Map Implementation with Duplicate Keys

Map implementation with duplicate keys

You are searching for a multimap, and indeed both commons-collections and Guava have several implementations for that. Multimaps allow for multiple keys by maintaining a collection of values per key, i.e. you can put a single object into the map, but you retrieve a collection.

If you can use Java 5, I would prefer Guava's Multimap as it is generics-aware.

Multimap implementation with duplicate keys

You can use groupingBy which is the ideal case for you I think

Map<String, List<String>> res =
                dataTypes.stream().collect(Collectors.groupingBy(Map.Entry::getKey,
                             Collectors.mapping(Map.Entry::getValue, Collectors.toList())));

Mapping duplicate values from Map against the keys in which they are found

What you're trying to do is to invert the map (values become keys and keys get grouped by old values). There should be libraries that do that, but a sample solution with streams:

result = map.entrySet()
.stream()
.filter(e -> Collections.frequency(map.values(), e.getValue()) > 1)
.collect(Collectors.groupingBy(Map.Entry::getValue, Collectors.mapping(Map.Entry::getKey, Collectors.toList())))

How can i have a HashMap in Java with duplicate keys?

you need MultiMap,
take a look at Google Guava Multimap

java map with duplicate keys

$ cat YourMap.java
public class YourMap extends HashMap<String, List<Integer>> {
    public void put(String key, Integer number) {
        List<Integer> current = get(key);
        if (current == null) {
            current = new ArrayList<Integer>();
            super.put(key, current);
        }
        current.add(number);
    }

    public static void main(String args[]) {
        YourMap m = new YourMap();
        m.put("a", 1);
        m.put("a", 2);
        m.put("b", 3);
        for(Map.Entry e : m.entrySet()) {
            System.out.println(e.getKey() + " -> " + e.getValue());
        }
    }
}

$ java map
b -> [3]
a -> [1, 2]

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