adding a key to HashMap without the value?

Question

Is there a way to add a key to a HashMap without also adding a value? I know it seems strange, but I have a HashMap> amd I wan

Answer 1

Since you're using a Map>, you're really looking for a multimap. I highly recommend using a third-party library such as Google Guava for this - see Guava's Multimaps.

Multimap myMultimap = ArrayListMultimap.create();

// fill it
myMultimap.put("hello", "hola");
myMultimap.put("hello", "buongiorno");
myMultimap.put("hello", "สวัสดี");

// retrieve
List greetings = myMultimap.get("hello");
                      // ["hola", "buongiorno", "สวัสดี"]

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Java 8 update: I'm no longer convinced that every Map> should be rewritten as a multimap. These days it's quite easy to get what you need without Guava, thanks to Map#computeIfAbsent().

Map> myMap = new HashMap<>();

// fill it
myMap.computeIfAbsent("hello", ignored -> new ArrayList<>())
  .addAll(Arrays.asList("hola", "buongiorno", "สวัสดี");

// retrieve
List greetings = myMap.get("hello");
                      // ["hola", "buongiorno", "สวัสดี"]