title: Defensive copy of Calendar
body: <p>Been trying to find the best way to implement a method that makes a defensive copy of a Calendar object.</p>
  
  <p>eg:</p>
  
  <pre><code>public void setDate(Calendar date) {
      // What to do.... 
  }
  </code></pre>
  
  <p>I am particularly concerned about interleaving of threads when checking for null input and making the copy or am I missing something very apparent?</p>