Adding necessary fake classes to allow Javac to compile lamda expressions against our emulation library.
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26 package java.lang.invoke;
32 * A method handle is a typed, directly executable reference to an underlying method,
33 * constructor, field, or similar low-level operation, with optional
34 * transformations of arguments or return values.
35 * These transformations are quite general, and include such patterns as
36 * {@linkplain #asType conversion},
37 * {@linkplain #bindTo insertion},
38 * {@linkplain java.lang.invoke.MethodHandles#dropArguments deletion},
39 * and {@linkplain java.lang.invoke.MethodHandles#filterArguments substitution}.
41 * <h1>Method handle contents</h1>
42 * Method handles are dynamically and strongly typed according to their parameter and return types.
43 * They are not distinguished by the name or the defining class of their underlying methods.
44 * A method handle must be invoked using a symbolic type descriptor which matches
45 * the method handle's own {@linkplain #type type descriptor}.
47 * Every method handle reports its type descriptor via the {@link #type type} accessor.
48 * This type descriptor is a {@link java.lang.invoke.MethodType MethodType} object,
49 * whose structure is a series of classes, one of which is
50 * the return type of the method (or {@code void.class} if none).
52 * A method handle's type controls the types of invocations it accepts,
53 * and the kinds of transformations that apply to it.
55 * A method handle contains a pair of special invoker methods
56 * called {@link #invokeExact invokeExact} and {@link #invoke invoke}.
57 * Both invoker methods provide direct access to the method handle's
58 * underlying method, constructor, field, or other operation,
59 * as modified by transformations of arguments and return values.
60 * Both invokers accept calls which exactly match the method handle's own type.
61 * The plain, inexact invoker also accepts a range of other call types.
63 * Method handles are immutable and have no visible state.
64 * Of course, they can be bound to underlying methods or data which exhibit state.
65 * With respect to the Java Memory Model, any method handle will behave
66 * as if all of its (internal) fields are final variables. This means that any method
67 * handle made visible to the application will always be fully formed.
68 * This is true even if the method handle is published through a shared
69 * variable in a data race.
71 * Method handles cannot be subclassed by the user.
72 * Implementations may (or may not) create internal subclasses of {@code MethodHandle}
73 * which may be visible via the {@link java.lang.Object#getClass Object.getClass}
74 * operation. The programmer should not draw conclusions about a method handle
75 * from its specific class, as the method handle class hierarchy (if any)
76 * may change from time to time or across implementations from different vendors.
78 * <h1>Method handle compilation</h1>
79 * A Java method call expression naming {@code invokeExact} or {@code invoke}
80 * can invoke a method handle from Java source code.
81 * From the viewpoint of source code, these methods can take any arguments
82 * and their result can be cast to any return type.
83 * Formally this is accomplished by giving the invoker methods
84 * {@code Object} return types and variable arity {@code Object} arguments,
85 * but they have an additional quality called <em>signature polymorphism</em>
86 * which connects this freedom of invocation directly to the JVM execution stack.
88 * As is usual with virtual methods, source-level calls to {@code invokeExact}
89 * and {@code invoke} compile to an {@code invokevirtual} instruction.
90 * More unusually, the compiler must record the actual argument types,
91 * and may not perform method invocation conversions on the arguments.
92 * Instead, it must push them on the stack according to their own unconverted types.
93 * The method handle object itself is pushed on the stack before the arguments.
94 * The compiler then calls the method handle with a symbolic type descriptor which
95 * describes the argument and return types.
97 * To issue a complete symbolic type descriptor, the compiler must also determine
98 * the return type. This is based on a cast on the method invocation expression,
99 * if there is one, or else {@code Object} if the invocation is an expression
100 * or else {@code void} if the invocation is a statement.
101 * The cast may be to a primitive type (but not {@code void}).
103 * As a corner case, an uncasted {@code null} argument is given
104 * a symbolic type descriptor of {@code java.lang.Void}.
105 * The ambiguity with the type {@code Void} is harmless, since there are no references of type
106 * {@code Void} except the null reference.
108 * <h1>Method handle invocation</h1>
109 * The first time a {@code invokevirtual} instruction is executed
110 * it is linked, by symbolically resolving the names in the instruction
111 * and verifying that the method call is statically legal.
112 * This is true of calls to {@code invokeExact} and {@code invoke}.
113 * In this case, the symbolic type descriptor emitted by the compiler is checked for
114 * correct syntax and names it contains are resolved.
115 * Thus, an {@code invokevirtual} instruction which invokes
116 * a method handle will always link, as long
117 * as the symbolic type descriptor is syntactically well-formed
118 * and the types exist.
120 * When the {@code invokevirtual} is executed after linking,
121 * the receiving method handle's type is first checked by the JVM
122 * to ensure that it matches the symbolic type descriptor.
123 * If the type match fails, it means that the method which the
124 * caller is invoking is not present on the individual
125 * method handle being invoked.
127 * In the case of {@code invokeExact}, the type descriptor of the invocation
128 * (after resolving symbolic type names) must exactly match the method type
129 * of the receiving method handle.
130 * In the case of plain, inexact {@code invoke}, the resolved type descriptor
131 * must be a valid argument to the receiver's {@link #asType asType} method.
132 * Thus, plain {@code invoke} is more permissive than {@code invokeExact}.
134 * After type matching, a call to {@code invokeExact} directly
135 * and immediately invoke the method handle's underlying method
136 * (or other behavior, as the case may be).
138 * A call to plain {@code invoke} works the same as a call to
139 * {@code invokeExact}, if the symbolic type descriptor specified by the caller
140 * exactly matches the method handle's own type.
141 * If there is a type mismatch, {@code invoke} attempts
142 * to adjust the type of the receiving method handle,
143 * as if by a call to {@link #asType asType},
144 * to obtain an exactly invokable method handle {@code M2}.
145 * This allows a more powerful negotiation of method type
146 * between caller and callee.
148 * (<em>Note:</em> The adjusted method handle {@code M2} is not directly observable,
149 * and implementations are therefore not required to materialize it.)
151 * <h1>Invocation checking</h1>
152 * In typical programs, method handle type matching will usually succeed.
153 * But if a match fails, the JVM will throw a {@link WrongMethodTypeException},
154 * either directly (in the case of {@code invokeExact}) or indirectly as if
155 * by a failed call to {@code asType} (in the case of {@code invoke}).
157 * Thus, a method type mismatch which might show up as a linkage error
158 * in a statically typed program can show up as
159 * a dynamic {@code WrongMethodTypeException}
160 * in a program which uses method handles.
162 * Because method types contain "live" {@code Class} objects,
163 * method type matching takes into account both types names and class loaders.
164 * Thus, even if a method handle {@code M} is created in one
165 * class loader {@code L1} and used in another {@code L2},
166 * method handle calls are type-safe, because the caller's symbolic type
167 * descriptor, as resolved in {@code L2},
168 * is matched against the original callee method's symbolic type descriptor,
169 * as resolved in {@code L1}.
170 * The resolution in {@code L1} happens when {@code M} is created
171 * and its type is assigned, while the resolution in {@code L2} happens
172 * when the {@code invokevirtual} instruction is linked.
174 * Apart from the checking of type descriptors,
175 * a method handle's capability to call its underlying method is unrestricted.
176 * If a method handle is formed on a non-public method by a class
177 * that has access to that method, the resulting handle can be used
178 * in any place by any caller who receives a reference to it.
180 * Unlike with the Core Reflection API, where access is checked every time
181 * a reflective method is invoked,
182 * method handle access checking is performed
183 * <a href="MethodHandles.Lookup.html#access">when the method handle is created</a>.
184 * In the case of {@code ldc} (see below), access checking is performed as part of linking
185 * the constant pool entry underlying the constant method handle.
187 * Thus, handles to non-public methods, or to methods in non-public classes,
188 * should generally be kept secret.
189 * They should not be passed to untrusted code unless their use from
190 * the untrusted code would be harmless.
192 * <h1>Method handle creation</h1>
193 * Java code can create a method handle that directly accesses
194 * any method, constructor, or field that is accessible to that code.
195 * This is done via a reflective, capability-based API called
196 * {@link java.lang.invoke.MethodHandles.Lookup MethodHandles.Lookup}
197 * For example, a static method handle can be obtained
198 * from {@link java.lang.invoke.MethodHandles.Lookup#findStatic Lookup.findStatic}.
199 * There are also conversion methods from Core Reflection API objects,
200 * such as {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
202 * Like classes and strings, method handles that correspond to accessible
203 * fields, methods, and constructors can also be represented directly
204 * in a class file's constant pool as constants to be loaded by {@code ldc} bytecodes.
205 * A new type of constant pool entry, {@code CONSTANT_MethodHandle},
206 * refers directly to an associated {@code CONSTANT_Methodref},
207 * {@code CONSTANT_InterfaceMethodref}, or {@code CONSTANT_Fieldref}
208 * constant pool entry.
209 * (For full details on method handle constants,
210 * see sections 4.4.8 and 5.4.3.5 of the Java Virtual Machine Specification.)
212 * Method handles produced by lookups or constant loads from methods or
213 * constructors with the variable arity modifier bit ({@code 0x0080})
214 * have a corresponding variable arity, as if they were defined with
215 * the help of {@link #asVarargsCollector asVarargsCollector}.
217 * A method reference may refer either to a static or non-static method.
218 * In the non-static case, the method handle type includes an explicit
219 * receiver argument, prepended before any other arguments.
220 * In the method handle's type, the initial receiver argument is typed
221 * according to the class under which the method was initially requested.
222 * (E.g., if a non-static method handle is obtained via {@code ldc},
223 * the type of the receiver is the class named in the constant pool entry.)
225 * Method handle constants are subject to the same link-time access checks
226 * their corresponding bytecode instructions, and the {@code ldc} instruction
227 * will throw corresponding linkage errors if the bytecode behaviors would
230 * As a corollary of this, access to protected members is restricted
231 * to receivers only of the accessing class, or one of its subclasses,
232 * and the accessing class must in turn be a subclass (or package sibling)
233 * of the protected member's defining class.
234 * If a method reference refers to a protected non-static method or field
235 * of a class outside the current package, the receiver argument will
236 * be narrowed to the type of the accessing class.
238 * When a method handle to a virtual method is invoked, the method is
239 * always looked up in the receiver (that is, the first argument).
241 * A non-virtual method handle to a specific virtual method implementation
242 * can also be created. These do not perform virtual lookup based on
243 * receiver type. Such a method handle simulates the effect of
244 * an {@code invokespecial} instruction to the same method.
246 * <h1>Usage examples</h1>
247 * Here are some examples of usage:
248 * <blockquote><pre>{@code
249 Object x, y; String s; int i;
250 MethodType mt; MethodHandle mh;
251 MethodHandles.Lookup lookup = MethodHandles.lookup();
252 // mt is (char,char)String
253 mt = MethodType.methodType(String.class, char.class, char.class);
254 mh = lookup.findVirtual(String.class, "replace", mt);
255 s = (String) mh.invokeExact("daddy",'d','n');
256 // invokeExact(Ljava/lang/String;CC)Ljava/lang/String;
257 assertEquals(s, "nanny");
258 // weakly typed invocation (using MHs.invoke)
259 s = (String) mh.invokeWithArguments("sappy", 'p', 'v');
260 assertEquals(s, "savvy");
261 // mt is (Object[])List
262 mt = MethodType.methodType(java.util.List.class, Object[].class);
263 mh = lookup.findStatic(java.util.Arrays.class, "asList", mt);
264 assert(mh.isVarargsCollector());
265 x = mh.invoke("one", "two");
266 // invoke(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/Object;
267 assertEquals(x, java.util.Arrays.asList("one","two"));
268 // mt is (Object,Object,Object)Object
269 mt = MethodType.genericMethodType(3);
271 x = mh.invokeExact((Object)1, (Object)2, (Object)3);
272 // invokeExact(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
273 assertEquals(x, java.util.Arrays.asList(1,2,3));
275 mt = MethodType.methodType(int.class);
276 mh = lookup.findVirtual(java.util.List.class, "size", mt);
277 i = (int) mh.invokeExact(java.util.Arrays.asList(1,2,3));
278 // invokeExact(Ljava/util/List;)I
280 mt = MethodType.methodType(void.class, String.class);
281 mh = lookup.findVirtual(java.io.PrintStream.class, "println", mt);
282 mh.invokeExact(System.out, "Hello, world.");
283 // invokeExact(Ljava/io/PrintStream;Ljava/lang/String;)V
284 * }</pre></blockquote>
285 * Each of the above calls to {@code invokeExact} or plain {@code invoke}
286 * generates a single invokevirtual instruction with
287 * the symbolic type descriptor indicated in the following comment.
288 * In these examples, the helper method {@code assertEquals} is assumed to
289 * be a method which calls {@link java.util.Objects#equals(Object,Object) Objects.equals}
290 * on its arguments, and asserts that the result is true.
292 * <h1>Exceptions</h1>
293 * The methods {@code invokeExact} and {@code invoke} are declared
294 * to throw {@link java.lang.Throwable Throwable},
295 * which is to say that there is no static restriction on what a method handle
296 * can throw. Since the JVM does not distinguish between checked
297 * and unchecked exceptions (other than by their class, of course),
298 * there is no particular effect on bytecode shape from ascribing
299 * checked exceptions to method handle invocations. But in Java source
300 * code, methods which perform method handle calls must either explicitly
301 * throw {@code Throwable}, or else must catch all
302 * throwables locally, rethrowing only those which are legal in the context,
303 * and wrapping ones which are illegal.
305 * <h1><a name="sigpoly"></a>Signature polymorphism</h1>
306 * The unusual compilation and linkage behavior of
307 * {@code invokeExact} and plain {@code invoke}
308 * is referenced by the term <em>signature polymorphism</em>.
309 * As defined in the Java Language Specification,
310 * a signature polymorphic method is one which can operate with
311 * any of a wide range of call signatures and return types.
313 * In source code, a call to a signature polymorphic method will
314 * compile, regardless of the requested symbolic type descriptor.
315 * As usual, the Java compiler emits an {@code invokevirtual}
316 * instruction with the given symbolic type descriptor against the named method.
317 * The unusual part is that the symbolic type descriptor is derived from
318 * the actual argument and return types, not from the method declaration.
320 * When the JVM processes bytecode containing signature polymorphic calls,
321 * it will successfully link any such call, regardless of its symbolic type descriptor.
322 * (In order to retain type safety, the JVM will guard such calls with suitable
323 * dynamic type checks, as described elsewhere.)
325 * Bytecode generators, including the compiler back end, are required to emit
326 * untransformed symbolic type descriptors for these methods.
327 * Tools which determine symbolic linkage are required to accept such
328 * untransformed descriptors, without reporting linkage errors.
330 * <h1>Interoperation between method handles and the Core Reflection API</h1>
331 * Using factory methods in the {@link java.lang.invoke.MethodHandles.Lookup Lookup} API,
332 * any class member represented by a Core Reflection API object
333 * can be converted to a behaviorally equivalent method handle.
334 * For example, a reflective {@link java.lang.reflect.Method Method} can
335 * be converted to a method handle using
336 * {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
337 * The resulting method handles generally provide more direct and efficient
338 * access to the underlying class members.
341 * when the Core Reflection API is used to view the signature polymorphic
342 * methods {@code invokeExact} or plain {@code invoke} in this class,
343 * they appear as ordinary non-polymorphic methods.
344 * Their reflective appearance, as viewed by
345 * {@link java.lang.Class#getDeclaredMethod Class.getDeclaredMethod},
346 * is unaffected by their special status in this API.
347 * For example, {@link java.lang.reflect.Method#getModifiers Method.getModifiers}
348 * will report exactly those modifier bits required for any similarly
349 * declared method, including in this case {@code native} and {@code varargs} bits.
351 * As with any reflected method, these methods (when reflected) may be
352 * invoked via {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.
353 * However, such reflective calls do not result in method handle invocations.
354 * Such a call, if passed the required argument
355 * (a single one, of type {@code Object[]}), will ignore the argument and
356 * will throw an {@code UnsupportedOperationException}.
358 * Since {@code invokevirtual} instructions can natively
359 * invoke method handles under any symbolic type descriptor, this reflective view conflicts
360 * with the normal presentation of these methods via bytecodes.
361 * Thus, these two native methods, when reflectively viewed by
362 * {@code Class.getDeclaredMethod}, may be regarded as placeholders only.
364 * In order to obtain an invoker method for a particular type descriptor,
365 * use {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker},
366 * or {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}.
367 * The {@link java.lang.invoke.MethodHandles.Lookup#findVirtual Lookup.findVirtual}
368 * API is also able to return a method handle
369 * to call {@code invokeExact} or plain {@code invoke},
370 * for any specified type descriptor .
372 * <h1>Interoperation between method handles and Java generics</h1>
373 * A method handle can be obtained on a method, constructor, or field
374 * which is declared with Java generic types.
375 * As with the Core Reflection API, the type of the method handle
376 * will constructed from the erasure of the source-level type.
377 * When a method handle is invoked, the types of its arguments
378 * or the return value cast type may be generic types or type instances.
379 * If this occurs, the compiler will replace those
380 * types by their erasures when it constructs the symbolic type descriptor
381 * for the {@code invokevirtual} instruction.
383 * Method handles do not represent
384 * their function-like types in terms of Java parameterized (generic) types,
385 * because there are three mismatches between function-like types and parameterized
388 * <li>Method types range over all possible arities,
389 * from no arguments to up to the <a href="MethodHandle.html#maxarity">maximum number</a> of allowed arguments.
390 * Generics are not variadic, and so cannot represent this.</li>
391 * <li>Method types can specify arguments of primitive types,
392 * which Java generic types cannot range over.</li>
393 * <li>Higher order functions over method handles (combinators) are
394 * often generic across a wide range of function types, including
395 * those of multiple arities. It is impossible to represent such
396 * genericity with a Java type parameter.</li>
399 * <h1><a name="maxarity"></a>Arity limits</h1>
400 * The JVM imposes on all methods and constructors of any kind an absolute
401 * limit of 255 stacked arguments. This limit can appear more restrictive
404 * <li>A {@code long} or {@code double} argument counts (for purposes of arity limits) as two argument slots.
405 * <li>A non-static method consumes an extra argument for the object on which the method is called.
406 * <li>A constructor consumes an extra argument for the object which is being constructed.
407 * <li>Since a method handle’s {@code invoke} method (or other signature-polymorphic method) is non-virtual,
408 * it consumes an extra argument for the method handle itself, in addition to any non-virtual receiver object.
410 * These limits imply that certain method handles cannot be created, solely because of the JVM limit on stacked arguments.
411 * For example, if a static JVM method accepts exactly 255 arguments, a method handle cannot be created for it.
412 * Attempts to create method handles with impossible method types lead to an {@link IllegalArgumentException}.
413 * In particular, a method handle’s type must not have an arity of the exact maximum 255.
417 * @author John Rose, JSR 292 EG
419 public abstract class MethodHandle {
421 * Internal marker interface which distinguishes (to the Java compiler)
422 * those methods which are <a href="MethodHandle.html#sigpoly">signature polymorphic</a>.
424 @java.lang.annotation.Target({java.lang.annotation.ElementType.METHOD})
425 @java.lang.annotation.Retention(java.lang.annotation.RetentionPolicy.RUNTIME)
426 @interface PolymorphicSignature { }
429 * Reports the type of this method handle.
430 * Every invocation of this method handle via {@code invokeExact} must exactly match this type.
431 * @return the method handle type
433 public MethodType type() {
434 throw new IllegalStateException();
438 * Invokes the method handle, allowing any caller type descriptor, but requiring an exact type match.
439 * The symbolic type descriptor at the call site of {@code invokeExact} must
440 * exactly match this method handle's {@link #type type}.
441 * No conversions are allowed on arguments or return values.
443 * When this method is observed via the Core Reflection API,
444 * it will appear as a single native method, taking an object array and returning an object.
445 * If this native method is invoked directly via
446 * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
447 * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
448 * it will throw an {@code UnsupportedOperationException}.
449 * @param args the signature-polymorphic parameter list, statically represented using varargs
450 * @return the signature-polymorphic result, statically represented using {@code Object}
451 * @throws WrongMethodTypeException if the target's type is not identical with the caller's symbolic type descriptor
452 * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
454 public final native @PolymorphicSignature Object invokeExact(Object... args) throws Throwable;
457 * Invokes the method handle, allowing any caller type descriptor,
458 * and optionally performing conversions on arguments and return values.
460 * If the call site's symbolic type descriptor exactly matches this method handle's {@link #type type},
461 * the call proceeds as if by {@link #invokeExact invokeExact}.
463 * Otherwise, the call proceeds as if this method handle were first
464 * adjusted by calling {@link #asType asType} to adjust this method handle
465 * to the required type, and then the call proceeds as if by
466 * {@link #invokeExact invokeExact} on the adjusted method handle.
468 * There is no guarantee that the {@code asType} call is actually made.
469 * If the JVM can predict the results of making the call, it may perform
470 * adaptations directly on the caller's arguments,
471 * and call the target method handle according to its own exact type.
473 * The resolved type descriptor at the call site of {@code invoke} must
474 * be a valid argument to the receivers {@code asType} method.
475 * In particular, the caller must specify the same argument arity
476 * as the callee's type,
477 * if the callee is not a {@linkplain #asVarargsCollector variable arity collector}.
479 * When this method is observed via the Core Reflection API,
480 * it will appear as a single native method, taking an object array and returning an object.
481 * If this native method is invoked directly via
482 * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
483 * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
484 * it will throw an {@code UnsupportedOperationException}.
485 * @param args the signature-polymorphic parameter list, statically represented using varargs
486 * @return the signature-polymorphic result, statically represented using {@code Object}
487 * @throws WrongMethodTypeException if the target's type cannot be adjusted to the caller's symbolic type descriptor
488 * @throws ClassCastException if the target's type can be adjusted to the caller, but a reference cast fails
489 * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
491 public final native @PolymorphicSignature Object invoke(Object... args) throws Throwable;
494 * Private method for trusted invocation of a method handle respecting simplified signatures.
495 * Type mismatches will not throw {@code WrongMethodTypeException}, but could crash the JVM.
497 * The caller signature is restricted to the following basic types:
498 * Object, int, long, float, double, and void return.
500 * The caller is responsible for maintaining type correctness by ensuring
501 * that the each outgoing argument value is a member of the range of the corresponding
502 * callee argument type.
503 * (The caller should therefore issue appropriate casts and integer narrowing
504 * operations on outgoing argument values.)
505 * The caller can assume that the incoming result value is part of the range
506 * of the callee's return type.
507 * @param args the signature-polymorphic parameter list, statically represented using varargs
508 * @return the signature-polymorphic result, statically represented using {@code Object}
510 /*non-public*/ final native @PolymorphicSignature Object invokeBasic(Object... args) throws Throwable;
513 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeVirtual}.
514 * The caller signature is restricted to basic types as with {@code invokeBasic}.
515 * The trailing (not leading) argument must be a MemberName.
516 * @param args the signature-polymorphic parameter list, statically represented using varargs
517 * @return the signature-polymorphic result, statically represented using {@code Object}
519 /*non-public*/ static native @PolymorphicSignature Object linkToVirtual(Object... args) throws Throwable;
522 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeStatic}.
523 * The caller signature is restricted to basic types as with {@code invokeBasic}.
524 * The trailing (not leading) argument must be a MemberName.
525 * @param args the signature-polymorphic parameter list, statically represented using varargs
526 * @return the signature-polymorphic result, statically represented using {@code Object}
528 /*non-public*/ static native @PolymorphicSignature Object linkToStatic(Object... args) throws Throwable;
531 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeSpecial}.
532 * The caller signature is restricted to basic types as with {@code invokeBasic}.
533 * The trailing (not leading) argument must be a MemberName.
534 * @param args the signature-polymorphic parameter list, statically represented using varargs
535 * @return the signature-polymorphic result, statically represented using {@code Object}
537 /*non-public*/ static native @PolymorphicSignature Object linkToSpecial(Object... args) throws Throwable;
540 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeInterface}.
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}
546 /*non-public*/ static native @PolymorphicSignature Object linkToInterface(Object... args) throws Throwable;
549 * Performs a variable arity invocation, passing the arguments in the given list
550 * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
551 * which mentions only the type {@code Object}, and whose arity is the length
552 * of the argument list.
554 * Specifically, execution proceeds as if by the following steps,
555 * although the methods are not guaranteed to be called if the JVM
556 * can predict their effects.
558 * <li>Determine the length of the argument array as {@code N}.
559 * For a null reference, {@code N=0}. </li>
560 * <li>Determine the general type {@code TN} of {@code N} arguments as
561 * as {@code TN=MethodType.genericMethodType(N)}.</li>
562 * <li>Force the original target method handle {@code MH0} to the
563 * required type, as {@code MH1 = MH0.asType(TN)}. </li>
564 * <li>Spread the array into {@code N} separate arguments {@code A0, ...}. </li>
565 * <li>Invoke the type-adjusted method handle on the unpacked arguments:
566 * MH1.invokeExact(A0, ...). </li>
567 * <li>Take the return value as an {@code Object} reference. </li>
570 * Because of the action of the {@code asType} step, the following argument
571 * conversions are applied as necessary:
573 * <li>reference casting
575 * <li>widening primitive conversions
578 * The result returned by the call is boxed if it is a primitive,
579 * or forced to null if the return type is void.
581 * This call is equivalent to the following code:
582 * <blockquote><pre>{@code
583 * MethodHandle invoker = MethodHandles.spreadInvoker(this.type(), 0);
584 * Object result = invoker.invokeExact(this, arguments);
585 * }</pre></blockquote>
587 * Unlike the signature polymorphic methods {@code invokeExact} and {@code invoke},
588 * {@code invokeWithArguments} can be accessed normally via the Core Reflection API and JNI.
589 * It can therefore be used as a bridge between native or reflective code and method handles.
591 * @param arguments the arguments to pass to the target
592 * @return the result returned by the target
593 * @throws ClassCastException if an argument cannot be converted by reference casting
594 * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
595 * @throws Throwable anything thrown by the target method invocation
596 * @see MethodHandles#spreadInvoker
598 public Object invokeWithArguments(Object... arguments) throws Throwable {
599 throw new IllegalStateException();
603 * Performs a variable arity invocation, passing the arguments in the given array
604 * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
605 * which mentions only the type {@code Object}, and whose arity is the length
606 * of the argument array.
608 * This method is also equivalent to the following code:
609 * <blockquote><pre>{@code
610 * invokeWithArguments(arguments.toArray()
611 * }</pre></blockquote>
613 * @param arguments the arguments to pass to the target
614 * @return the result returned by the target
615 * @throws NullPointerException if {@code arguments} is a null reference
616 * @throws ClassCastException if an argument cannot be converted by reference casting
617 * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
618 * @throws Throwable anything thrown by the target method invocation
620 public Object invokeWithArguments(java.util.List<?> arguments) throws Throwable {
621 return invokeWithArguments(arguments.toArray());
625 * Produces an adapter method handle which adapts the type of the
626 * current method handle to a new type.
627 * The resulting method handle is guaranteed to report a type
628 * which is equal to the desired new type.
630 * If the original type and new type are equal, returns {@code this}.
632 * The new method handle, when invoked, will perform the following
635 * <li>Convert the incoming argument list to match the original
636 * method handle's argument list.
637 * <li>Invoke the original method handle on the converted argument list.
638 * <li>Convert any result returned by the original method handle
639 * to the return type of new method handle.
642 * This method provides the crucial behavioral difference between
643 * {@link #invokeExact invokeExact} and plain, inexact {@link #invoke invoke}.
645 * perform the same steps when the caller's type descriptor exactly m atches
646 * the callee's, but when the types differ, plain {@link #invoke invoke}
647 * also calls {@code asType} (or some internal equivalent) in order
648 * to match up the caller's and callee's types.
650 * If the current method is a variable arity method handle
651 * argument list conversion may involve the conversion and collection
652 * of several arguments into an array, as
653 * {@linkplain #asVarargsCollector described elsewhere}.
654 * In every other case, all conversions are applied <em>pairwise</em>,
655 * which means that each argument or return value is converted to
656 * exactly one argument or return value (or no return value).
657 * The applied conversions are defined by consulting the
658 * the corresponding component types of the old and new
659 * method handle types.
661 * Let <em>T0</em> and <em>T1</em> be corresponding new and old parameter types,
662 * or old and new return types. Specifically, for some valid index {@code i}, let
663 * <em>T0</em>{@code =newType.parameterType(i)} and <em>T1</em>{@code =this.type().parameterType(i)}.
664 * Or else, going the other way for return values, let
665 * <em>T0</em>{@code =this.type().returnType()} and <em>T1</em>{@code =newType.returnType()}.
666 * If the types are the same, the new method handle makes no change
667 * to the corresponding argument or return value (if any).
668 * Otherwise, one of the following conversions is applied
671 * <li>If <em>T0</em> and <em>T1</em> are references, then a cast to <em>T1</em> is applied.
672 * (The types do not need to be related in any particular way.
673 * This is because a dynamic value of null can convert to any reference type.)
674 * <li>If <em>T0</em> and <em>T1</em> are primitives, then a Java method invocation
675 * conversion (JLS 5.3) is applied, if one exists.
676 * (Specifically, <em>T0</em> must convert to <em>T1</em> by a widening primitive conversion.)
677 * <li>If <em>T0</em> is a primitive and <em>T1</em> a reference,
678 * a Java casting conversion (JLS 5.5) is applied if one exists.
679 * (Specifically, the value is boxed from <em>T0</em> to its wrapper class,
680 * which is then widened as needed to <em>T1</em>.)
681 * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
682 * conversion will be applied at runtime, possibly followed
683 * by a Java method invocation conversion (JLS 5.3)
684 * on the primitive value. (These are the primitive widening conversions.)
685 * <em>T0</em> must be a wrapper class or a supertype of one.
686 * (In the case where <em>T0</em> is Object, these are the conversions
687 * allowed by {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.)
688 * The unboxing conversion must have a possibility of success, which means that
689 * if <em>T0</em> is not itself a wrapper class, there must exist at least one
690 * wrapper class <em>TW</em> which is a subtype of <em>T0</em> and whose unboxed
691 * primitive value can be widened to <em>T1</em>.
692 * <li>If the return type <em>T1</em> is marked as void, any returned value is discarded
693 * <li>If the return type <em>T0</em> is void and <em>T1</em> a reference, a null value is introduced.
694 * <li>If the return type <em>T0</em> is void and <em>T1</em> a primitive,
695 * a zero value is introduced.
697 * (<em>Note:</em> Both <em>T0</em> and <em>T1</em> may be regarded as static types,
698 * because neither corresponds specifically to the <em>dynamic type</em> of any
699 * actual argument or return value.)
701 * The method handle conversion cannot be made if any one of the required
702 * pairwise conversions cannot be made.
704 * At runtime, the conversions applied to reference arguments
705 * or return values may require additional runtime checks which can fail.
706 * An unboxing operation may fail because the original reference is null,
707 * causing a {@link java.lang.NullPointerException NullPointerException}.
708 * An unboxing operation or a reference cast may also fail on a reference
709 * to an object of the wrong type,
710 * causing a {@link java.lang.ClassCastException ClassCastException}.
711 * Although an unboxing operation may accept several kinds of wrappers,
712 * if none are available, a {@code ClassCastException} will be thrown.
714 * @param newType the expected type of the new method handle
715 * @return a method handle which delegates to {@code this} after performing
716 * any necessary argument conversions, and arranges for any
717 * necessary return value conversions
718 * @throws NullPointerException if {@code newType} is a null reference
719 * @throws WrongMethodTypeException if the conversion cannot be made
720 * @see MethodHandles#explicitCastArguments
722 public MethodHandle asType(MethodType newType) {
723 throw new IllegalStateException();
727 * Makes an <em>array-spreading</em> method handle, which accepts a trailing array argument
728 * and spreads its elements as positional arguments.
729 * The new method handle adapts, as its <i>target</i>,
730 * the current method handle. The type of the adapter will be
731 * the same as the type of the target, except that the final
732 * {@code arrayLength} parameters of the target's type are replaced
733 * by a single array parameter of type {@code arrayType}.
735 * If the array element type differs from any of the corresponding
736 * argument types on the original target,
737 * the original target is adapted to take the array elements directly,
738 * as if by a call to {@link #asType asType}.
740 * When called, the adapter replaces a trailing array argument
741 * by the array's elements, each as its own argument to the target.
742 * (The order of the arguments is preserved.)
743 * They are converted pairwise by casting and/or unboxing
744 * to the types of the trailing parameters of the target.
745 * Finally the target is called.
746 * What the target eventually returns is returned unchanged by the adapter.
748 * Before calling the target, the adapter verifies that the array
749 * contains exactly enough elements to provide a correct argument count
750 * to the target method handle.
751 * (The array may also be null when zero elements are required.)
753 * If, when the adapter is called, the supplied array argument does
754 * not have the correct number of elements, the adapter will throw
755 * an {@link IllegalArgumentException} instead of invoking the target.
757 * Here are some simple examples of array-spreading method handles:
758 * <blockquote><pre>{@code
759 MethodHandle equals = publicLookup()
760 .findVirtual(String.class, "equals", methodType(boolean.class, Object.class));
761 assert( (boolean) equals.invokeExact("me", (Object)"me"));
762 assert(!(boolean) equals.invokeExact("me", (Object)"thee"));
763 // spread both arguments from a 2-array:
764 MethodHandle eq2 = equals.asSpreader(Object[].class, 2);
765 assert( (boolean) eq2.invokeExact(new Object[]{ "me", "me" }));
766 assert(!(boolean) eq2.invokeExact(new Object[]{ "me", "thee" }));
767 // try to spread from anything but a 2-array:
768 for (int n = 0; n <= 10; n++) {
769 Object[] badArityArgs = (n == 2 ? null : new Object[n]);
770 try { assert((boolean) eq2.invokeExact(badArityArgs) && false); }
771 catch (IllegalArgumentException ex) { } // OK
773 // spread both arguments from a String array:
774 MethodHandle eq2s = equals.asSpreader(String[].class, 2);
775 assert( (boolean) eq2s.invokeExact(new String[]{ "me", "me" }));
776 assert(!(boolean) eq2s.invokeExact(new String[]{ "me", "thee" }));
777 // spread second arguments from a 1-array:
778 MethodHandle eq1 = equals.asSpreader(Object[].class, 1);
779 assert( (boolean) eq1.invokeExact("me", new Object[]{ "me" }));
780 assert(!(boolean) eq1.invokeExact("me", new Object[]{ "thee" }));
781 // spread no arguments from a 0-array or null:
782 MethodHandle eq0 = equals.asSpreader(Object[].class, 0);
783 assert( (boolean) eq0.invokeExact("me", (Object)"me", new Object[0]));
784 assert(!(boolean) eq0.invokeExact("me", (Object)"thee", (Object[])null));
785 // asSpreader and asCollector are approximate inverses:
786 for (int n = 0; n <= 2; n++) {
787 for (Class<?> a : new Class<?>[]{Object[].class, String[].class, CharSequence[].class}) {
788 MethodHandle equals2 = equals.asSpreader(a, n).asCollector(a, n);
789 assert( (boolean) equals2.invokeWithArguments("me", "me"));
790 assert(!(boolean) equals2.invokeWithArguments("me", "thee"));
793 MethodHandle caToString = publicLookup()
794 .findStatic(Arrays.class, "toString", methodType(String.class, char[].class));
795 assertEquals("[A, B, C]", (String) caToString.invokeExact("ABC".toCharArray()));
796 MethodHandle caString3 = caToString.asCollector(char[].class, 3);
797 assertEquals("[A, B, C]", (String) caString3.invokeExact('A', 'B', 'C'));
798 MethodHandle caToString2 = caString3.asSpreader(char[].class, 2);
799 assertEquals("[A, B, C]", (String) caToString2.invokeExact('A', "BC".toCharArray()));
800 * }</pre></blockquote>
801 * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments
802 * @param arrayLength the number of arguments to spread from an incoming array argument
803 * @return a new method handle which spreads its final array argument,
804 * before calling the original method handle
805 * @throws NullPointerException if {@code arrayType} is a null reference
806 * @throws IllegalArgumentException if {@code arrayType} is not an array type,
807 * or if target does not have at least
808 * {@code arrayLength} parameter types,
809 * or if {@code arrayLength} is negative,
810 * or if the resulting method handle's type would have
811 * <a href="MethodHandle.html#maxarity">too many parameters</a>
812 * @throws WrongMethodTypeException if the implied {@code asType} call fails
815 public MethodHandle asSpreader(Class<?> arrayType, int arrayLength) {
816 throw new IllegalStateException();
820 * Makes an <em>array-collecting</em> method handle, which accepts a given number of trailing
821 * positional arguments and collects them into an array argument.
822 * The new method handle adapts, as its <i>target</i>,
823 * the current method handle. The type of the adapter will be
824 * the same as the type of the target, except that a single trailing
825 * parameter (usually of type {@code arrayType}) is replaced by
826 * {@code arrayLength} parameters whose type is element type of {@code arrayType}.
828 * If the array type differs from the final argument type on the original target,
829 * the original target is adapted to take the array type directly,
830 * as if by a call to {@link #asType asType}.
832 * When called, the adapter replaces its trailing {@code arrayLength}
833 * arguments by a single new array of type {@code arrayType}, whose elements
834 * comprise (in order) the replaced arguments.
835 * Finally the target is called.
836 * What the target eventually returns is returned unchanged by the adapter.
838 * (The array may also be a shared constant when {@code arrayLength} is zero.)
840 * (<em>Note:</em> The {@code arrayType} is often identical to the last
841 * parameter type of the original target.
842 * It is an explicit argument for symmetry with {@code asSpreader}, and also
843 * to allow the target to use a simple {@code Object} as its last parameter type.)
845 * In order to create a collecting adapter which is not restricted to a particular
846 * number of collected arguments, use {@link #asVarargsCollector asVarargsCollector} instead.
848 * Here are some examples of array-collecting method handles:
849 * <blockquote><pre>{@code
850 MethodHandle deepToString = publicLookup()
851 .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
852 assertEquals("[won]", (String) deepToString.invokeExact(new Object[]{"won"}));
853 MethodHandle ts1 = deepToString.asCollector(Object[].class, 1);
854 assertEquals(methodType(String.class, Object.class), ts1.type());
855 //assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"})); //FAIL
856 assertEquals("[[won]]", (String) ts1.invokeExact((Object) new Object[]{"won"}));
857 // arrayType can be a subtype of Object[]
858 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
859 assertEquals(methodType(String.class, String.class, String.class), ts2.type());
860 assertEquals("[two, too]", (String) ts2.invokeExact("two", "too"));
861 MethodHandle ts0 = deepToString.asCollector(Object[].class, 0);
862 assertEquals("[]", (String) ts0.invokeExact());
863 // collectors can be nested, Lisp-style
864 MethodHandle ts22 = deepToString.asCollector(Object[].class, 3).asCollector(String[].class, 2);
865 assertEquals("[A, B, [C, D]]", ((String) ts22.invokeExact((Object)'A', (Object)"B", "C", "D")));
866 // arrayType can be any primitive array type
867 MethodHandle bytesToString = publicLookup()
868 .findStatic(Arrays.class, "toString", methodType(String.class, byte[].class))
869 .asCollector(byte[].class, 3);
870 assertEquals("[1, 2, 3]", (String) bytesToString.invokeExact((byte)1, (byte)2, (byte)3));
871 MethodHandle longsToString = publicLookup()
872 .findStatic(Arrays.class, "toString", methodType(String.class, long[].class))
873 .asCollector(long[].class, 1);
874 assertEquals("[123]", (String) longsToString.invokeExact((long)123));
875 * }</pre></blockquote>
876 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
877 * @param arrayLength the number of arguments to collect into a new array argument
878 * @return a new method handle which collects some trailing argument
879 * into an array, before calling the original method handle
880 * @throws NullPointerException if {@code arrayType} is a null reference
881 * @throws IllegalArgumentException if {@code arrayType} is not an array type
882 * or {@code arrayType} is not assignable to this method handle's trailing parameter type,
883 * or {@code arrayLength} is not a legal array size,
884 * or the resulting method handle's type would have
885 * <a href="MethodHandle.html#maxarity">too many parameters</a>
886 * @throws WrongMethodTypeException if the implied {@code asType} call fails
888 * @see #asVarargsCollector
890 public MethodHandle asCollector(Class<?> arrayType, int arrayLength) {
891 throw new IllegalStateException();
895 * Makes a <em>variable arity</em> adapter which is able to accept
896 * any number of trailing positional arguments and collect them
897 * into an array argument.
899 * The type and behavior of the adapter will be the same as
900 * the type and behavior of the target, except that certain
901 * {@code invoke} and {@code asType} requests can lead to
902 * trailing positional arguments being collected into target's
903 * trailing parameter.
904 * Also, the last parameter type of the adapter will be
905 * {@code arrayType}, even if the target has a different
906 * last parameter type.
908 * This transformation may return {@code this} if the method handle is
909 * already of variable arity and its trailing parameter type
910 * is identical to {@code arrayType}.
912 * When called with {@link #invokeExact invokeExact}, the adapter invokes
913 * the target with no argument changes.
914 * (<em>Note:</em> This behavior is different from a
915 * {@linkplain #asCollector fixed arity collector},
916 * since it accepts a whole array of indeterminate length,
917 * rather than a fixed number of arguments.)
919 * When called with plain, inexact {@link #invoke invoke}, if the caller
920 * type is the same as the adapter, the adapter invokes the target as with
921 * {@code invokeExact}.
922 * (This is the normal behavior for {@code invoke} when types match.)
924 * Otherwise, if the caller and adapter arity are the same, and the
925 * trailing parameter type of the caller is a reference type identical to
926 * or assignable to the trailing parameter type of the adapter,
927 * the arguments and return values are converted pairwise,
928 * as if by {@link #asType asType} on a fixed arity
931 * Otherwise, the arities differ, or the adapter's trailing parameter
932 * type is not assignable from the corresponding caller type.
933 * In this case, the adapter replaces all trailing arguments from
934 * the original trailing argument position onward, by
935 * a new array of type {@code arrayType}, whose elements
936 * comprise (in order) the replaced arguments.
938 * The caller type must provides as least enough arguments,
939 * and of the correct type, to satisfy the target's requirement for
940 * positional arguments before the trailing array argument.
941 * Thus, the caller must supply, at a minimum, {@code N-1} arguments,
942 * where {@code N} is the arity of the target.
943 * Also, there must exist conversions from the incoming arguments
944 * to the target's arguments.
945 * As with other uses of plain {@code invoke}, if these basic
946 * requirements are not fulfilled, a {@code WrongMethodTypeException}
949 * In all cases, what the target eventually returns is returned unchanged by the adapter.
951 * In the final case, it is exactly as if the target method handle were
952 * temporarily adapted with a {@linkplain #asCollector fixed arity collector}
953 * to the arity required by the caller type.
954 * (As with {@code asCollector}, if the array length is zero,
955 * a shared constant may be used instead of a new array.
956 * If the implied call to {@code asCollector} would throw
957 * an {@code IllegalArgumentException} or {@code WrongMethodTypeException},
958 * the call to the variable arity adapter must throw
959 * {@code WrongMethodTypeException}.)
961 * The behavior of {@link #asType asType} is also specialized for
962 * variable arity adapters, to maintain the invariant that
963 * plain, inexact {@code invoke} is always equivalent to an {@code asType}
964 * call to adjust the target type, followed by {@code invokeExact}.
965 * Therefore, a variable arity adapter responds
966 * to an {@code asType} request by building a fixed arity collector,
967 * if and only if the adapter and requested type differ either
968 * in arity or trailing argument type.
969 * The resulting fixed arity collector has its type further adjusted
970 * (if necessary) to the requested type by pairwise conversion,
971 * as if by another application of {@code asType}.
973 * When a method handle is obtained by executing an {@code ldc} instruction
974 * of a {@code CONSTANT_MethodHandle} constant, and the target method is marked
975 * as a variable arity method (with the modifier bit {@code 0x0080}),
976 * the method handle will accept multiple arities, as if the method handle
977 * constant were created by means of a call to {@code asVarargsCollector}.
979 * In order to create a collecting adapter which collects a predetermined
980 * number of arguments, and whose type reflects this predetermined number,
981 * use {@link #asCollector asCollector} instead.
983 * No method handle transformations produce new method handles with
984 * variable arity, unless they are documented as doing so.
985 * Therefore, besides {@code asVarargsCollector},
986 * all methods in {@code MethodHandle} and {@code MethodHandles}
987 * will return a method handle with fixed arity,
988 * except in the cases where they are specified to return their original
989 * operand (e.g., {@code asType} of the method handle's own type).
991 * Calling {@code asVarargsCollector} on a method handle which is already
992 * of variable arity will produce a method handle with the same type and behavior.
993 * It may (or may not) return the original variable arity method handle.
995 * Here is an example, of a list-making variable arity method handle:
996 * <blockquote><pre>{@code
997 MethodHandle deepToString = publicLookup()
998 .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
999 MethodHandle ts1 = deepToString.asVarargsCollector(Object[].class);
1000 assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"}));
1001 assertEquals("[won]", (String) ts1.invoke( new Object[]{"won"}));
1002 assertEquals("[won]", (String) ts1.invoke( "won" ));
1003 assertEquals("[[won]]", (String) ts1.invoke((Object) new Object[]{"won"}));
1004 // findStatic of Arrays.asList(...) produces a variable arity method handle:
1005 MethodHandle asList = publicLookup()
1006 .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class));
1007 assertEquals(methodType(List.class, Object[].class), asList.type());
1008 assert(asList.isVarargsCollector());
1009 assertEquals("[]", asList.invoke().toString());
1010 assertEquals("[1]", asList.invoke(1).toString());
1011 assertEquals("[two, too]", asList.invoke("two", "too").toString());
1012 String[] argv = { "three", "thee", "tee" };
1013 assertEquals("[three, thee, tee]", asList.invoke(argv).toString());
1014 assertEquals("[three, thee, tee]", asList.invoke((Object[])argv).toString());
1015 List ls = (List) asList.invoke((Object)argv);
1016 assertEquals(1, ls.size());
1017 assertEquals("[three, thee, tee]", Arrays.toString((Object[])ls.get(0)));
1018 * }</pre></blockquote>
1019 * <p style="font-size:smaller;">
1020 * <em>Discussion:</em>
1021 * These rules are designed as a dynamically-typed variation
1022 * of the Java rules for variable arity methods.
1023 * In both cases, callers to a variable arity method or method handle
1024 * can either pass zero or more positional arguments, or else pass
1025 * pre-collected arrays of any length. Users should be aware of the
1026 * special role of the final argument, and of the effect of a
1027 * type match on that final argument, which determines whether
1028 * or not a single trailing argument is interpreted as a whole
1029 * array or a single element of an array to be collected.
1030 * Note that the dynamic type of the trailing argument has no
1031 * effect on this decision, only a comparison between the symbolic
1032 * type descriptor of the call site and the type descriptor of the method handle.)
1034 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
1035 * @return a new method handle which can collect any number of trailing arguments
1036 * into an array, before calling the original method handle
1037 * @throws NullPointerException if {@code arrayType} is a null reference
1038 * @throws IllegalArgumentException if {@code arrayType} is not an array type
1039 * or {@code arrayType} is not assignable to this method handle's trailing parameter type
1041 * @see #isVarargsCollector
1042 * @see #asFixedArity
1044 public MethodHandle asVarargsCollector(Class<?> arrayType) {
1045 throw new IllegalStateException();
1049 * Determines if this method handle
1050 * supports {@linkplain #asVarargsCollector variable arity} calls.
1051 * Such method handles arise from the following sources:
1053 * <li>a call to {@linkplain #asVarargsCollector asVarargsCollector}
1054 * <li>a call to a {@linkplain java.lang.invoke.MethodHandles.Lookup lookup method}
1055 * which resolves to a variable arity Java method or constructor
1056 * <li>an {@code ldc} instruction of a {@code CONSTANT_MethodHandle}
1057 * which resolves to a variable arity Java method or constructor
1059 * @return true if this method handle accepts more than one arity of plain, inexact {@code invoke} calls
1060 * @see #asVarargsCollector
1061 * @see #asFixedArity
1063 public boolean isVarargsCollector() {
1068 * Makes a <em>fixed arity</em> method handle which is otherwise
1069 * equivalent to the current method handle.
1071 * If the current method handle is not of
1072 * {@linkplain #asVarargsCollector variable arity},
1073 * the current method handle is returned.
1074 * This is true even if the current method handle
1075 * could not be a valid input to {@code asVarargsCollector}.
1077 * Otherwise, the resulting fixed-arity method handle has the same
1078 * type and behavior of the current method handle,
1079 * except that {@link #isVarargsCollector isVarargsCollector}
1081 * The fixed-arity method handle may (or may not) be the
1082 * a previous argument to {@code asVarargsCollector}.
1084 * Here is an example, of a list-making variable arity method handle:
1085 * <blockquote><pre>{@code
1086 MethodHandle asListVar = publicLookup()
1087 .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class))
1088 .asVarargsCollector(Object[].class);
1089 MethodHandle asListFix = asListVar.asFixedArity();
1090 assertEquals("[1]", asListVar.invoke(1).toString());
1091 Exception caught = null;
1092 try { asListFix.invoke((Object)1); }
1093 catch (Exception ex) { caught = ex; }
1094 assert(caught instanceof ClassCastException);
1095 assertEquals("[two, too]", asListVar.invoke("two", "too").toString());
1096 try { asListFix.invoke("two", "too"); }
1097 catch (Exception ex) { caught = ex; }
1098 assert(caught instanceof WrongMethodTypeException);
1099 Object[] argv = { "three", "thee", "tee" };
1100 assertEquals("[three, thee, tee]", asListVar.invoke(argv).toString());
1101 assertEquals("[three, thee, tee]", asListFix.invoke(argv).toString());
1102 assertEquals(1, ((List) asListVar.invoke((Object)argv)).size());
1103 assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString());
1104 * }</pre></blockquote>
1106 * @return a new method handle which accepts only a fixed number of arguments
1107 * @see #asVarargsCollector
1108 * @see #isVarargsCollector
1110 public MethodHandle asFixedArity() {
1111 assert(!isVarargsCollector());
1116 * Binds a value {@code x} to the first argument of a method handle, without invoking it.
1117 * The new method handle adapts, as its <i>target</i>,
1118 * the current method handle by binding it to the given argument.
1119 * The type of the bound handle will be
1120 * the same as the type of the target, except that a single leading
1121 * reference parameter will be omitted.
1123 * When called, the bound handle inserts the given value {@code x}
1124 * as a new leading argument to the target. The other arguments are
1125 * also passed unchanged.
1126 * What the target eventually returns is returned unchanged by the bound handle.
1128 * The reference {@code x} must be convertible to the first parameter
1129 * type of the target.
1131 * (<em>Note:</em> Because method handles are immutable, the target method handle
1132 * retains its original type and behavior.)
1133 * @param x the value to bind to the first argument of the target
1134 * @return a new method handle which prepends the given value to the incoming
1135 * argument list, before calling the original method handle
1136 * @throws IllegalArgumentException if the target does not have a
1137 * leading parameter type that is a reference type
1138 * @throws ClassCastException if {@code x} cannot be converted
1139 * to the leading parameter type of the target
1140 * @see MethodHandles#insertArguments
1142 public MethodHandle bindTo(Object x) {
1143 throw new IllegalStateException();
1147 * Returns a string representation of the method handle,
1148 * starting with the string {@code "MethodHandle"} and
1149 * ending with the string representation of the method handle's type.
1150 * In other words, this method returns a string equal to the value of:
1151 * <blockquote><pre>{@code
1152 * "MethodHandle" + type().toString()
1153 * }</pre></blockquote>
1155 * (<em>Note:</em> Future releases of this API may add further information
1156 * to the string representation.
1157 * Therefore, the present syntax should not be parsed by applications.)
1159 * @return a string representation of the method handle
1162 public String toString() {
1163 return standardString();
1165 String standardString() {
1166 throw new IllegalStateException();