Convert Map<String, Object> to Map<String, List<Object>>

Question

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.

  protected Subscriber updateSubscriberAttributes(Subscriber subscriber, Collection<SubscriberAttributeDto> subscriberAttributesCollection)
        throws ErrorException {
    // Do we have any custom attributes to store?
    if ((subscriberAttributesCollection != null) && !subscriberAttributesCollection.isEmpty()) {
        // Yes! Convert to a Map first.
        Map<String, Object> subscriberAttributesMap = new HashMap<String, Object>(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;
    }
}

Sorry the code is little messy. So the trouble I am facing here is, If I change the declaration of subscribersAttributeMap to Map<String,List<Object>> I have to change the method declaration and throwing me so many errors. I did try it from long time.

Answer 1

probably something like:

 Map<String,List<Object>>subscriberAttributesMap = new HashMap<String, List<Object>>(subscriberAttributesCollection.size());
                for (SubscriberAttributeDto subscriberAttribute : subscriberAttributesCollection) {
                    // Convert the input attribute value to a database-appropriate value
                    SubscriberAttributeMetadata attrMetadata = subscriberAttribute.getAttributeMetadata();
                    if (attrMetadata != null) {
                        List<Object> lst = subscriberAttributesMap.get(attrMetadata.getColumnName());
                        if(lst == null){
                            lst = new ArrayList<Object>();
                            subscriberAttributesMap.put(attrMetadata.getColumnName(), lst);
                        }
                        lst.add(attrMetadata.convertToDatabaseValue((String) subscriberAttribute.getValue()));

                    }
                }

Answer 2

protected Subscriber updateSubscriberAttributes(Subscriber subscriber, Collection<SubscriberAttributeDto> subscriberAttributesCollection) throws ErrorException {
    // Do we have any custom attributes to store?
    if ((subscriberAttributesCollection != null) && !subscriberAttributesCollection.isEmpty()) {
        // Yes! Convert to a Map first.
        Map<String, List<Object>> subscriberAttributesMap = new HashMap<String, List<Object>>(subscriberAttributesCollection.size());
        for (SubscriberAttributeDto subscriberAttribute : subscriberAttributesCollection) {
            // Convert the input attribute value to a database-appropriate value
            SubscriberAttributeMetadata attrMetadata = subscriberAttribute.getAttributeMetadata();
            if (attrMetadata != null) {
                //
                String columnName = attrMetadata.getColumnName();
                List<Object> list = subscriberAttributesMap.get(columnName);
                if (list == null) {
                    list = new ArrayList<Object>();
                    subscriberAttributesMap.put(columnName, list);
                }
                //
                list.add(attrMetadata.convertToDatabaseValue((String) subscriberAttribute.getValue()));
            }
        }

        // Perform the update
        return updateSubscriberAttributes(subscriber, subscriberAttributesMap);
    } else {
        return subscriber;
    }
}

protected Subscriber updateSubscriberAttributes(Subscriber subscriber,  Map<String, List<Object>> map) throws ErrorException {
    // do some logic hiere ...
    return new Subscriber();
}

Answer 3

You may want using/re-implementing/copying Guava's Multimap , which has the functionality you need and the interface similar to java.util.Map . So this would minimize changes in the code. Example from their documentation:

ListMultimap<String, Object> multimap = ArrayListMultimap.create();

for (President pres : US_PRESIDENTS_IN_ORDER) {
    multimap.put(pres.firstName(), pres.lastName());
}

for (String firstName : multimap.keySet()) {
    List<Object> lastNames = multimap.get(firstName);
    out.println(firstName + ": " + lastNames);
}

Multimap can be converted back to a Map :

// java.util.Map representation
Map<String,Collection<Object>> stringCollectionMap = multimap.asMap();