Flatten nested Map containing with unknown level of nested Arrays and Maps recursively

Question

I have a nested HashMap with String keys that contains either List, Map, or String values. I would like to f

Answer 1

You can use recursion to flatten the Map. Each time you encounter a Map, recurse by flattening that Map; when you encounter a List, iterate over it and add the index to the current key. A single value can be trivially set otherwise. See the below code in action here.

public static Map flatten(final Map map) {
    return flatten("", map, new HashMap<>());
    //use new TreeMap<>() to order map based on key
}

@SuppressWarnings("unchecked")//recursive helper method
private static Map flatten(final String key, final Map map,
        final Map result) {
    final Set> entries = map.entrySet();
    if (!entries.isEmpty()) {
        for (final Map.Entry entry : entries) {
            //iterate over entries
            final String currKey = key + (key.isEmpty() ? "" : '.') + entry.getKey();
           //append current key to previous key, adding a dot if the previous key was not an empty String
            final Object value = entry.getValue();
            if (value instanceof Map) {//current value is a Map
                flatten(currKey, (Map) value, result);//flatten Map
            } else if (value instanceof List) {//current value is a List
                final List