title: Group by two fields then summing BigDecimal
body: <p>I have a list of taxes:</p>
  
  <pre><code>TaxLine = title:"New York Tax", rate:0.20, price:20.00
  TaxLine = title:"New York Tax", rate:0.20, price:20.00
  TaxLine = title:"County Tax", rate:0.10, price:10.00
  </code></pre>
  
  <p>TaxLine class is</p>
  
  <pre><code>public class TaxLine {
      private BigDecimal price;
      private BigDecimal rate;
      private String title;
  }
  </code></pre>
  
  <p>I would like to combine them base on unique <code>title</code> and <code>rate</code>, then add the <code>price</code>, expected:</p>
  
  <pre><code> TaxLine = title:"New York Tax", rate:0.20, price:40.00
   TaxLine = title:"County Tax", rate:0.10, price:10.00
  </code></pre>
  
  <p>How can I do this in Java 8? </p>
  
  <p><a href="https://stackoverflow.com/questions/28342814">Group by multiple field names in java 8</a>, is not summing a field, it can only group by the two fields.</p>