Question

I have two maps with key as an integer and value as a double.

I want to create a third map sorted on key and value would be a list of doubles from both the maps.

map1:

1, 90.00
5, 75.45
8, 76.50

map2:

4, 12.00
5, 322.09
8, 11.09
9, 21.00

final map:

1, (90.00,0.00)
5, (75.45,322.09)
8, (76.50,11.09)
9, (0.00, 21.00)

As clear from above, if a key in one of the map is missing in the other map, the value in the final map for the other map should be defaulted to 0.00

Map firstMap = new HashMap<Integer, Double>();
Map secondMap = new HashMap<Integer, Double>();

Map finalMap = new HashMap<Integer, List<Double>>();

firstMap.put(1, 90.00);
firstMap.put(5, 75.45);
firstMap.put(8, 76.50);

secondMap.put(4, 12.00);
secondMap.put(5, 322.09);
secondMap.put(8, 11.09);
secondMap.put(9, 21.00);

I can use putAll method to put all keys into the third map. But how to put the values as I want ?

Thanks for reading!

Answer 1

Simply do

// Create a sorted map
Map<Integer, List<Double>> finalMap = new TreeMap<Integer, List<Double>>();

Set<Integer> keys = new HashSet<Integer>();
keys.addAll(firstMap.keySet());
keys.addAll(secondMap.keySet());
for (Integer key : keys) {
    double first  = firstMap.containsKey(key)  ? firstMap.get(key)  : 0.0;
    double second = secondMap.containsKey(key) ? secondMap.get(key) : 0.0;
    finalMap.put(key, Arrays.asList(first, second));
}

Using Guava it’s as simple as

Multimap<Integer, Double> finalMultiMap = ArrayListMultimap.create();
finalMultiMap.putAll(Multimaps.forMap(firstMap));
finalMultiMap.putAll(Multimaps.forMap(secondMap));

Map<Integer, Collection<Double>> finalMap = finalMultiMap.asMap();