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

@SuppressWarnings("unchecked")//recursive helper method
private static Map<String, Object> flatten(final String key, final Map<String, Object> map,
        final Map<String, Object> result) {
    final Set<Map.Entry<String, Object>> entries = map.entrySet();
    if (!entries.isEmpty()) {
        for (final Map.Entry<String, Object> 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<String, Object>) value, result);//flatten Map
            } else if (value instanceof List) {//current value is a List
                final List<Object> list = (List<Object>) value;
                for (int i = 0, size = list.size(); i < size; i++) {
                    result.put(currKey + '.' + (i + 1), list.get(i));
                }
                //iterate over the List and append the index to the current key when setting value
            } else {
                result.put(currKey, value);//set normal value
            }
        }
    }
    return result;
}
public static void main(final String[] args){
    final Map<String, Object> flattened = flatten(dates);
    System.out.println(flattened);
}

Answer 2

You can iterate over this map, and process each entry value, depending on its instance of: Map, List, or String. Since the level of nested arrays or maps is unknown, I have modified a little your code example and flat map format for clarity, also I used TreeMap instead of HashMap for entries ordering.

public static void main(String[] args) {
    TreeMap<String, Object> treeMap = new TreeMap<String, Object>() {{
        put("1999", new TreeMap<String, Object>() {{
            put("3", Arrays.asList("23", "24", "25"));
            put("4", Arrays.asList("1", "2", new TreeMap<String, Object>() {{
                put("10", "42");
            }}));
        }});
        put("2001", new TreeMap<String, Object>() {{
            put("11", new TreeMap<String, Object>() {{
                put("7", Arrays.asList("23", "24", "25"));
                put("9", Arrays.asList("1", "2", "3"));
            }});
            put("12", "45");
        }});
    }};

    TreeMap<String, String> flatMap = new TreeMap<>();
    processMap("", treeMap, flatMap);

    System.out.println(treeMap);
    System.out.println(flatMap);
}

private static void processMap(String prefix,
                               Map<String, Object> map,
                               Map<String, String> flatMap) {
    for (Map.Entry<String, Object> entry : map.entrySet()) {
        String key = entry.getKey();
        Object value = entry.getValue();
        processEntry(prefix, key, value, flatMap);
    }
}

private static void processList(String prefix,
                                List<Object> list,
                                Map<String, String> flatMap) {
    for (int i = 0; i < list.size(); i++) {
        String key = String.valueOf(i + 1);
        Object value = list.get(i);
        processEntry(prefix, key, value, flatMap);
    }
}

@SuppressWarnings("unchecked")
private static void processEntry(String prefix,
                                 String key,
                                 Object value,
                                 Map<String, String> flatMap) {
    if (value instanceof Map) {
        processMap(prefix + key + ".", (Map<String, Object>) value, flatMap);
    } else if (value instanceof List) {
        processList(prefix + key + ":", (List<Object>) value, flatMap);
    } else if (value instanceof String) {
        flatMap.put(prefix + key, (String) value);
    }
}

---

Sample map:

{1999={3=[23, 24, 25], 4=[1, 2, {10=42}]},
 2001={11={7=[23, 24, 25], 9=[1, 2, 3]}, 12=45}}

Flattened map:

{1999.3:1=23, 1999.3:2=24, 1999.3:3=25, 1999.4:1=1, 1999.4:2=2, 1999.4:3.10=42,
 2001.11.7:1=23, 2001.11.7:2=24, 2001.11.7:3=25,
 2001.11.9:1=1, 2001.11.9:2=2, 2001.11.9:3=3, 2001.12=45}

---

^{Opposite: Restoring a value tree from its flat map representation.}