Multiple values for a key in HashMap in Java

Question

Is it possible to keep multiple values corresponding to a key in a HashMap? If yes, how?

Solution 1

Yes, this is called chaining. You will want to avoid chaining as much as possible, especially if the size of the chain starts increasing. Longer chain size will counter the whole purpose of using a hash structure because the goal is to come as close to O(1) as possible.

Map<String, List<String>> hm = new HashMap<String, List<String>>();
List<String> values = new ArrayList<String>();
values.add("Value 1");
values.add("Value 2");
hm.put("Key1", values);

Solution 2

You could give a shot at Guava library (former Google collections). It has implementations of Multimaps which can store multiple values for a single key.

For example ListMultimap implementations allow duplicate key/value pairs which are kept in insertion order.

Here's how you'd use it:

ListMultimap<String, Integer> numberClasses = ArrayListMultimap.create();
numberClasses.put("odd", 1);
numberClasses.put("odd", 3);
numberClasses.put("odd", 5);

numberClasses.put("even", 2);
numberClasses.put("even", 4);
numberClasses.put("even", 6);

assertEquals(Arrays.asList(1,3,5), numberClasses.get("odd"));
assertEquals(Arrays.asList(2,4,6), numberClasses.get("even"));

Another cool example would be SetMultimap, which is very similar to ListMultimap except that values for a key are kept in a set. (From user perspective, I don't know how exactly it is implemented.)

SetMultimap<String, Integer> setMultimap= HashMultimap.create();
setMultimap.put("key1", 1);
setMultimap.put("key1", 1);
setMultimap.put("key1", 1);
setMultimap.put("key1", 2);

setMultimap.put("key2", 1);
setMultimap.put("key2", 3);

assertEquals(ImmutableSet.of(1,2), setMultimap.get("key1"));
assertEquals(ImmutableSet.of(1,3), setMultimap.get("key2"));

Solution 3

Use Map<String, List<String>>.

Comments

[Victor Sorokin]

or any other container, better suited for your task. For example, Set. Or just plain old array.

[Neo]

best answer i found for the problem :)

[rajarajan2809]

how can i get the values of "List" for key "Key1"