Merging combine and composing Maps

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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

Found a mistake? Have a question or improvement idea? Let me know.

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