pes of the form <code>Future<?></code> and
 return <code>null</code> as a result of the underlying task.

 <p>
 <b>Sample Usage</b> (Note that the following classes are all
 made-up.) <p>
  <pre> <code>interface ArchiveSearcher { String search(String target); </code>
 class App {
   ExecutorService executor = ...
   ArchiveSearcher searcher = ...
   void showSearch(final String target)
       throws InterruptedException {
     Future<String> future
       = executor.submit(new Callable<String>() {
         