Recursively Flatten values of nested maps in Java 8

Question

Given a Map<String, Object>, where the values are either a String or another Map<String, Object>, how would one, using Java 8, flatten the maps to a single list of values?

Example:

Map - "key1" -> "value1"
    - "key2" -> "value2"
    - "key3" -> Map - "key3.1" -> "value3.1"
                    - "key3.2" -> "value3.2"
                    - "key3.3" -> Map - "key3.3.1" -> "value3.3.1"
                                      - "key3.3.2" -> "value3.3.2" 

For the above example, I would like the following list:

value1
value2
value3.1
value3.2
value3.3.1
value3.3.2

I know it can be done like this:

public static void main(String args[]) throws Exception {
    //Map with nested maps with nested maps with nested maps with nested......
    Map<String, Object> map = getSomeMapWithNestedMaps();

    List<Object> values = new ArrayList<>();
    addToList(map, values);

    for (Object o:values) {
        System.out.println(o);
    }
}

static void addToList(Map<String, Object>map, List<Object> list) {
    for (Object o:map.values()) {
        if (o instanceof Map) {
            addToList((Map<String, Object>)o, list);
        } else {
            list.add(o);
        }
    }
}

How can I do this with a Stream?

Edit:

After some playing around I figured it out:

public static void main(String args[]) throws Exception {
    //Map with nested maps with nested maps with nested maps with nested......
    Map<String, Object> map = getSomeMapWithNestedMaps();
    //Recursively flatten maps and print out all values
    List<Object> list= flatten(map.values().stream()).collect(Collectors.toList());
}

static Stream<Object> flatten(Stream<Object> stream) {
    return stream.flatMap((o) ->
        (o instanceof Map) ? flatten(((Map<String, Object>)o).values().stream()) : Stream.of(o)
    );
}

Answer 1

You could define a recursive method which flattens one map and use it as a function for Stream#flatMap or use it by calling it directly.

Example:

public class FlatMap {

    public static Stream<Object> flatten(Object o) {
        if (o instanceof Map<?, ?>) {
            return ((Map<?, ?>) o).values().stream().flatMap(FlatMap::flatten);
        }
        return Stream.of(o);
    }

    public static void main(String[] args) {
        Map<String, Object> map0 = new TreeMap<>();
        map0.put("key1", "value1");
        map0.put("key2", "value2");
        Map<String, Object> map1 = new TreeMap<>();
        map0.put("key3", map1);
        map1.put("key3.1", "value3.1");
        map1.put("key3.2", "value3.2");
        Map<String, Object> map2 = new TreeMap<>();
        map1.put("key3.3", map2);
        map2.put("key3.3.1", "value3.3.1");
        map2.put("key3.3.2", "value3.3.2");

        List<Object> collect = map0.values().stream()
                                            .flatMap(FlatMap::flatten)
                                            .collect(Collectors.toList());
        // or
        List<Object> collect2 = flatten(map0).collect(Collectors.toList());
        System.out.println(collect); 
    }
}

For the given nested map, it prints

[value1, value2, value3.1, value3.2, value3.3.1, value3.3.2]