[JAVA] Create a multi-key map with the standard library

If you need a map with multiple keys, you can use a third-party library, or you can nest the standard library Map. Here, an example is a map with three keys. The first is get (). Returns null if no value with the specified key exists.

public static <A, B, C, D> D get(Map<A, Map<B, Map<C, D>>> map, A a, B b, C c) {
    return Optional.ofNullable(map.get(a))
        .map(m -> m.get(b))
        .map(m -> m.get(c))

Next is put ().

public static <A, B, C, D> void put(Map<A, Map<B, Map<C, D>>> map, A a, B b, C c, D d) {
    map.computeIfAbsent(a, t -> new HashMap<>())
        .computeIfAbsent(b, t -> new HashMap<>())
        .put(c, d);

If you know the pattern, you can easily make a map with 4 or more keys. Below is the test code.

public void testMultiKeyMap() {
    Map<Integer, Map<Integer, Map<Integer, Integer>>> map = new HashMap<>();
    for (int a = 0, value = 0; a < 2; ++a)
        for (int b = 0; b < 2; ++b)
            for (int c = 0; c < 2; ++c)
                put(map, a, b, c, value++);
    Map<Integer, Map<Integer, Map<Integer, Integer>>> expected =
        Map.of(0, Map.of(0, Map.of(0, 0,
                                   1, 1),
                         1, Map.of(0, 2,
                                   1, 3)),
               1, Map.of(0, Map.of(0, 4,
                                   1, 5),
                         1, Map.of(0, 6,
                                   1, 7)));
    assertEquals(expected, map);
    for (int a = 0, value = 0; a < 2; ++a)
        for (int b = 0; b < 2; ++b)
            for (int c = 0; c < 2; ++c)
                assertEquals(value++, get(map, a, b, c));
    assertEquals(null, get(map, 2, 2, 2));

map.toString () looks like this:

{0={0={0=0, 1=1}, 1={0=2, 1=3}}, 1={0={0=4, 1=5}, 1={0=6, 1=7}}}

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