title: Group map values but keys are same
body: <p>I have a map like this. <code>Map&lt;long,List&lt;Student&gt;&gt; studentMap</code></p>
  <p>Key is a number 1,2,3,4...
  Student object is :</p>
  <pre><code>public class Student {
   private long addressNo;
   private String code;
   private BigDecimal tax;
   private String name;
   private String city;
  
   // getter and setters` 
  }
  </code></pre>
  <p>What i want to do is to convert it <code>Map&lt;long,List&lt;StudentInfo&gt;&gt; studentInfoMap</code> object and group id, addressNo and code fields.I want key are same for both maps.</p>
  <p>I can group the map by using these codes but summingDouble is not working for BigDecimal.Also I cannot convert my studentMap to studentInfoMap.:(</p>
  <pre><code> studentInfoMap.values().stream()
              .collect(
                       Collectors.groupingBy(StudentInfo::getCode, 
                       Collectors.groupingBy(StudentInfo::getAddressNo, 
                       Collectors.summingDouble(StudentInfo::getTax))));
  </code></pre>
  <p>My studentInfo object is :</p>
  <pre><code>public class StudentInfo {
    private long addressNo;
    private String code;
    private BigDecimal tax;
  
    // getter and setters` 
  }
  </code></pre>