title: Java groupingBy: sum multiple fields
body: <p>This question is an extension to the post: <a href="https://stackoverflow.com/questions/48728341/java-8-groupingby-with-returning-multiple-field">Java 8 groupingby with returning multiple field</a>.</p>
  
  <p>For the same problem, how do you return a list of <code>Customer</code>? For example, it should return:</p>
  
  <pre><code>Customer("A",4500,6500)
  Customer("B",3000,3500)
  Customer("C",4000,4500)
  </code></pre>