Join two maps by key

Question

I have two maps:

Map<Integer, String> mapOne = {(1,"a"), (2, "b")};
Map<Integer, Double> mapTwo = {(1,10.0), (2,20.0)};

and I want to combine this maps into one by Integer value, so the result map is

Map<String, Double> mapResult = {("a",10.0), ("b",20.0)};

Is there any way to do this easier than iterate over entry set?

Answer 1

Assuming that the keys of the two maps match and that the maps have the same number of entries, with Java 8 you can write it in one line with:

Map<String, Double> map = mapOne.entrySet().stream()
                            .collect(toMap(e -> e.getValue(),
                                           e -> mapTwo.get(e.getKey())));

So you start from the first map and create a new map where the keys are the values of mapOne and the values are the corresponding values in mapTwo.

Technically this is somewhat equivalent to iterating over the entry set of the first map though.

_{Note: requires import static java.util.stream.Collectors.toMap;}

Answer 2

Looks like only iteration:

@Test
public void testCollection() {
    Map<Integer, String> mapOne = new HashMap<Integer, String>();
    mapOne.put(1, "a");
    mapOne.put(2, "b");
    Map<Integer, Double> mapTwo = new HashMap<Integer, Double>();
    mapTwo.put(1, 10.0);
    mapTwo.put(2, 20.0);


    Map<String, Double> mapResult = new HashMap<String, Double>();
    Set<Integer> keySet = mapOne.keySet();
    keySet.retainAll(mapTwo.keySet());
    for (Integer value : keySet) {
        mapResult.put(mapOne.get(value), mapTwo.get(value));
    }
    System.out.println(mapResult);
}