title: java thread reuse
body: <p>I have always read that creating threads is expensive.<br>
  I also know that you cannot rerun a thread.</p>
  
  <p>I see in the doc of <code>Executors</code> class:</p>
  
  <blockquote>
    <p>Creates a thread pool that creates new threads as needed, but will reuse previously constructed threads when they are available.</p>
  </blockquote>
  
  <p>Mind the word 'reuse'.</p>
  
  <p>How do thread pools 'reuse' threads? </p>