title: Java 8 Streams - Group by parent property
body: <p>I have the following model:</p>
  
  <pre><code>public class Employee {
      Set&lt;Position&gt; positions;
  }
  
  public class Position {
      Area area;
  }
  </code></pre>
  
  <p>and then in my controller I have a <code>List&lt;Employee&gt;</code> and I want to get with lambda stream operations a <code>Map&lt;Area, List&lt;Employee&gt;&gt;</code> that result of knowing the area of every position of the employee.</p>
  
  <p>How can I achieve this?</p>