java.lang.invoke

## Class MethodHandles

- java.lang.Object
- - java.lang.invoke.MethodHandles

-     public class MethodHandles
      extends Object

  This class consists exclusively of static methods that operate on or return method handles. They fall into several categories:
  - Lookup methods which help create method handles for methods and fields.
  - Combinator methods, which combine or transform pre-existing method handles into new ones.
  - Other factory methods to create method handles that emulate other common JVM operations or control flow patterns.
  - Wrapper methods which can convert between method handles and interface types.

- - ### Nested Class Summary

    Nested Classes
    Modifier and Type | Class and Description
    --- | ---
    `static class` | `MethodHandles.Lookup` A lookup object is a factory for creating method handles, when the creation requires access checking.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `static MethodHandle` | `arrayElementGetter(Class<?> arrayClass)` Produces a method handle giving read access to elements of an array.
    `static MethodHandle` | `arrayElementSetter(Class<?> arrayClass)` Produces a method handle giving write access to elements of an array.
    `static MethodHandle` | `catchException(MethodHandle target, Class<? extends Throwable> exType, MethodHandle handler)` Makes a method handle which adapts a target method handle, by running it inside an exception handler.
    `static MethodHandle` | `constant(Class<?> type, Object value)` Produces a method handle of the requested return type which returns the given constant value every time it is invoked.
    `static MethodHandle` | `dropArguments(MethodHandle target, int pos, Class<?>... valueTypes)` Produces a method handle which will discard some dummy arguments before calling some other specified target method handle.
    `static MethodHandle` | `dropArguments(MethodHandle target, int pos, List<Class<?>> valueTypes)` Produces a method handle which will discard some dummy arguments before calling some other specified target method handle.
    `static MethodHandle` | `exactInvoker(MethodType type)` Produces a special invoker method handle which can be used to invoke any method handle of the given type, as if by `invokeExact`.
    `static MethodHandle` | `explicitCastArguments(MethodHandle target, MethodType newType)` Produces a method handle which adapts the type of the given method handle to a new type by pairwise argument and return type conversion.
    `static MethodHandle` | `filterArguments(MethodHandle target, int pos, MethodHandle... filters)` Adapts a target method handle by pre-processing one or more of its arguments, each with its own unary filter function, and then calling the target with each pre-processed argument replaced by the result of its corresponding filter function.
    `static MethodHandle` | `filterReturnValue(MethodHandle target, MethodHandle filter)` Adapts a target method handle by post-processing its return value (if any) with a filter (another method handle).
    `static MethodHandle` | `foldArguments(MethodHandle target, MethodHandle combiner)` Adapts a target method handle by pre-processing some of its arguments, and then calling the target with the result of the pre-processing, inserted into the original sequence of arguments.
    `static MethodHandle` | `guardWithTest(MethodHandle test, MethodHandle target, MethodHandle fallback)` Makes a method handle which adapts a target method handle, by guarding it with a test, a boolean-valued method handle.
    `static MethodHandle` | `identity(Class<?> type)` Produces a method handle which returns its sole argument when invoked.
    `static MethodHandle` | `insertArguments(MethodHandle target, int pos, Object... values)` Provides a target method handle with one or more bound arguments in advance of the method handle's invocation.
    `static MethodHandle` | `invoker(MethodType type)` Produces a special invoker method handle which can be used to invoke any method handle compatible with the given type, as if by `invoke`.
    `static MethodHandles.Lookup` | `lookup()` Returns a `lookup object` on the caller, which has the capability to access any method handle that the caller has access to, including direct method handles to private fields and methods.
    `static MethodHandle` | `permuteArguments(MethodHandle target, MethodType newType, int... reorder)` Produces a method handle which adapts the calling sequence of the given method handle to a new type, by reordering the arguments.
    `static MethodHandles.Lookup` | `publicLookup()` Returns a `lookup object` which is trusted minimally.
    `static MethodHandle` | `spreadInvoker(MethodType type, int leadingArgCount)` Produces a method handle which will invoke any method handle of the given `type`, with a given number of trailing arguments replaced by a single trailing `Object[]` array.
    `static MethodHandle` | `throwException(Class<?> returnType, Class<? extends Throwable> exType)` Produces a method handle which will throw exceptions of the given `exType`.

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Method Detail

    - #### lookup

          public static MethodHandles.Lookup lookup()

      Returns a `lookup object` on the caller, which has the capability to access any method handle that the caller has access to, including direct method handles to private fields and methods. This lookup object is a capability which may be delegated to trusted agents. Do not store it in place where untrusted code can access it.

    - #### publicLookup

          public static MethodHandles.Lookup publicLookup()

      Returns a `lookup object` which is trusted minimally. It can only be used to create method handles to publicly accessible fields and methods.

      As a matter of pure convention, the lookup class of this lookup object will be `Object`.

      The lookup class can be changed to any other class `C` using an expression of the form `publicLookup().in(C.class)`. Since all classes have equal access to public names, such a change would confer no new access rights.

    - #### arrayElementGetter

          public static MethodHandle arrayElementGetter(Class<?> arrayClass)
                                                 throws IllegalArgumentException

      Produces a method handle giving read access to elements of an array. The type of the method handle will have a return type of the array's element type. Its first argument will be the array type, and the second will be `int`.
      Parameters:
        `arrayClass` - an array type
      Returns:
        a method handle which can load values from the given array type
      Throws:
        `NullPointerException` - if the argument is null
        `IllegalArgumentException` - if arrayClass is not an array type

    - #### arrayElementSetter

          public static MethodHandle arrayElementSetter(Class<?> arrayClass)
                                                 throws IllegalArgumentException

      Produces a method handle giving write access to elements of an array. The type of the method handle will have a void return type. Its last argument will be the array's element type. The first and second arguments will be the array type and int.
      Returns:
        a method handle which can store values into the array type
      Throws:
        `NullPointerException` - if the argument is null
        `IllegalArgumentException` - if arrayClass is not an array type

    - #### spreadInvoker

          public static MethodHandle spreadInvoker(MethodType type,
                                   int leadingArgCount)

      Produces a method handle which will invoke any method handle of the given `type`, with a given number of trailing arguments replaced by a single trailing `Object[]` array. The resulting invoker will be a method handle with the following arguments:
      - a single `MethodHandle` target
      - zero or more leading values (counted by `leadingArgCount`)
      - an `Object[]` array containing trailing arguments

      The invoker will invoke its target like a call to `invoke` with the indicated `type`. That is, if the target is exactly of the given `type`, it will behave like `invokeExact`; otherwise it behave as if `asType` is used to convert the target to the required `type`.

      The type of the returned invoker will not be the given `type`, but rather will have all parameters except the first `leadingArgCount` replaced by a single array of type `Object[]`, which will be the final parameter.

      Before invoking its target, the invoker will spread the final array, apply reference casts as necessary, and unbox and widen primitive arguments. If, when the invoker is called, the supplied array argument does not have the correct number of elements, the invoker will throw an `IllegalArgumentException` instead of invoking the target.

      This method is equivalent to the following code (though it may be more efficient):

      >     MethodHandle invoker = MethodHandles.invoker(type);
      >     int spreadArgCount = type.parameterCount() - leadingArgCount;
      >     invoker = invoker.asSpreader(Object[].class, spreadArgCount);
      >     return invoker;

      This method throws no reflective or security exceptions.

      Parameters:
        `type` - the desired target type
        `leadingArgCount` - number of fixed arguments, to be passed unchanged to the target
      Returns:
        a method handle suitable for invoking any method handle of the given type
      Throws:
        `NullPointerException` - if `type` is null
        `IllegalArgumentException` - if `leadingArgCount` is not in the range from 0 to `type.parameterCount()` inclusive

    - #### exactInvoker

          public static MethodHandle exactInvoker(MethodType type)

      Produces a special invoker method handle which can be used to invoke any method handle of the given type, as if by `invokeExact`. The resulting invoker will have a type which is exactly equal to the desired type, except that it will accept an additional leading argument of type `MethodHandle`.

      This method is equivalent to the following code (though it may be more efficient):

      >     publicLookup().findVirtual(MethodHandle.class, "invokeExact", type)

      Discussion: Invoker method handles can be useful when working with variable method handles of unknown types. For example, to emulate an `invokeExact` call to a variable method handle `M`, extract its type `T`, look up the invoker method `X` for `T`, and call the invoker method, as `X.invoke(T, A...)`. (It would not work to call `X.invokeExact`, since the type `T` is unknown.) If spreading, collecting, or other argument transformations are required, they can be applied once to the invoker `X` and reused on many `M` method handle values, as long as they are compatible with the type of `X`.

      (Note: The invoker method is not available via the Core Reflection API. An attempt to call java.lang.reflect.Method.invoke on the declared `invokeExact` or `invoke` method will raise an `UnsupportedOperationException`.)

      This method throws no reflective or security exceptions.

      Parameters:
        `type` - the desired target type
      Returns:
        a method handle suitable for invoking any method handle of the given type

    - #### invoker

          public static MethodHandle invoker(MethodType type)

      Produces a special invoker method handle which can be used to invoke any method handle compatible with the given type, as if by `invoke`. The resulting invoker will have a type which is exactly equal to the desired type, except that it will accept an additional leading argument of type `MethodHandle`.

      Before invoking its target, if the target differs from the expected type, the invoker will apply reference casts as necessary and box, unbox, or widen primitive values, as if by `asType`. Similarly, the return value will be converted as necessary. If the target is a variable arity method handle, the required arity conversion will be made, again as if by `asType`.

      A general method type, mentions only `Object` arguments and return values. An invoker for such a type is capable of calling any method handle of the same arity as the general type.

      This method is equivalent to the following code (though it may be more efficient):

      >     publicLookup().findVirtual(MethodHandle.class, "invoke", type)

      This method throws no reflective or security exceptions.

      Parameters:
        `type` - the desired target type
      Returns:
        a method handle suitable for invoking any method handle convertible to the given type

    - #### explicitCastArguments

          public static MethodHandle explicitCastArguments(MethodHandle target,
                                           MethodType newType)

      Produces a method handle which adapts the type of the given method handle to a new type by pairwise argument and return type conversion. The original type and new type must have the same number of arguments. The resulting method handle is guaranteed to report a type which is equal to the desired new type.

      If the original type and new type are equal, returns target.

      The same conversions are allowed as for `MethodHandle.asType`, and some additional conversions are also applied if those conversions fail. Given types T0, T1, one of the following conversions is applied if possible, before or instead of any conversions done by `asType`:

      - If T0 and T1 are references, and T1 is an interface type, then the value of type T0 is passed as a T1 without a cast. (This treatment of interfaces follows the usage of the bytecode verifier.)
      - If T0 is boolean and T1 is another primitive, the boolean is converted to a byte value, 1 for true, 0 for false. (This treatment follows the usage of the bytecode verifier.)
      - If T1 is boolean and T0 is another primitive, T0 is converted to byte via Java casting conversion (JLS 5.5), and the low order bit of the result is tested, as if by `(x & 1) != 0`.
      - If T0 and T1 are primitives other than boolean, then a Java casting conversion (JLS 5.5) is applied. (Specifically, T0 will convert to T1 by widening and/or narrowing.)
      - If T0 is a reference and T1 a primitive, an unboxing conversion will be applied at runtime, possibly followed by a Java casting conversion (JLS 5.5) on the primitive value, possibly followed by a conversion from byte to boolean by testing the low-order bit.
      - If T0 is a reference and T1 a primitive, and if the reference is null at runtime, a zero value is introduced.
      Parameters:
        `target` - the method handle to invoke after arguments are retyped
        `newType` - the expected type of the new method handle
      Returns:
        a method handle which delegates to the target after performing any necessary argument conversions, and arranges for any necessary return value conversions
      Throws:
        `NullPointerException` - if either argument is null
        `WrongMethodTypeException` - if the conversion cannot be made
      See Also:
        `MethodHandle.asType(java.lang.invoke.MethodType)`

    - #### permuteArguments

          public static MethodHandle permuteArguments(MethodHandle target,
                                      MethodType newType,
                                      int... reorder)

      Produces a method handle which adapts the calling sequence of the given method handle to a new type, by reordering the arguments. The resulting method handle is guaranteed to report a type which is equal to the desired new type.

      The given array controls the reordering. Call `#I` the number of incoming parameters (the value `newType.parameterCount()`, and call `#O` the number of outgoing parameters (the value `target.type().parameterCount()`). Then the length of the reordering array must be `#O`, and each element must be a non-negative number less than `#I`. For every `N` less than `#O`, the `N`-th outgoing argument will be taken from the `I`-th incoming argument, where `I` is `reorder[N]`.

      No argument or return value conversions are applied. The type of each incoming argument, as determined by `newType`, must be identical to the type of the corresponding outgoing parameter or parameters in the target method handle. The return type of `newType` must be identical to the return type of the original target.

      The reordering array need not specify an actual permutation. An incoming argument will be duplicated if its index appears more than once in the array, and an incoming argument will be dropped if its index does not appear in the array. As in the case of `dropArguments`, incoming arguments which are not mentioned in the reordering array are may be any type, as determined only by `newType`.

      >     import static java.lang.invoke.MethodHandles.*;
      >     import static java.lang.invoke.MethodType.*;
      >     ...
      >     MethodType intfn1 = methodType(int.class, int.class);
      >     MethodType intfn2 = methodType(int.class, int.class, int.class);
      >     MethodHandle sub = ... {int x, int y => x-y} ...;
      >     assert(sub.type().equals(intfn2));
      >     MethodHandle sub1 = permuteArguments(sub, intfn2, 0, 1);
      >     MethodHandle rsub = permuteArguments(sub, intfn2, 1, 0);
      >     assert((int)rsub.invokeExact(1, 100) == 99);
      >     MethodHandle add = ... {int x, int y => x+y} ...;
      >     assert(add.type().equals(intfn2));
      >     MethodHandle twice = permuteArguments(add, intfn1, 0, 0);
      >     assert(twice.type().equals(intfn1));
      >     assert((int)twice.invokeExact(21) == 42);

      Parameters:
        `target` - the method handle to invoke after arguments are reordered
        `newType` - the expected type of the new method handle
        `reorder` - an index array which controls the reordering
      Returns:
        a method handle which delegates to the target after it drops unused arguments and moves and/or duplicates the other arguments
      Throws:
        `NullPointerException` - if any argument is null
        `IllegalArgumentException` - if the index array length is not equal to the arity of the target, or if any index array element not a valid index for a parameter of `newType`, or if two corresponding parameter types in `target.type()` and `newType` are not identical,

    - #### constant

          public static MethodHandle constant(Class<?> type,
                              Object value)

      Produces a method handle of the requested return type which returns the given constant value every time it is invoked.

      Before the method handle is returned, the passed-in value is converted to the requested type. If the requested type is primitive, widening primitive conversions are attempted, else reference conversions are attempted.

      The returned method handle is equivalent to `identity(type).bindTo(value)`.

      Parameters:
        `type` - the return type of the desired method handle
        `value` - the value to return
      Returns:
        a method handle of the given return type and no arguments, which always returns the given value
      Throws:
        `NullPointerException` - if the `type` argument is null
        `ClassCastException` - if the value cannot be converted to the required return type
        `IllegalArgumentException` - if the given type is `void.class`

    - #### identity

          public static MethodHandle identity(Class<?> type)

      Produces a method handle which returns its sole argument when invoked.
      Parameters:
        `type` - the type of the sole parameter and return value of the desired method handle
      Returns:
        a unary method handle which accepts and returns the given type
      Throws:
        `NullPointerException` - if the argument is null
        `IllegalArgumentException` - if the given type is `void.class`

    - #### insertArguments

          public static MethodHandle insertArguments(MethodHandle target,
                                     int pos,
                                     Object... values)

      Provides a target method handle with one or more bound arguments in advance of the method handle's invocation. The formal parameters to the target corresponding to the bound arguments are called bound parameters. Returns a new method handle which saves away the bound arguments. When it is invoked, it receives arguments for any non-bound parameters, binds the saved arguments to their corresponding parameters, and calls the original target.

      The type of the new method handle will drop the types for the bound parameters from the original target type, since the new method handle will no longer require those arguments to be supplied by its callers.

      Each given argument object must match the corresponding bound parameter type. If a bound parameter type is a primitive, the argument object must be a wrapper, and will be unboxed to produce the primitive value.

      The `pos` argument selects which parameters are to be bound. It may range between zero and N-L (inclusively), where N is the arity of the target method handle and L is the length of the values array.

      Parameters:
        `target` - the method handle to invoke after the argument is inserted
        `pos` - where to insert the argument (zero for the first)
        `values` - the series of arguments to insert
      Returns:
        a method handle which inserts an additional argument, before calling the original method handle
      Throws:
        `NullPointerException` - if the target or the `values` array is null
      See Also:
        `MethodHandle.bindTo(java.lang.Object)`

    - #### dropArguments

          public static MethodHandle dropArguments(MethodHandle target,
                                   int pos,
                                   List<Class<?>> valueTypes)

      Produces a method handle which will discard some dummy arguments before calling some other specified target method handle. The type of the new method handle will be the same as the target's type, except it will also include the dummy argument types, at some given position.

      The `pos` argument may range between zero and N, where N is the arity of the target. If `pos` is zero, the dummy arguments will precede the target's real arguments; if `pos` is N they will come after.

      Example:

      >     import static java.lang.invoke.MethodHandles.*;
      >     import static java.lang.invoke.MethodType.*;
      >     ...
      >     MethodHandle cat = lookup().findVirtual(String.class,
      >       "concat", methodType(String.class, String.class));
      >     assertEquals("xy", (String) cat.invokeExact("x", "y"));
      >     MethodType bigType = cat.type().insertParameterTypes(0, int.class, String.class);
      >     MethodHandle d0 = dropArguments(cat, 0, bigType.parameterList().subList(0,2));
      >     assertEquals(bigType, d0.type());
      >     assertEquals("yz", (String) d0.invokeExact(123, "x", "y", "z"));

      This method is also equivalent to the following code:

      >      dropArguments(target, pos, valueTypes.toArray(new Class[0]))

      Parameters:
        `target` - the method handle to invoke after the arguments are dropped
        `valueTypes` - the type(s) of the argument(s) to drop
        `pos` - position of first argument to drop (zero for the leftmost)
      Returns:
        a method handle which drops arguments of the given types, before calling the original method handle
      Throws:
        `NullPointerException` - if the target is null, or if the `valueTypes` list or any of its elements is null
        `IllegalArgumentException` - if any element of `valueTypes` is `void.class`, or if `pos` is negative or greater than the arity of the target, or if the new method handle's type would have too many parameters

    - #### dropArguments

          public static MethodHandle dropArguments(MethodHandle target,
                                   int pos,
                                   Class<?>... valueTypes)

      Produces a method handle which will discard some dummy arguments before calling some other specified target method handle. The type of the new method handle will be the same as the target's type, except it will also include the dummy argument types, at some given position.

      The `pos` argument may range between zero and N, where N is the arity of the target. If `pos` is zero, the dummy arguments will precede the target's real arguments; if `pos` is N they will come after.

      Example:

      >     import static java.lang.invoke.MethodHandles.*;
      >     import static java.lang.invoke.MethodType.*;
      >     ...
      >     MethodHandle cat = lookup().findVirtual(String.class,
      >       "concat", methodType(String.class, String.class));
      >     assertEquals("xy", (String) cat.invokeExact("x", "y"));
      >     MethodHandle d0 = dropArguments(cat, 0, String.class);
      >     assertEquals("yz", (String) d0.invokeExact("x", "y", "z"));
      >     MethodHandle d1 = dropArguments(cat, 1, String.class);
      >     assertEquals("xz", (String) d1.invokeExact("x", "y", "z"));
      >     MethodHandle d2 = dropArguments(cat, 2, String.class);
      >     assertEquals("xy", (String) d2.invokeExact("x", "y", "z"));
      >     MethodHandle d12 = dropArguments(cat, 1, int.class, boolean.class);
      >     assertEquals("xz", (String) d12.invokeExact("x", 12, true, "z"));

      This method is also equivalent to the following code:

      >      dropArguments(target, pos, Arrays.asList(valueTypes))

      Parameters:
        `target` - the method handle to invoke after the arguments are dropped
        `valueTypes` - the type(s) of the argument(s) to drop
        `pos` - position of first argument to drop (zero for the leftmost)
      Returns:
        a method handle which drops arguments of the given types, before calling the original method handle
      Throws:
        `NullPointerException` - if the target is null, or if the `valueTypes` array or any of its elements is null
        `IllegalArgumentException` - if any element of `valueTypes` is `void.class`, or if `pos` is negative or greater than the arity of the target, or if the new method handle's type would have too many parameters

    - #### filterArguments

          public static MethodHandle filterArguments(MethodHandle target,
                                     int pos,
                                     MethodHandle... filters)

      Adapts a target method handle by pre-processing one or more of its arguments, each with its own unary filter function, and then calling the target with each pre-processed argument replaced by the result of its corresponding filter function.

      The pre-processing is performed by one or more method handles, specified in the elements of the `filters` array. The first element of the filter array corresponds to the `pos` argument of the target, and so on in sequence.

      Null arguments in the array are treated as identity functions, and the corresponding arguments left unchanged. (If there are no non-null elements in the array, the original target is returned.) Each filter is applied to the corresponding argument of the adapter.

      If a filter `F` applies to the `N`th argument of the target, then `F` must be a method handle which takes exactly one argument. The type of `F`'s sole argument replaces the corresponding argument type of the target in the resulting adapted method handle. The return type of `F` must be identical to the corresponding parameter type of the target.

      It is an error if there are elements of `filters` (null or not) which do not correspond to argument positions in the target. Example:

      >     import static java.lang.invoke.MethodHandles.*;
      >     import static java.lang.invoke.MethodType.*;
      >     ...
      >     MethodHandle cat = lookup().findVirtual(String.class,
      >       "concat", methodType(String.class, String.class));
      >     MethodHandle upcase = lookup().findVirtual(String.class,
      >       "toUpperCase", methodType(String.class));
      >     assertEquals("xy", (String) cat.invokeExact("x", "y"));
      >     MethodHandle f0 = filterArguments(cat, 0, upcase);
      >     assertEquals("Xy", (String) f0.invokeExact("x", "y")); // Xy
      >     MethodHandle f1 = filterArguments(cat, 1, upcase);
      >     assertEquals("xY", (String) f1.invokeExact("x", "y")); // xY
      >     MethodHandle f2 = filterArguments(cat, 0, upcase, upcase);
      >     assertEquals("XY", (String) f2.invokeExact("x", "y")); // XY

      Here is pseudocode for the resulting adapter:

      >      V target(P... p, A[i]... a[i], B... b);
      >      A[i] filter[i](V[i]);
      >      T adapter(P... p, V[i]... v[i], B... b) {
      >        return target(p..., f[i](v[i])..., b...);
      >      }

      Parameters:
        `target` - the method handle to invoke after arguments are filtered
        `pos` - the position of the first argument to filter
        `filters` - method handles to call initially on filtered arguments
      Returns:
        method handle which incorporates the specified argument filtering logic
      Throws:
        `NullPointerException` - if the target is null or if the `filters` array is null
        `IllegalArgumentException` - if a non-null element of `filters` does not match a corresponding argument type of target as described above, or if the `pos+filters.length` is greater than `target.type().parameterCount()`

    - #### filterReturnValue

          public static MethodHandle filterReturnValue(MethodHandle target,
                                       MethodHandle filter)

      Adapts a target method handle by post-processing its return value (if any) with a filter (another method handle). The result of the filter is returned from the adapter.

      If the target returns a value, the filter must accept that value as its only argument. If the target returns void, the filter must accept no arguments.

      The return type of the filter replaces the return type of the target in the resulting adapted method handle. The argument type of the filter (if any) must be identical to the return type of the target. Example:

      >     import static java.lang.invoke.MethodHandles.*;
      >     import static java.lang.invoke.MethodType.*;
      >     ...
      >     MethodHandle cat = lookup().findVirtual(String.class,
      >       "concat", methodType(String.class, String.class));
      >     MethodHandle length = lookup().findVirtual(String.class,
      >       "length", methodType(int.class));
      >     System.out.println((String) cat.invokeExact("x", "y")); // xy
      >     MethodHandle f0 = filterReturnValue(cat, length);
      >     System.out.println((int) f0.invokeExact("x", "y")); // 2

      Here is pseudocode for the resulting adapter:

      >      V target(A...);
      >      T filter(V);
      >      T adapter(A... a) {
      >        V v = target(a...);
      >        return filter(v);
      >      }
      >      // and if the target has a void return:
      >      void target2(A...);
      >      T filter2();
      >      T adapter2(A... a) {
      >        target2(a...);
      >        return filter2();
      >      }
      >      // and if the filter has a void return:
      >      V target3(A...);
      >      void filter3(V);
      >      void adapter3(A... a) {
      >        V v = target3(a...);
      >        filter3(v);
      >      }

      Parameters:
        `target` - the method handle to invoke before filtering the return value
        `filter` - method handle to call on the return value
      Returns:
        method handle which incorporates the specified return value filtering logic
      Throws:
        `NullPointerException` - if either argument is null
        `IllegalArgumentException` - if the argument list of `filter` does not match the return type of target as described above

    - #### foldArguments

          public static MethodHandle foldArguments(MethodHandle target,
                                   MethodHandle combiner)

      Adapts a target method handle by pre-processing some of its arguments, and then calling the target with the result of the pre-processing, inserted into the original sequence of arguments.

      The pre-processing is performed by `combiner`, a second method handle. Of the arguments passed to the adapter, the first `N` arguments are copied to the combiner, which is then called. (Here, `N` is defined as the parameter count of the combiner.) After this, control passes to the target, with any result from the combiner inserted before the original `N` incoming arguments.

      If the combiner returns a value, the first parameter type of the target must be identical with the return type of the combiner, and the next `N` parameter types of the target must exactly match the parameters of the combiner.

      If the combiner has a void return, no result will be inserted, and the first `N` parameter types of the target must exactly match the parameters of the combiner.

      The resulting adapter is the same type as the target, except that the first parameter type is dropped, if it corresponds to the result of the combiner.

      (Note that `dropArguments` can be used to remove any arguments that either the combiner or the target does not wish to receive. If some of the incoming arguments are destined only for the combiner, consider using `asCollector` instead, since those arguments will not need to be live on the stack on entry to the target.) Example:

      >     import static java.lang.invoke.MethodHandles.*;
      >     import static java.lang.invoke.MethodType.*;
      >     ...
      >     MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
      >       "println", methodType(void.class, String.class))
      >         .bindTo(System.out);
      >     MethodHandle cat = lookup().findVirtual(String.class,
      >       "concat", methodType(String.class, String.class));
      >     assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
      >     MethodHandle catTrace = foldArguments(cat, trace);
      >     // also prints "boo":
      >     assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));

      Here is pseudocode for the resulting adapter:

      >      // there are N arguments in A...
      >      T target(V, A[N]..., B...);
      >      V combiner(A...);
      >      T adapter(A... a, B... b) {
      >        V v = combiner(a...);
      >        return target(v, a..., b...);
      >      }
      >      // and if the combiner has a void return:
      >      T target2(A[N]..., B...);
      >      void combiner2(A...);
      >      T adapter2(A... a, B... b) {
      >        combiner2(a...);
      >        return target2(a..., b...);
      >      }

      Parameters:
        `target` - the method handle to invoke after arguments are combined
        `combiner` - method handle to call initially on the incoming arguments
      Returns:
        method handle which incorporates the specified argument folding logic
      Throws:
        `NullPointerException` - if either argument is null
        `IllegalArgumentException` - if `combiner`'s return type is non-void and not the same as the first argument type of the target, or if the initial `N` argument types of the target (skipping one matching the `combiner`'s return type) are not identical with the argument types of `combiner`

    - #### guardWithTest

          public static MethodHandle guardWithTest(MethodHandle test,
                                   MethodHandle target,
                                   MethodHandle fallback)

      Makes a method handle which adapts a target method handle, by guarding it with a test, a boolean-valued method handle. If the guard fails, a fallback handle is called instead. All three method handles must have the same corresponding argument and return types, except that the return type of the test must be boolean, and the test is allowed to have fewer arguments than the other two method handles.

      Here is pseudocode for the resulting adapter:

      >      boolean test(A...);
      >      T target(A...,B...);
      >      T fallback(A...,B...);
      >      T adapter(A... a,B... b) {
      >        if (test(a...))
      >          return target(a..., b...);
      >        else
      >          return fallback(a..., b...);
      >      }

      Note that the test arguments (`a...` in the pseudocode) cannot be modified by execution of the test, and so are passed unchanged from the caller to the target or fallback as appropriate.
      Parameters:
        `test` - method handle used for test, must return boolean
        `target` - method handle to call if test passes
        `fallback` - method handle to call if test fails
      Returns:
        method handle which incorporates the specified if/then/else logic
      Throws:
        `NullPointerException` - if any argument is null
        `IllegalArgumentException` - if `test` does not return boolean, or if all three method types do not match (with the return type of `test` changed to match that of the target).

    - #### catchException

          public static MethodHandle catchException(MethodHandle target,
                                    Class<? extends Throwable> exType,
                                    MethodHandle handler)

      Makes a method handle which adapts a target method handle, by running it inside an exception handler. If the target returns normally, the adapter returns that value. If an exception matching the specified type is thrown, the fallback handle is called instead on the exception, plus the original arguments.

      The target and handler must have the same corresponding argument and return types, except that handler may omit trailing arguments (similarly to the predicate in `guardWithTest`). Also, the handler must have an extra leading parameter of `exType` or a supertype.

      Here is pseudocode for the resulting adapter:

      >      T target(A..., B...);
      >      T handler(ExType, A...);
      >      T adapter(A... a, B... b) {
      >        try {
      >          return target(a..., b...);
      >        } catch (ExType ex) {
      >          return handler(ex, a...);
      >        }
      >      }

      Note that the saved arguments (`a...` in the pseudocode) cannot be modified by execution of the target, and so are passed unchanged from the caller to the handler, if the handler is invoked.

      The target and handler must return the same type, even if the handler always throws. (This might happen, for instance, because the handler is simulating a `finally` clause). To create such a throwing handler, compose the handler creation logic with `throwException`, in order to create a method handle of the correct return type.

      Parameters:
        `target` - method handle to call
        `exType` - the type of exception which the handler will catch
        `handler` - method handle to call if a matching exception is thrown
      Returns:
        method handle which incorporates the specified try/catch logic
      Throws:
        `NullPointerException` - if any argument is null
        `IllegalArgumentException` - if `handler` does not accept the given exception type, or if the method handle types do not match in their return types and their corresponding parameters

    - #### throwException

          public static MethodHandle throwException(Class<?> returnType,
                                    Class<? extends Throwable> exType)

      Produces a method handle which will throw exceptions of the given `exType`. The method handle will accept a single argument of `exType`, and immediately throw it as an exception. The method type will nominally specify a return of `returnType`. The return type may be anything convenient: It doesn't matter to the method handle's behavior, since it will never return normally.
      Returns:
        method handle which can throw the given exceptions
      Throws:
        `NullPointerException` - if either argument is null

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