title: Join a list of object's properties into a String
body: <p>I'm learning lambda right now, and I wonder how can I write this code by a single line with lambda.</p>
  
  <p>I have a <code>Person</code> class which includes an <code>ID</code> and <code>name</code> fields</p>
  
  <p>Currently, I have a <code>List&lt;Person&gt;</code> which stores these <code>Person</code> objects. What I want to accomplish is to get a string consisting of person's id just like.</p>
  
  <p><strong>"id1,id2,id3"</strong>. </p>
  
  <p>How can I accomplish this with lambda?</p>