title: Convert Map<String, Object> to Map<String, List<Object>>
body: <p>I am trying to change the Map value Object to List Value Object, so that a single key can store multiple values. Please find the code snippet.</p>
  
  <pre><code>  protected Subscriber updateSubscriberAttributes(Subscriber subscriber, Collection&lt;SubscriberAttributeDto&gt; subscriberAttributesCollection)
          throws ErrorException {
      // Do we have any custom attributes to store?
      if ((subscriberAttributesCollection != null) &amp;&amp; !subscriberAttributesCollection.isEmpty()) {
          // Yes! Convert to a Map first.
          Map&lt;String, Object&gt; subscriberAttributesMap = new HashMap&lt;String, Object&gt;(subscriberAttributesCollection.size());
          for (SubscriberAttributeDto subscriberAttribute : subscriberAttributesCollection) {
              // Convert the input attribute value to a database-appropriate value
              SubscriberAttributeMetadata attrMetadata = subscriberAttribute.getAttributeMetadata();
              if (attrMetadata != null) {
                  subscriberAttributesMap.put(attrMetadata.getColumnName(),
                          attrMetadata.convertToDatabaseValue((String) subscriberAttribute.getValue()));
              }
          }
  
          // Perform the update
          return updateSubscriberAttributes(subscriber, subscriberAttributesMap);
      } else {
          return subscriber;
      }
  }
  </code></pre>
  
  <p>Sorry the code is little messy. So the trouble I am facing here is, If I change the declaration of <code>subscribersAttributeMap</code> to <code>Map&lt;String,List&lt;Object&gt;&gt;</code> I have to change the method declaration and throwing me so many errors. I did try it from long time.</p>