HashMap with multiple values under the same key

Question

Is it possible for us to implement a HashMap with one key and two values. Just as HashMap?

Please do help me, also by telling (if there is no way) any other way to

Answer 1

Just for the record, the pure JDK8 solution would be to use Map::compute method:

map.compute(key, (s, strings) -> strings == null ? new ArrayList<>() : strings).add(value);

Such as

public static void main(String[] args) {
    Map> map = new HashMap<>();

    put(map, "first", "hello");
    put(map, "first", "foo");
    put(map, "bar", "foo");
    put(map, "first", "hello");

    map.forEach((s, strings) -> {
        System.out.print(s + ": ");
        System.out.println(strings.stream().collect(Collectors.joining(", ")));
    });
}

private static  void put(Map> map, KEY key, VALUE value) {
    map.compute(key, (s, strings) -> strings == null ? new ArrayList<>() : strings).add(value);
}

with output:

bar: foo
first: hello, foo, hello

Note that to ensure consistency in case multiple threads access this data structure, ConcurrentHashMap and CopyOnWriteArrayList for instance need to be used.