title: Fastest way to put contents of Set<String> to a single String with words separated by a whitespace?
body: <p>I have a few <code>Set&lt;String&gt;</code>s and want to transform each of these into a single <code>String</code> where each element of the original <code>Set</code> is separated by a whitespace " ".
  A naive first approach is doing it like this</p>
  
  <pre><code>Set&lt;String&gt; set_1;
  Set&lt;String&gt; set_2;
  
  StringBuilder builder = new StringBuilder();
  for (String str : set_1) {
    builder.append(str).append(" ");
  }
  
  this.string_1 = builder.toString();
  
  builder = new StringBuilder();
  for (String str : set_2) {
    builder.append(str).append(" ");
  }
  
  this.string_2 = builder.toString();
  </code></pre>
  
  <p>Can anyone think of a faster, prettier or more efficient way to do this?</p>