title: Collect to set with joining Java 8
body: <p>Hi I'm trying to have a string represents concatenation of set of names for each teacher, 
  thus I need to use both <code>Collectors.toSet</code> and  <code>Collectors.joining(", ")</code> how can I use them in 1 combine line ?
  I can only make each one of them separately  how can i do both of them ?</p>
  
  <pre><code>students.stream().collect(Collectors.groupingBy(student -&gt; student.getTeacherName(), mapping(student -&gt; student.getName(), toSet())
  
  
  students.stream().collect(Collectors.groupingBy(student -&gt; student.getTeacherName(), mapping(student -&gt; student.getName(), joining(", "))
  </code></pre>