title: how to use java stream to group fields and create a summary
body: <pre><code>public class Call {
      private String status;
      private String callName;
  }
  </code></pre>
  
  <p>I have a list of calls  and i have to create a summary, like this:</p>
  
  <pre><code>public class CallSummary {
      private String callName;
      private List&lt;ItemSummary&gt; items;
  }
  public class itemSummary {
      private String status;
      private Integer percentage;
  }
  </code></pre>
  
  <p>My goal is show a percentage of calls with some status
  like :</p>
  
  <pre><code>INBOUND_CALL : {
  FAILED = 30%
  SUCCESS = 70%
  }
  </code></pre>
  
  <p>how can i do it using java 8 stream and Collectors ?</p>