Java Language Merging, combine and composing Maps


Example

Use putAll to put every member of one map into another. Keys already present in the map will have their corresponding values overwritten.

Map<String, Integer> numbers = new HashMap<>();
numbers.put("One", 1)
numbers.put("Three", 3)
Map<String, Integer> other_numbers = new HashMap<>();
other_numbers.put("Two", 2)
other_numbers.put("Three", 4)

numbers.putAll(other_numbers)

This yields the following mapping in numbers:

"One" -> 1
"Two" -> 2
"Three" -> 4 //old value 3 was overwritten by new value 4

If you want to combine values instead of overwriting them, you can use Map.merge, added in Java 8, which uses a user-provided BiFunction to merge values for duplicate keys. merge operates on individual keys and values, so you'll need to use a loop or Map.forEach. Here we concatenate strings for duplicate keys:

for (Map.Entry<String, Integer> e : other_numbers.entrySet())
    numbers.merge(e.getKey(), e.getValue(), Integer::sum);
//or instead of the above loop
other_numbers.forEach((k, v) -> numbers.merge(k, v, Integer::sum));

If you want to enforce the constraint there are no duplicate keys, you can use a merge function that throws an AssertionError:

mapA.forEach((k, v) ->
    mapB.merge(k, v, (v1, v2) ->
        {throw new AssertionError("duplicate values for key: "+k);}));

Composing Map<X,Y> and Map<Y,Z> to get Map<X,Z>

If you want to compose two mappings, you can do it as follows

    Map<String, Integer> map1 = new HashMap<String, Integer>();
    map1.put("key1", 1);
    map1.put("key2", 2);
    map1.put("key3", 3);
    
    Map<Integer, Double> map2 = new HashMap<Integer, Double>();
    map2.put(1, 1.0);
    map2.put(2, 2.0);
    map2.put(3, 3.0);

    Map<String, Double> map3 = new new HashMap<String, Double>();
    map1.forEach((key,value)->map3.put(key,map2.get(value)));

This yields the following mapping

    "key1" -> 1.0
    "key2" -> 2.0
    "key3" -> 3.0