Looping through hashmap to group the values of same key into a <key, list<values>> pair

Question

I've been having a hard time trying to think of a way create a HashMap that groups values (into a list) that has the same key. This is what I mean:

Say I have the following keys and values:

Value     Key  *Sorry I got the columns swapped
1         10 
1         11 
1         12 
2         20 
3         30 
3         31 

I would like to put these values into a

Hashmap <Integer, List<Integer>>

So that it will group the values into the List Integer that has the same key, something like this:

(1, {10, 11, 12}),(2, {20}), (3, {30,31})

Right now the key and the value are stored in a

Hashmap <Integer, Integer>

And I'm lost at how to loop through this Hashmap to create the new Hashmap with the key: List of Values pair. Does anyone have a good approach to this topic?

Answer 1

Suppose you create a HashMap<Integer, List<Integer>>, and you want to add a key-value pair to it the way you asked, you can use the following method:

public void addToMap(HashMap<Integer, List<Integer>> map, Integer key, Integer value){
  if(!map.containsKey(key)){
    map.put(key, new ArrayList<>());
  }
  map.get(key).add(value);
}

Using this method with your example data:

HashMap<Integer, List<Integer>> map = new HashMap<Integer, List<Integer>>();
addToMap(map, 1, 10); 
addToMap(map, 1, 11);
addToMap(map, 2, 20);
addToMap(map, 3, 30);
addToMap(map, 3, 31);

Answer 2

Instead of a plain Map use Google Guava's Multimap.

A Multimap is a

...collection that maps keys to values, similar to Map, but in which each key may be associated with multiple values.

This concept has of course been implemented in other libraries as well, Guava is just my personal preference.

Answer 3

HashMap will only store 1 value for each Integer. So iterating over it should only gives you the following values:

Key      Value 
1         12 
2         20 
3         31 

To iterate over the content of a Map you can use the entrySet() method:

for(Map.Entry<Integer, Integer> entry : map.entrySet()) {
    System.out.println(entry.getKey() + " = " + entry.getValue());
}

To build a Map of List, I recommand doing this:

List<Integer> list = map.get(key);
if(list == null) {
    list = new ArrayList<Integer>();
    map.put(key, list);
}
list.add(value);