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 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 treeMap = new TreeMap() {{
        put("1999", new TreeMap() {{
            put("3", Arrays.asList("23", "24", "25"));
            put("4", Arrays.asList("1", "2", new TreeMap() {{
                put("10", "42");
            }}));
        }});
        put("2001", new TreeMap() {{
            put("11", new TreeMap() {{
                put("7", Arrays.asList("23", "24", "25"));
                put("9", Arrays.asList("1", "2", "3"));
            }});
            put("12", "45");
        }});
    }};

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

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

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

private static void processList(String prefix,
                                List