Appending data to Map in Java

Question

I have a Map

Map<Integer, List<Object>> entireData;

Now to this I'm adding some data using putAll like

entireData.putAll(someData);

where someData returns Map<Integer, List<Object>>

Now, I have another line which says

entireData.putAll(someMoreData);

which also returns Map<Integer, List<Object>>, but by doing this it over-writes the content of the existing entireData, how do I append?

Answer 1

First line of the Java Map Class Reference:

An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.

Answer 2

You want a Multimap from Google Guava. Rewriting your example with Guava Multimap:

ListMultimap<Integer, Object> entireData = ArrayListMultimap.create();

entireData.get(key) returns a List<Object>. putAll will not override old keys but will append the values of those keys to the existing values. This is also much nicer than dealing with initializing the List instances yourself.