Restoring a value tree from its flat map representation

Question

I have a flat map of type Map<String, String> from which I would like to restore the original value tree Map<String, Object>. Elements of the tree can be either Map<String, Object>, or List<Object>, or simply String. The depth of the tree is not limited. For example:

public static void main(String[] args) {
    TreeMap<String, String> flatMap = new TreeMap<String, String>() {{
        put("1999.3:1", "23");
        put("1999.3:2", "24");
        put("1999.3:3", "25");
        put("1999.4:1", "1");
        put("1999.4:2", "2");
        put("1999.4:3.10", "42");
        put("2001.11.7:1", "23");
        put("2001.11.7:2", "24");
        put("2001.11.7:3", "25");
        put("2001.11.9:1", "1");
        put("2001.11.9:2", "2");
        put("2001.11.9:3", "3");
        put("2001.12", "45");
    }};
    System.out.println(flatMap);
}

Flat 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}

Original value tree:

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

---

^{Opposite: Flatten nested Map containing with unknown level of nested Arrays and Maps recursively.}

Answer 1

Iterate over flat map keys and restore original tree. For each key left to right find delimiter characters:

• Dot . - nested object is a map. The next number is the key in the map.
• Colon : - nested object is a list. The next number is the index in the list (starts fom 1).

Then recursively process nested objects, shifting to the right by the key of the flat map:

public static void main(String[] args) {
    TreeMap<String, String> flatMap = new TreeMap<String, String>() {{
        put("1999.3:1", "23");
        put("1999.3:2", "24");
        put("1999.3:3", "25");
        put("1999.4:1", "1");
        put("1999.4:2", "2");
        put("1999.4:3.10", "42");
        put("2001.11.7:1", "23");
        put("2001.11.7:2", "24");
        put("2001.11.7:3", "25");
        put("2001.11.9:1", "1");
        put("2001.11.9:2", "2");
        put("2001.11.9:3", "3");
        put("2001.12", "45");
    }};
    TreeMap<String, Object> originalMap = new TreeMap<>();
    flatMap.forEach((key, value) -> processMap(key, value, originalMap));

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

@SuppressWarnings("unchecked")
private static void processMap(String flatMapKey,
                               String flatMapValue,
                               Map<String, Object> map) {
    int dot = flatMapKey.indexOf('.');
    int colon = flatMapKey.indexOf(':');

    // nested map
    if (dot > -1 && (dot < colon || colon == -1)) {
        String key = flatMapKey.substring(0, dot);
        Object nestedMap = map.get(key);
        if (nestedMap == null) {
            map.put(key, new TreeMap<>());
        }
        nestedMap = map.get(key);
        processMap(flatMapKey.substring(dot + 1),
                flatMapValue, (Map<String, Object>) nestedMap);
    }
    // nested list
    else if (colon > -1 && (colon < dot || dot == -1)) {
        String key = flatMapKey.substring(0, colon);
        Object nestedList = map.get(key);
        if (nestedList == null) {
            map.put(key, new ArrayList<>());
        }
        nestedList = map.get(key);
        processList(flatMapKey.substring(colon + 1),
                flatMapValue, (List<Object>) nestedList);
    }
    // insert value
    else if (dot == colon) {
        map.put(flatMapKey, flatMapValue);
    }
}

@SuppressWarnings("unchecked")
private static void processList(String flatMapKey,
                                String flatMapValue,
                                List<Object> list) {
    int dot = flatMapKey.indexOf('.');
    int colon = flatMapKey.indexOf(':');

    // nested map
    if (dot > -1 && (dot < colon || colon == -1)) {
        int index = Integer.parseInt(flatMapKey.substring(0, dot));
        while (list.size() < index) {
            list.add(null);
        }
        Object nestedMap = list.get(index - 1);
        if (nestedMap == null) {
            list.set(index - 1, new TreeMap<>());
        }
        nestedMap = list.get(index - 1);
        processMap(flatMapKey.substring(dot + 1),
                flatMapValue, (Map<String, Object>) nestedMap);
    }
    // nested list
    else if (colon > -1 && (colon < dot || dot == -1)) {
        int index = Integer.parseInt(flatMapKey.substring(0, colon));
        while (list.size() < index) {
            list.add(null);
        }
        Object nestedList = list.get(index - 1);
        if (nestedList == null) {
            list.set(index - 1, new ArrayList<>());
        }
        nestedList = list.get(index - 1);
        processList(flatMapKey.substring(colon + 1),
                flatMapValue, (List<Object>) nestedList);
    }
    // insert value
    else if (dot == colon) {
        int index = Integer.parseInt(flatMapKey);
        while (list.size() < index) {
            list.add(null);
        }
        list.set(index - 1, flatMapValue);
    }
}

---

Flat 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}

Original tree:

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