title: Java Stream API: how to convert to a Map of Sets
body: <p>Lets say I have  following an class named <code>Employee</code> and a List of these <code>Employee</code>-classes:</p>
  
  <pre><code>class Employee {
  
  private String praefix;
  private String middleFix;
  private String postfix;
  private String name;
  
  public Employee(String praefix, String middleFix, String postfix, String name) {
      this.praefix = praefix;
      this.middleFix = middleFix;
      this.postfix = postfix;
      this.name = name;
  }
  
  public String getPraefix() {
      return praefix;
  }
  
  public void setPraefix(String praefix) {
      this.praefix = praefix;
  }
  
  public String getMiddleFix() {
      return middleFix;
  }
  
  public void setMiddleFix(String middleFix) {
      this.middleFix = middleFix;
  }
  
  public String getPostfix() {
      return postfix;
  }
  
  public void setPostfix(String postfix) {
      this.postfix = postfix;
  }
  
  public String getName() {
      return name;
  }
  
  public void setName(String name) {
      this.name = name;
  }
  
  
  
  List&lt;Employee&gt; employees = new ArrayList&lt;&gt;();
  employees.add(new Employee("A", "B", "C", "Michael Phelps"));
  employees.add(new Employee("A", "B", "C", "Cristiano Ronaldo"));
  employees.add(new Employee("D", "E", "F", "Usain Bolton"));
  employees.add(new Employee("D", "E", "F", "Diego Armando Maradona"));
  employees.add(new Employee("D", "E", "F", "Lionel Messi"));
  </code></pre>
  
  <p>Is it possible to convert it to following Map with Java Stream-API?</p>
  
  <pre><code>{A.B.C=[Cristiano Ronaldo, Michael Phelps], D.E.F=[Aydin Korkmaz, Diego Armando Maradona, Usain Bolton]} 
  </code></pre>