1

I have several temporary TreeMaps which I would like to combine into one Super TreeMap, a union of the smaller TreeMaps.

The generic type of my TreeMaps is

TreeMap<String, Set<Integer>>

When I try to call

SuperTreeMap.addALL(temp)

I received the following error

Error: cannot find symbol.
zach
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2 Answers2

0

You canot union all Set as values with putAll. Each new Set will replace existed one. You have to do it manyally:

Map<String, Set<Integer>> map = new TreeMap<>();
Map<String, Set<Integer>> map1 = new TreeMap<>();
Map<String, Set<Integer>> map2 = new TreeMap<>();

map1.put("one", new HashSet<>());
map1.get("one").add(1);
map1.get("one").add(2);
map1.get("one").add(3);

map1.put("two", new HashSet<>());
map1.get("two").add(1);
map1.get("two").add(2);
map1.get("two").add(3);

map2.put("one", new HashSet<>());
map2.get("one").add(4);
map2.get("one").add(5);
map2.get("one").add(6);

map2.put("two", new HashSet<>());
map2.get("two").add(4);
map2.get("two").add(5);
map2.get("two").add(6);

for (Map<String, Set<Integer>> m : Arrays.asList(map1, map2)) {
    for (Map.Entry<String, Set<Integer>> entry : m.entrySet()) {
        if (!map.containsKey(entry.getKey()))
            map.put(entry.getKey(), new HashSet<>());
        map.get(entry.getKey()).addAll(entry.getValue());
    }
}
Oleg Cherednik
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0

You can create a custom addALL method to combine the entries of two maps and inside them the elements of two collections:

/**
 * Combines the entries of several maps into the {@link TreeMap}
 * and the elements of sets inside them into the {@link TreeSet}.
 *
 * @param maps the collection of maps to be combined
 * @param <T>  the type of keys of maps
 * @param <U>  the type of elements of sets
 * @return combined map of sets
 */
public static <T, U> Map<T, Set<U>> addAll(Collection<Map<T, Set<U>>> maps) {
    return maps.stream()
            // Stream<Map.Entry<String,Set<Integer>>>
            .flatMap(map -> map.entrySet().stream())
            // stream of map entries into one map
            .collect(Collectors.toMap(
                    // key is the same
                    Map.Entry::getKey,
                    // value is the same
                    Map.Entry::getValue,
                    // combining elements of two collections
                    (set1, set2) -> Stream.concat(set1.stream(), set2.stream())
                            // collectionFactory
                            .collect(Collectors.toCollection(TreeSet::new)),
                    // mapFactory
                    TreeMap::new));
}

// test
public static void main(String[] args) {
    Map<String, Set<Integer>>
            map1 = Map.of("one", Set.of(1, 2, 3), "two", Set.of(1, 2, 3)),
            map2 = Map.of("one", Set.of(4, 5, 6), "two", Set.of(4, 5, 6)),
            // combine the entries of two maps into one map
            map3 = addAll(Set.of(map1, map2));
    // output
    System.out.println(map3);
    // {one=[1, 2, 3, 4, 5, 6], two=[1, 2, 3, 4, 5, 6]}
}

See also: How do I take the union of sets?