test/java/lang/invoke/JavaDocExamplesTest.java

/*
 * Copyright (c) 2009, 2012, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

/* @test
 * @summary example code used in javadoc for java.lang.invoke API
 * @compile JavaDocExamplesTest.java
 * @run testng/othervm test.java.lang.invoke.JavaDocExamplesTest
 */

package test.java.lang.invoke;

import java.lang.invoke.*;
import static java.lang.invoke.MethodHandles.*;
import static java.lang.invoke.MethodType.*;

import java.util.*;

import org.testng.*;
import static org.testng.AssertJUnit.*;
import org.testng.annotations.*;

/**
 * @author jrose
 */
public class JavaDocExamplesTest {
    /** Wrapper for running the TestNG tests in this module.
     *  Put TestNG on the classpath!
     */
    public static void main(String... ignore) throws Throwable {
        new JavaDocExamplesTest().run();
    }

    public void run() throws Throwable {
        testFindVirtual();
        testPermuteArguments();
        testDropArguments();
        testFilterArguments();
        testFoldArguments();
        testFoldArguments2();
        testMethodHandlesSummary();
        testAsSpreader();
        testAsCollector();
        testAsVarargsCollector();
        testAsFixedArity();
        testAsTypeCornerCases();
        testMutableCallSite();
    }
    // How much output?
    static final Class<?> THIS_CLASS = JavaDocExamplesTest.class;
    static int verbosity = Integer.getInteger(THIS_CLASS.getSimpleName()+".verbosity", 0);


{}
static final private Lookup LOOKUP = lookup();
// static final private MethodHandle CONCAT_1 = LOOKUP.findVirtual(String.class,
//     "concat", methodType(String.class, String.class));
// static final private MethodHandle HASHCODE_1 = LOOKUP.findVirtual(Object.class,
//     "hashCode", methodType(int.class));

// form required if ReflectiveOperationException is intercepted:
    static final private MethodHandle CONCAT_2, HASHCODE_2, ADD_2, SUB_2;
static {
  try {
    Class<?> THIS_CLASS = LOOKUP.lookupClass();
    CONCAT_2 = LOOKUP.findVirtual(String.class,
      "concat", methodType(String.class, String.class));
    HASHCODE_2 = LOOKUP.findVirtual(Object.class,
      "hashCode", methodType(int.class));
    ADD_2 = LOOKUP.findStatic(THIS_CLASS, "add", methodType(int.class, int.class, int.class));
    SUB_2 = LOOKUP.findStatic(THIS_CLASS, "sub", methodType(int.class, int.class, int.class));
   } catch (ReflectiveOperationException ex) {
     throw new RuntimeException(ex);
   }
}
    static int add(int x, int y) { return x + y; }
    static int sub(int x, int y) { return x - y; }

{}

    @Test public void testFindVirtual() throws Throwable {
{}
MethodHandle CONCAT_3 = LOOKUP.findVirtual(String.class,
  "concat", methodType(String.class, String.class));
MethodHandle HASHCODE_3 = LOOKUP.findVirtual(Object.class,
  "hashCode", methodType(int.class));
//assertEquals("xy", (String) CONCAT_1.invokeExact("x", "y"));
assertEquals("xy", (String) CONCAT_2.invokeExact("x", "y"));
assertEquals("xy", (String) CONCAT_3.invokeExact("x", "y"));
//assertEquals("xy".hashCode(), (int) HASHCODE_1.invokeExact((Object)"xy"));
assertEquals("xy".hashCode(), (int) HASHCODE_2.invokeExact((Object)"xy"));
assertEquals("xy".hashCode(), (int) HASHCODE_3.invokeExact((Object)"xy"));
{}
    }

    @Test public void testPermuteArguments() throws Throwable {
        {{
{} /// JAVADOC
MethodType intfn1 = methodType(int.class, int.class);
MethodType intfn2 = methodType(int.class, int.class, int.class);
MethodHandle sub = SUB_2;// ... {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 = ADD_2;// ... {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);
            }}
        {{
{} /// JAVADOC
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"));
            }}
    }

    @Test public void testDropArguments() throws Throwable {
        {{
{} /// JAVADOC
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"));
            }}
        {{
{} /// JAVADOC
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"));
            }}
    }

    @Test public void testFilterArguments() throws Throwable {
        {{
{} /// JAVADOC
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
            }}
    }

    @Test public void testFoldArguments() throws Throwable {
        {{
{} /// JAVADOC
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"));
            }}
    }

    static void assertEquals(Object exp, Object act) {
        if (verbosity > 0)
            System.out.println("result: "+act);
        Assert.assertEquals(exp, act);
    }

    @Test public void testMethodHandlesSummary() throws Throwable {
        {{
{} /// JAVADOC
Object x, y; String s; int i;
MethodType mt; MethodHandle mh;
MethodHandles.Lookup lookup = MethodHandles.lookup();
// mt is (char,char)String
mt = MethodType.methodType(String.class, char.class, char.class);
mh = lookup.findVirtual(String.class, "replace", mt);
s = (String) mh.invokeExact("daddy",'d','n');
// invokeExact(Ljava/lang/String;CC)Ljava/lang/String;
assertEquals(s, "nanny");
// weakly typed invocation (using MHs.invoke)
s = (String) mh.invokeWithArguments("sappy", 'p', 'v');
assertEquals(s, "savvy");
// mt is (Object[])List
mt = MethodType.methodType(java.util.List.class, Object[].class);
mh = lookup.findStatic(java.util.Arrays.class, "asList", mt);
assert(mh.isVarargsCollector());
x = mh.invoke("one", "two");
// invoke(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/Object;
assertEquals(x, java.util.Arrays.asList("one","two"));
// mt is (Object,Object,Object)Object
mt = MethodType.genericMethodType(3);
mh = mh.asType(mt);
x = mh.invokeExact((Object)1, (Object)2, (Object)3);
// invokeExact(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
assertEquals(x, java.util.Arrays.asList(1,2,3));
// mt is ()int
mt = MethodType.methodType(int.class);
mh = lookup.findVirtual(java.util.List.class, "size", mt);
i = (int) mh.invokeExact(java.util.Arrays.asList(1,2,3));
// invokeExact(Ljava/util/List;)I
assert(i == 3);
mt = MethodType.methodType(void.class, String.class);
mh = lookup.findVirtual(java.io.PrintStream.class, "println", mt);
mh.invokeExact(System.out, "Hello, world.");
// invokeExact(Ljava/io/PrintStream;Ljava/lang/String;)V
{}
            }}
    }

    @Test public void testAsSpreader() throws Throwable {
        {{
{} /// JAVADOC
MethodHandle equals = publicLookup()
  .findVirtual(String.class, "equals", methodType(boolean.class, Object.class));
assert( (boolean) equals.invokeExact("me", (Object)"me"));
assert(!(boolean) equals.invokeExact("me", (Object)"thee"));
// spread both arguments from a 2-array:
MethodHandle eq2 = equals.asSpreader(Object[].class, 2);
assert( (boolean) eq2.invokeExact(new Object[]{ "me", "me" }));
assert(!(boolean) eq2.invokeExact(new Object[]{ "me", "thee" }));
// spread both arguments from a String array:
MethodHandle eq2s = equals.asSpreader(String[].class, 2);
assert( (boolean) eq2s.invokeExact(new String[]{ "me", "me" }));
assert(!(boolean) eq2s.invokeExact(new String[]{ "me", "thee" }));
// spread second arguments from a 1-array:
MethodHandle eq1 = equals.asSpreader(Object[].class, 1);
assert( (boolean) eq1.invokeExact("me", new Object[]{ "me" }));
assert(!(boolean) eq1.invokeExact("me", new Object[]{ "thee" }));
// spread no arguments from a 0-array or null:
MethodHandle eq0 = equals.asSpreader(Object[].class, 0);
assert( (boolean) eq0.invokeExact("me", (Object)"me", new Object[0]));
assert(!(boolean) eq0.invokeExact("me", (Object)"thee", (Object[])null));
// asSpreader and asCollector are approximate inverses:
for (int n = 0; n <= 2; n++) {
    for (Class<?> a : new Class<?>[]{Object[].class, String[].class, CharSequence[].class}) {
        MethodHandle equals2 = equals.asSpreader(a, n).asCollector(a, n);
        assert( (boolean) equals2.invokeWithArguments("me", "me"));
        assert(!(boolean) equals2.invokeWithArguments("me", "thee"));
    }
}
MethodHandle caToString = publicLookup()
  .findStatic(Arrays.class, "toString", methodType(String.class, char[].class));
assertEquals("[A, B, C]", (String) caToString.invokeExact("ABC".toCharArray()));
MethodHandle caString3 = caToString.asCollector(char[].class, 3);
assertEquals("[A, B, C]", (String) caString3.invokeExact('A', 'B', 'C'));
MethodHandle caToString2 = caString3.asSpreader(char[].class, 2);
assertEquals("[A, B, C]", (String) caToString2.invokeExact('A', "BC".toCharArray()));
            }}
    }

    @Test public void testAsCollector() throws Throwable {
        {{
{} /// JAVADOC
MethodHandle deepToString = publicLookup()
  .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
assertEquals("[won]",   (String) deepToString.invokeExact(new Object[]{"won"}));
MethodHandle ts1 = deepToString.asCollector(Object[].class, 1);
assertEquals(methodType(String.class, Object.class), ts1.type());
//assertEquals("[won]", (String) ts1.invokeExact(         new Object[]{"won"})); //FAIL
assertEquals("[[won]]", (String) ts1.invokeExact((Object) new Object[]{"won"}));
// arrayType can be a subtype of Object[]
MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
assertEquals(methodType(String.class, String.class, String.class), ts2.type());
assertEquals("[two, too]", (String) ts2.invokeExact("two", "too"));
MethodHandle ts0 = deepToString.asCollector(Object[].class, 0);
assertEquals("[]", (String) ts0.invokeExact());
// collectors can be nested, Lisp-style
MethodHandle ts22 = deepToString.asCollector(Object[].class, 3).asCollector(String[].class, 2);
assertEquals("[A, B, [C, D]]", ((String) ts22.invokeExact((Object)'A', (Object)"B", "C", "D")));
// arrayType can be any primitive array type
MethodHandle bytesToString = publicLookup()
  .findStatic(Arrays.class, "toString", methodType(String.class, byte[].class))
  .asCollector(byte[].class, 3);
assertEquals("[1, 2, 3]", (String) bytesToString.invokeExact((byte)1, (byte)2, (byte)3));
MethodHandle longsToString = publicLookup()
  .findStatic(Arrays.class, "toString", methodType(String.class, long[].class))
  .asCollector(long[].class, 1);
assertEquals("[123]", (String) longsToString.invokeExact((long)123));
            }}
    }

    @SuppressWarnings("rawtypes")
    @Test public void testAsVarargsCollector() throws Throwable {
        {{
{} /// JAVADOC
MethodHandle deepToString = publicLookup()
  .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
MethodHandle ts1 = deepToString.asVarargsCollector(Object[].class);
assertEquals("[won]",   (String) ts1.invokeExact(    new Object[]{"won"}));
assertEquals("[won]",   (String) ts1.invoke(         new Object[]{"won"}));
assertEquals("[won]",   (String) ts1.invoke(                      "won" ));
assertEquals("[[won]]", (String) ts1.invoke((Object) new Object[]{"won"}));
// findStatic of Arrays.asList(...) produces a variable arity method handle:
MethodHandle asList = publicLookup()
  .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class));
assertEquals(methodType(List.class, Object[].class), asList.type());
assert(asList.isVarargsCollector());
assertEquals("[]", asList.invoke().toString());
assertEquals("[1]", asList.invoke(1).toString());
assertEquals("[two, too]", asList.invoke("two", "too").toString());
String[] argv = { "three", "thee", "tee" };
assertEquals("[three, thee, tee]", asList.invoke(argv).toString());
assertEquals("[three, thee, tee]", asList.invoke((Object[])argv).toString());
List ls = (List) asList.invoke((Object)argv);
assertEquals(1, ls.size());
assertEquals("[three, thee, tee]", Arrays.toString((Object[])ls.get(0)));
            }}
    }

    @Test public void testAsFixedArity() throws Throwable {
        {{
{} /// JAVADOC
MethodHandle asListVar = publicLookup()
  .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class))
  .asVarargsCollector(Object[].class);
MethodHandle asListFix = asListVar.asFixedArity();
assertEquals("[1]", asListVar.invoke(1).toString());
Exception caught = null;
try { asListFix.invoke((Object)1); }
catch (Exception ex) { caught = ex; }
assert(caught instanceof ClassCastException);
assertEquals("[two, too]", asListVar.invoke("two", "too").toString());
try { asListFix.invoke("two", "too"); }
catch (Exception ex) { caught = ex; }
assert(caught instanceof WrongMethodTypeException);
Object[] argv = { "three", "thee", "tee" };
assertEquals("[three, thee, tee]", asListVar.invoke(argv).toString());
assertEquals("[three, thee, tee]", asListFix.invoke(argv).toString());
assertEquals(1, ((List) asListVar.invoke((Object)argv)).size());
assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString());
            }}
    }

    @Test public void testAsTypeCornerCases() throws Throwable {
        {{
{} /// JAVADOC
MethodHandle i2s = publicLookup()
  .findVirtual(Integer.class, "toString", methodType(String.class));
i2s = i2s.asType(i2s.type().unwrap());
MethodHandle l2s = publicLookup()
  .findVirtual(Long.class, "toString", methodType(String.class));
l2s = l2s.asType(l2s.type().unwrap());

Exception caught = null;
try { i2s.asType(methodType(String.class, String.class)); }
catch (Exception ex) { caught = ex; }
assert(caught instanceof WrongMethodTypeException);

i2s.asType(methodType(String.class, byte.class));
i2s.asType(methodType(String.class, Byte.class));
i2s.asType(methodType(String.class, Character.class));
i2s.asType(methodType(String.class, Integer.class));
l2s.asType(methodType(String.class, byte.class));
l2s.asType(methodType(String.class, Byte.class));
l2s.asType(methodType(String.class, Character.class));
l2s.asType(methodType(String.class, Integer.class));
l2s.asType(methodType(String.class, Long.class));

caught = null;
try { i2s.asType(methodType(String.class, Long.class)); }
catch (Exception ex) { caught = ex; }
assert(caught instanceof WrongMethodTypeException);

MethodHandle i2sGen = i2s.asType(methodType(String.class, Object.class));
MethodHandle l2sGen = l2s.asType(methodType(String.class, Object.class));

i2sGen.invoke(42);  // int -> Integer -> Object -> Integer -> int
i2sGen.invoke((byte)4);  // byte -> Byte -> Object -> Byte -> byte -> int
l2sGen.invoke(42);  // int -> Integer -> Object -> Integer -> int
l2sGen.invoke((byte)4);  // byte -> Byte -> Object -> Byte -> byte -> int
l2sGen.invoke(0x420000000L);

caught = null;
try { i2sGen.invoke(0x420000000L); } // long -> Long -> Object -> Integer CCE
catch (Exception ex) { caught = ex; }
assert(caught instanceof ClassCastException);

caught = null;
try { i2sGen.invoke("asdf"); } // String -> Object -> Integer CCE
catch (Exception ex) { caught = ex; }
assert(caught instanceof ClassCastException);
{}
            }}
    }

    @Test public void testMutableCallSite() throws Throwable {
        {{
{} /// JAVADOC
MutableCallSite name = new MutableCallSite(MethodType.methodType(String.class));
MethodHandle MH_name = name.dynamicInvoker();
MethodType MT_str1 = MethodType.methodType(String.class);
MethodHandle MH_upcase = MethodHandles.lookup()
    .findVirtual(String.class, "toUpperCase", MT_str1);
MethodHandle worker1 = MethodHandles.filterReturnValue(MH_name, MH_upcase);
name.setTarget(MethodHandles.constant(String.class, "Rocky"));
assertEquals("ROCKY", (String) worker1.invokeExact());
name.setTarget(MethodHandles.constant(String.class, "Fred"));
assertEquals("FRED", (String) worker1.invokeExact());
// (mutation can be continued indefinitely)
/*
 * </pre></blockquote>
 * <p>
 * The same call site may be used in several places at once.
 * <blockquote><pre>
 */
MethodType MT_str2 = MethodType.methodType(String.class, String.class);
MethodHandle MH_cat = lookup().findVirtual(String.class,
  "concat", methodType(String.class, String.class));
MethodHandle MH_dear = MethodHandles.insertArguments(MH_cat, 1, ", dear?");
MethodHandle worker2 = MethodHandles.filterReturnValue(MH_name, MH_dear);
assertEquals("Fred, dear?", (String) worker2.invokeExact());
name.setTarget(MethodHandles.constant(String.class, "Wilma"));
assertEquals("WILMA", (String) worker1.invokeExact());
assertEquals("Wilma, dear?", (String) worker2.invokeExact());
{}
            }}
    }

    @Test public void testSwitchPoint() throws Throwable {
        {{
{} /// JAVADOC
MethodHandle MH_strcat = MethodHandles.lookup()
    .findVirtual(String.class, "concat", MethodType.methodType(String.class, String.class));
SwitchPoint spt = new SwitchPoint();
assert(!spt.hasBeenInvalidated());
// the following steps may be repeated to re-use the same switch point:
MethodHandle worker1 = MH_strcat;
MethodHandle worker2 = MethodHandles.permuteArguments(MH_strcat, MH_strcat.type(), 1, 0);
MethodHandle worker = spt.guardWithTest(worker1, worker2);
assertEquals("method", (String) worker.invokeExact("met", "hod"));
SwitchPoint.invalidateAll(new SwitchPoint[]{ spt });
assert(spt.hasBeenInvalidated());
assertEquals("hodmet", (String) worker.invokeExact("met", "hod"));
{}
            }}
    }

    @Test public void testFoldArguments2() throws Throwable {
        {{
{} /// JAVADOC
// argument-based dispatch for methods of the form boolean x.___(y: String)
Lookup lookup = publicLookup();
// first, a tracing hack:
MethodHandle println = lookup.findVirtual(java.io.PrintStream.class, "println", methodType(void.class, String.class));
MethodHandle arrayToString = lookup.findStatic(Arrays.class, "toString", methodType(String.class, Object[].class));
MethodHandle concat = lookup.findVirtual(String.class, "concat", methodType(String.class, String.class));
MethodHandle arrayToString_DIS = filterReturnValue(arrayToString, concat.bindTo("DIS:"));
MethodHandle arrayToString_INV = filterReturnValue(arrayToString, concat.bindTo("INV:"));
MethodHandle printArgs_DIS = filterReturnValue(arrayToString_DIS, println.bindTo(System.out)).asVarargsCollector(Object[].class);
MethodHandle printArgs_INV = filterReturnValue(arrayToString_INV, println.bindTo(System.out)).asVarargsCollector(Object[].class);
// metaobject protocol:
MethodType mtype = methodType(boolean.class, String.class);
MethodHandle findVirtual = lookup.findVirtual(Lookup.class,
  "findVirtual", methodType(MethodHandle.class, Class.class, String.class, MethodType.class));
MethodHandle getClass = lookup.findVirtual(Object.class,
  "getClass", methodType(Class.class));
MethodHandle dispatch = findVirtual;
dispatch = filterArguments(dispatch, 1, getClass);
dispatch = insertArguments(dispatch, 3, mtype);
dispatch = dispatch.bindTo(lookup);
assertEquals(methodType(MethodHandle.class, Object.class, String.class), dispatch.type());
MethodHandle invoker = invoker(mtype.insertParameterTypes(0, Object.class));
// wrap tracing around the dispatch and invoke steps:
dispatch = foldArguments(dispatch, printArgs_DIS.asType(dispatch.type().changeReturnType(void.class)));
invoker = foldArguments(invoker, printArgs_INV.asType(invoker.type().changeReturnType(void.class)));
invoker = dropArguments(invoker, 2, String.class);  // ignore selector
// compose the dispatcher and the invoker:
MethodHandle invokeDispatched = foldArguments(invoker, dispatch);
Object x = "football", y = new java.util.Scanner("bar");
assert( (boolean) invokeDispatched.invokeExact(x, "startsWith", "foo"));
assert(!(boolean) invokeDispatched.invokeExact(x, "startsWith", "#"));
assert( (boolean) invokeDispatched.invokeExact(x, "endsWith", "all"));
assert(!(boolean) invokeDispatched.invokeExact(x, "endsWith", "foo"));
assert( (boolean) invokeDispatched.invokeExact(y, "hasNext", "[abc]+[rst]"));
assert(!(boolean) invokeDispatched.invokeExact(y, "hasNext", "[123]+[789]"));
            }}
    }

    /* ---- TEMPLATE ----
    @Test public void testFoo() throws Throwable {
        {{
{} /// JAVADOC
{}
            }}
    }
    */
}