### Question:
FlaggingVisitor extends AbstractFunction1<Object, Object> implements Serializable { @Nullable @Override public Object apply(@Nullable final Object value) { accumulable.$plus$eq(Boolean.TRUE); return value; } FlaggingVisitor(@Nonnull final Accumulable<Boolean, Boolean> accumulable); @Nullable @Override Object apply(@Nullable final Object value);  }### Answer:
@Test @SuppressWarnings("unchecked") public void test() { final Accumulator<Boolean> accumulator = Mockito.mock(Accumulator.class); final FlaggingVisitor visitor = new FlaggingVisitor(accumulator); Assert.assertEquals(Void.class, visitor.apply(Void.class)); Mockito.verify(accumulator).$plus$eq(true); }