977   .asCollector(long[].class, 1);
 978 assertEquals("[123]", (String) longsToString.invokeExact((long)123));
 979      * }</pre></blockquote>
 980      * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
 981      * @param arrayLength the number of arguments to collect into a new array argument
 982      * @return a new method handle which collects some trailing argument
 983      *         into an array, before calling the original method handle
 984      * @throws NullPointerException if {@code arrayType} is a null reference
 985      * @throws IllegalArgumentException if {@code arrayType} is not an array type
 986      *         or {@code arrayType} is not assignable to this method handle's trailing parameter type,
 987      *         or {@code arrayLength} is not a legal array size,
 988      *         or the resulting method handle's type would have
 989      *         <a href="MethodHandle.html#maxarity">too many parameters</a>
 990      * @throws WrongMethodTypeException if the implied {@code asType} call fails
 991      * @see #asSpreader
 992      * @see #asVarargsCollector
 993      */
 994     public MethodHandle asCollector(Class<?> arrayType, int arrayLength) {
 995         asCollectorChecks(arrayType, arrayLength);
 996         int collectArgPos = type().parameterCount() - 1;
 997         BoundMethodHandle mh = rebind();
 998         MethodType resultType = type().asCollectorType(arrayType, arrayLength);
 999         MethodHandle newArray = MethodHandleImpl.varargsArray(arrayType, arrayLength);
1000         LambdaForm lform = mh.editor().collectArgumentArrayForm(1 + collectArgPos, newArray);
1001         if (lform != null) {
1002             return mh.copyWith(resultType, lform);
1003         }
1004         lform = mh.editor().collectArgumentsForm(1 + collectArgPos, newArray.type().basicType());
1005         return mh.copyWithExtendL(resultType, lform, newArray);
1006     }
1007 
1008     /**
1009      * See if {@code asCollector} can be validly called with the given arguments.
1010      * Return false if the last parameter is not an exact match to arrayType.
1011      */
1012     /*non-public*/ boolean asCollectorChecks(Class<?> arrayType, int arrayLength) {
1013         spreadArrayChecks(arrayType, arrayLength);
1014         int nargs = type().parameterCount();
1015         if (nargs != 0) {
1016             Class<?> lastParam = type().parameterType(nargs-1);
1017             if (lastParam == arrayType)  return true;
1018             if (lastParam.isAssignableFrom(arrayType))  return false;
1019         }
1020         throw newIllegalArgumentException("array type not assignable to trailing argument", this, arrayType);
1021     }
1022 
1023     /**
1024      * Makes a <em>variable arity</em> adapter which is able to accept
1025      * any number of trailing positional arguments and collect them
1026      * into an array argument.
1027      * <p>
1028      * The type and behavior of the adapter will be the same as
1029      * the type and behavior of the target, except that certain
1030      * {@code invoke} and {@code asType} requests can lead to
1031      * trailing positional arguments being collected into target's
1032      * trailing parameter.
1033      * Also, the last parameter type of the adapter will be
1034      * {@code arrayType}, even if the target has a different
1035      * last parameter type.
1036      * <p>
1037      * This transformation may return {@code this} if the method handle is
1038      * already of variable arity and its trailing parameter type
1039      * is identical to {@code arrayType}.
1040      * <p>
1041      * When called with {@link #invokeExact invokeExact}, the adapter invokes
1042      * the target with no argument changes.
1043      * (<em>Note:</em> This behavior is different from a
1044      * {@linkplain #asCollector fixed arity collector},
1045      * since it accepts a whole array of indeterminate length,
1046      * rather than a fixed number of arguments.)
1047      * <p>
1048      * When called with plain, inexact {@link #invoke invoke}, if the caller
1049      * type is the same as the adapter, the adapter invokes the target as with
1050      * {@code invokeExact}.
1051      * (This is the normal behavior for {@code invoke} when types match.)
1052      * <p>
1053      * Otherwise, if the caller and adapter arity are the same, and the
1054      * trailing parameter type of the caller is a reference type identical to
1055      * or assignable to the trailing parameter type of the adapter,
1056      * the arguments and return values are converted pairwise,
1057      * as if by {@link #asType asType} on a fixed arity
1058      * method handle.
1059      * <p>
1060      * Otherwise, the arities differ, or the adapter's trailing parameter
1061      * type is not assignable from the corresponding caller type.
1062      * In this case, the adapter replaces all trailing arguments from
1063      * the original trailing argument position onward, by
1064      * a new array of type {@code arrayType}, whose elements
1065      * comprise (in order) the replaced arguments.
1066      * <p>
1067      * The caller type must provides as least enough arguments,
1068      * and of the correct type, to satisfy the target's requirement for
1069      * positional arguments before the trailing array argument.
1070      * Thus, the caller must supply, at a minimum, {@code N-1} arguments,
1071      * where {@code N} is the arity of the target.
1072      * Also, there must exist conversions from the incoming arguments
1073      * to the target's arguments.
1074      * As with other uses of plain {@code invoke}, if these basic
1075      * requirements are not fulfilled, a {@code WrongMethodTypeException}
1076      * may be thrown.
1077      * <p>
1078      * In all cases, what the target eventually returns is returned unchanged by the adapter.
1079      * <p>
1080      * In the final case, it is exactly as if the target method handle were
1081      * temporarily adapted with a {@linkplain #asCollector fixed arity collector}
1082      * to the arity required by the caller type.
1083      * (As with {@code asCollector}, if the array length is zero,
1084      * a shared constant may be used instead of a new array.
1085      * If the implied call to {@code asCollector} would throw
1086      * an {@code IllegalArgumentException} or {@code WrongMethodTypeException},
1087      * the call to the variable arity adapter must throw
1088      * {@code WrongMethodTypeException}.)
1089      * <p>
1090      * The behavior of {@link #asType asType} is also specialized for
1091      * variable arity adapters, to maintain the invariant that
1092      * plain, inexact {@code invoke} is always equivalent to an {@code asType}
1093      * call to adjust the target type, followed by {@code invokeExact}.
1094      * Therefore, a variable arity adapter responds
1095      * to an {@code asType} request by building a fixed arity collector,
1096      * if and only if the adapter and requested type differ either
1097      * in arity or trailing argument type.
1098      * The resulting fixed arity collector has its type further adjusted
1099      * (if necessary) to the requested type by pairwise conversion,
1100      * as if by another application of {@code asType}.
1101      * <p>
1102      * When a method handle is obtained by executing an {@code ldc} instruction
1103      * of a {@code CONSTANT_MethodHandle} constant, and the target method is marked
1104      * as a variable arity method (with the modifier bit {@code 0x0080}),
1105      * the method handle will accept multiple arities, as if the method handle
1106      * constant were created by means of a call to {@code asVarargsCollector}.
1107      * <p>
1108      * In order to create a collecting adapter which collects a predetermined
1109      * number of arguments, and whose type reflects this predetermined number,
1110      * use {@link #asCollector asCollector} instead.
1111      * <p>
1112      * No method handle transformations produce new method handles with
1113      * variable arity, unless they are documented as doing so.
1114      * Therefore, besides {@code asVarargsCollector},
1115      * all methods in {@code MethodHandle} and {@code MethodHandles}
1116      * will return a method handle with fixed arity,
1117      * except in the cases where they are specified to return their original
1118      * operand (e.g., {@code asType} of the method handle's own type).
1119      * <p>
1120      * Calling {@code asVarargsCollector} on a method handle which is already
1121      * of variable arity will produce a method handle with the same type and behavior.
1122      * It may (or may not) return the original variable arity method handle.
1123      * <p>
1124      * Here is an example, of a list-making variable arity method handle:
1125      * <blockquote><pre>{@code
1126 MethodHandle deepToString = publicLookup()
1127   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
1128 MethodHandle ts1 = deepToString.asVarargsCollector(Object[].class);
1129 assertEquals("[won]",   (String) ts1.invokeExact(    new Object[]{"won"}));
1130 assertEquals("[won]",   (String) ts1.invoke(         new Object[]{"won"}));
1131 assertEquals("[won]",   (String) ts1.invoke(                      "won" ));
1132 assertEquals("[[won]]", (String) ts1.invoke((Object) new Object[]{"won"}));
1133 // findStatic of Arrays.asList(...) produces a variable arity method handle:
1134 MethodHandle asList = publicLookup()
1135   .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class));
1136 assertEquals(methodType(List.class, Object[].class), asList.type());
1137 assert(asList.isVarargsCollector());
1138 assertEquals("[]", asList.invoke().toString());
1139 assertEquals("[1]", asList.invoke(1).toString());
1140 assertEquals("[two, too]", asList.invoke("two", "too").toString());
1141 String[] argv = { "three", "thee", "tee" };
1142 assertEquals("[three, thee, tee]", asList.invoke(argv).toString());
1143 assertEquals("[three, thee, tee]", asList.invoke((Object[])argv).toString());
1144 List ls = (List) asList.invoke((Object)argv);
1145 assertEquals(1, ls.size());
1146 assertEquals("[three, thee, tee]", Arrays.toString((Object[])ls.get(0)));
1147      * }</pre></blockquote>
1148      * <p style="font-size:smaller;">
1149      * <em>Discussion:</em>
1150      * These rules are designed as a dynamically-typed variation
1151      * of the Java rules for variable arity methods.
1152      * In both cases, callers to a variable arity method or method handle
1153      * can either pass zero or more positional arguments, or else pass
1154      * pre-collected arrays of any length.  Users should be aware of the
1155      * special role of the final argument, and of the effect of a
1156      * type match on that final argument, which determines whether
1157      * or not a single trailing argument is interpreted as a whole
1158      * array or a single element of an array to be collected.
1159      * Note that the dynamic type of the trailing argument has no
1160      * effect on this decision, only a comparison between the symbolic
1161      * type descriptor of the call site and the type descriptor of the method handle.)
1162      *
1163      * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
1164      * @return a new method handle which can collect any number of trailing arguments
1165      *         into an array, before calling the original method handle
1166      * @throws NullPointerException if {@code arrayType} is a null reference
1167      * @throws IllegalArgumentException if {@code arrayType} is not an array type
1168      *         or {@code arrayType} is not assignable to this method handle's trailing parameter type
1169      * @see #asCollector
1170      * @see #isVarargsCollector
1171      * @see #asFixedArity
1172      */
1173     public MethodHandle asVarargsCollector(Class<?> arrayType) {
1174         arrayType.getClass(); // explicit NPE
1175         boolean lastMatch = asCollectorChecks(arrayType, 0);
1176         if (isVarargsCollector() && lastMatch)
1177             return this;
1178         return MethodHandleImpl.makeVarargsCollector(this, arrayType);
1179     }
1180 
1181     /**
1182      * Determines if this method handle
1183      * supports {@linkplain #asVarargsCollector variable arity} calls.
1184      * Such method handles arise from the following sources:
1185      * <ul>
1186      * <li>a call to {@linkplain #asVarargsCollector asVarargsCollector}
1187      * <li>a call to a {@linkplain java.lang.invoke.MethodHandles.Lookup lookup method}
1188      *     which resolves to a variable arity Java method or constructor
1189      * <li>an {@code ldc} instruction of a {@code CONSTANT_MethodHandle}
1190      *     which resolves to a variable arity Java method or constructor
1191      * </ul>
1192      * @return true if this method handle accepts more than one arity of plain, inexact {@code invoke} calls
1193      * @see #asVarargsCollector
1194      * @see #asFixedArity
1195      */
1196     public boolean isVarargsCollector() {
1197         return false;
1198     }
1199 
1200     /**
1201      * Makes a <em>fixed arity</em> method handle which is otherwise
1202      * equivalent to the current method handle.
1203      * <p>
1204      * If the current method handle is not of
1205      * {@linkplain #asVarargsCollector variable arity},
1206      * the current method handle is returned.
1207      * This is true even if the current method handle
1208      * could not be a valid input to {@code asVarargsCollector}.
1209      * <p>
1210      * Otherwise, the resulting fixed-arity method handle has the same
1211      * type and behavior of the current method handle,
1212      * except that {@link #isVarargsCollector isVarargsCollector}
1213      * will be false.
1214      * The fixed-arity method handle may (or may not) be the
1215      * a previous argument to {@code asVarargsCollector}.
1216      * <p>
1217      * Here is an example, of a list-making variable arity method handle:
1218      * <blockquote><pre>{@code
1219 MethodHandle asListVar = publicLookup()
1220   .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class))
1221   .asVarargsCollector(Object[].class);
1222 MethodHandle asListFix = asListVar.asFixedArity();
1223 assertEquals("[1]", asListVar.invoke(1).toString());
1224 Exception caught = null;
1225 try { asListFix.invoke((Object)1); }
1226 catch (Exception ex) { caught = ex; }
1227 assert(caught instanceof ClassCastException);
1228 assertEquals("[two, too]", asListVar.invoke("two", "too").toString());
1229 try { asListFix.invoke("two", "too"); }
1230 catch (Exception ex) { caught = ex; }
1231 assert(caught instanceof WrongMethodTypeException);
1232 Object[] argv = { "three", "thee", "tee" };
1233 assertEquals("[three, thee, tee]", asListVar.invoke(argv).toString());
1234 assertEquals("[three, thee, tee]", asListFix.invoke(argv).toString());
1235 assertEquals(1, ((List) asListVar.invoke((Object)argv)).size());
1236 assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString());
1237      * }</pre></blockquote>
1238      *
1239      * @return a new method handle which accepts only a fixed number of arguments
1240      * @see #asVarargsCollector
1241      * @see #isVarargsCollector
1242      */
1243     public MethodHandle asFixedArity() {
1244         assert(!isVarargsCollector());
1245         return this;
1246     }
1247 
1248     /**
1249      * Binds a value {@code x} to the first argument of a method handle, without invoking it.
1250      * The new method handle adapts, as its <i>target</i>,
1251      * the current method handle by binding it to the given argument.
1252      * The type of the bound handle will be
1253      * the same as the type of the target, except that a single leading
1254      * reference parameter will be omitted.
1255      * <p>
1256      * When called, the bound handle inserts the given value {@code x}
1257      * as a new leading argument to the target.  The other arguments are
1258      * also passed unchanged.
1259      * What the target eventually returns is returned unchanged by the bound handle.
1260      * <p>
1261      * The reference {@code x} must be convertible to the first parameter
1262      * type of the target.
1263      * <p>
1264      * (<em>Note:</em>  Because method handles are immutable, the target method handle
1265      * retains its original type and behavior.)
1266      * @param x  the value to bind to the first argument of the target
1267      * @return a new method handle which prepends the given value to the incoming
1268      *         argument list, before calling the original method handle
1269      * @throws IllegalArgumentException if the target does not have a
1270      *         leading parameter type that is a reference type
1271      * @throws ClassCastException if {@code x} cannot be converted
1272      *         to the leading parameter type of the target
1273      * @see MethodHandles#insertArguments
1274      */
1275     public MethodHandle bindTo(Object x) {
1276         x = type.leadingReferenceParameter().cast(x);  // throw CCE if needed
1277         return bindArgumentL(0, x);
1278     }
1279 
1280     /**
1281      * Returns a string representation of the method handle,
1282      * starting with the string {@code "MethodHandle"} and
1283      * ending with the string representation of the method handle's type.
1284      * In other words, this method returns a string equal to the value of:
1285      * <blockquote><pre>{@code
1286      * "MethodHandle" + type().toString()
1287      * }</pre></blockquote>
1288      * <p>
1289      * (<em>Note:</em>  Future releases of this API may add further information
1290      * to the string representation.
1291      * Therefore, the present syntax should not be parsed by applications.)
1292      *
1293      * @return a string representation of the method handle
1294      */
1295     @Override
1296     public String toString() {
1297         if (DEBUG_METHOD_HANDLE_NAMES)  return "MethodHandle"+debugString();
1298         return standardString();
1299     }
1300     String standardString() {
1301         return "MethodHandle"+type;
1302     }
1303     /** Return a string with a several lines describing the method handle structure.
1304      *  This string would be suitable for display in an IDE debugger.
1305      */
1306     String debugString() {
1307         return type+" : "+internalForm()+internalProperties();
1308     }
1309 
1310     //// Implementation methods.
1311     //// Sub-classes can override these default implementations.
1312     //// All these methods assume arguments are already validated.
1313 
1314     // Other transforms to do:  convert, explicitCast, permute, drop, filter, fold, GWT, catch
1315 
1316     BoundMethodHandle bindArgumentL(int pos, Object value) {
1317         return rebind().bindArgumentL(pos, value);
1318     }
1319 
1320     /*non-public*/
1321     MethodHandle setVarargs(MemberName member) throws IllegalAccessException {
1322         if (!member.isVarargs())  return this;
1323         Class<?> arrayType = type().lastParameterType();
1324         if (arrayType.isArray()) {
1325             return MethodHandleImpl.makeVarargsCollector(this, arrayType);
1326         }
1327         throw member.makeAccessException("cannot make variable arity", null);
1328     }
1329 
1330     /*non-public*/
1331     MethodHandle viewAsType(MethodType newType, boolean strict) {
1332         // No actual conversions, just a new view of the same method.
1333         // Note that this operation must not produce a DirectMethodHandle,
1334         // because retyped DMHs, like any transformed MHs,
1335         // cannot be cracked into MethodHandleInfo.
1336         assert viewAsTypeChecks(newType, strict);
1337         BoundMethodHandle mh = rebind();
1338         assert(!((MethodHandle)mh instanceof DirectMethodHandle));
1339         return mh.copyWith(newType, mh.form);
1340     }
1341 
1342     /*non-public*/
1343     boolean viewAsTypeChecks(MethodType newType, boolean strict) {
1344         if (strict) {
1345             assert(type().isViewableAs(newType, true))
1346                 : Arrays.asList(this, newType);
1347         } else {
1348             assert(type().basicType().isViewableAs(newType.basicType(), true))
1349                 : Arrays.asList(this, newType);
1350         }
1351         return true;
1352     }
1353 
1354     // Decoding
1355 
1356     /*non-public*/
1357     LambdaForm internalForm() {
1358         return form;
1359     }
1360 
1361     /*non-public*/
1362     MemberName internalMemberName() {
1363         return null;  // DMH returns DMH.member
1364     }
1365 
1366     /*non-public*/
1367     Class<?> internalCallerClass() {
1368         return null;  // caller-bound MH for @CallerSensitive method returns caller
1369     }
1370 
1371     /*non-public*/
1372     MethodHandleImpl.Intrinsic intrinsicName() {
1373         // no special intrinsic meaning to most MHs
1374         return MethodHandleImpl.Intrinsic.NONE;
1375     }
1376 
1377     /*non-public*/
1378     MethodHandle withInternalMemberName(MemberName member, boolean isInvokeSpecial) {
1379         if (member != null) {
1380             return MethodHandleImpl.makeWrappedMember(this, member, isInvokeSpecial);
1381         } else if (internalMemberName() == null) {
1382             // The required internaMemberName is null, and this MH (like most) doesn't have one.
1383             return this;
1384         } else {
1385             // The following case is rare. Mask the internalMemberName by wrapping the MH in a BMH.
1386             MethodHandle result = rebind();
1387             assert (result.internalMemberName() == null);
1388             return result;
1389         }
1390     }
1391 
1392     /*non-public*/
1393     boolean isInvokeSpecial() {
1394         return false;  // DMH.Special returns true
1395     }
1396 
1397     /*non-public*/
1398     Object internalValues() {
1399         return null;
1400     }
1401 
1402     /*non-public*/
1403     Object internalProperties() {
1404         // Override to something to follow this.form, like "\n& FOO=bar"
1405         return "";
1406     }
1407 
1408     //// Method handle implementation methods.
1409     //// Sub-classes can override these default implementations.
1410     //// All these methods assume arguments are already validated.
1411 
1412     /*non-public*/
1413     abstract MethodHandle copyWith(MethodType mt, LambdaForm lf);
1414 
1415     /** Require this method handle to be a BMH, or else replace it with a "wrapper" BMH.
1416      *  Many transforms are implemented only for BMHs.
1417      *  @return a behaviorally equivalent BMH
1418      */
1419     abstract BoundMethodHandle rebind();
1420 
1421     /**
1422      * Replace the old lambda form of this method handle with a new one.
1423      * The new one must be functionally equivalent to the old one.
1424      * Threads may continue running the old form indefinitely,
1425      * but it is likely that the new one will be preferred for new executions.
1426      * Use with discretion.
1427      */
1428     /*non-public*/
1429     void updateForm(LambdaForm newForm) {
1430         if (form == newForm)  return;
1431         newForm.prepare();  // as in MethodHandle.<init>
1432         UNSAFE.putObject(this, FORM_OFFSET, newForm);
1433         UNSAFE.fullFence();
1434     }
1435 
1436     /** Craft a LambdaForm customized for this particular MethodHandle */
1437     /*non-public*/
1438     void customize() {
1439         if (form.customized == null) {
1440             LambdaForm newForm = new LambdaForm(form.debugName, form.arity, form.names, form.result, form.forceInline, this);
1441             updateForm(newForm);
1442         } else {
1443             assert(form.customized == this);
1444         }
1445     }
1446 
1447     private static final long FORM_OFFSET;
1448     static {
1449         try {
1450             FORM_OFFSET = UNSAFE.objectFieldOffset(MethodHandle.class.getDeclaredField("form"));
1451         } catch (ReflectiveOperationException ex) {
1452             throw newInternalError(ex);
1453         }
1454     }
1455 }