title: How to convert Object list into a map of <String, List<String>>?
body: <p>I have a list of users</p>
  <p><code>Collection&lt;User&gt; userList = groupManager.getUserList(groupName);</code></p>
  <p>Not knowing the Java-Streams API that well, I want to map them into a map as follows: the key would be the user key, and the value would be a list the User object's other properties.</p>
  <pre><code>Map&lt;String, List&lt;String&gt;&gt; userDsiplayData = userList.stream()
       .collect(Collectors.toMap(User::getKey, **.......**))
  </code></pre>
  <p>I tried to replace **..... ** with <code>Arrays.asList(User::getUsername(), User::getDisplayName(), User::getEmail())</code> or similar constructs but I can't seem to get it to work.</p>