Java 8: Multimaps

A multimap is a Map which maps a single key to multiple values e.g. HashMap<String, List<String>>.

Java 8 introduces a Map.computeIfAbsent method, which makes inserting values into a multimap much simpler:

Map<String, List<String>> multimap = new HashMap<>();
multimap.computeIfAbsent(key, k -> new ArrayList<>()).add(value);

Prior to Java 8, a multimap was usually created as follows:

// create the map
Map<String, List<String>> multimap = new HashMap<>();

// put a key/value into the map
List<String> list = multimap.get(key);
if (list == null) {
  multimap.put(key, list = new ArrayList<>());
}
list.add(value);

Or with Guava's Multimap class:

ListMultimap<String, String> multimap = ArrayListMultimap.create();
multimap.put(key, value);

You can read more about Java 8 updates to the Map class in my previous blog post.