Json – How to flatten a java object?

I've found a way using following code:

convertToFlatMap("", obj, new HashMap<>())

Implementation:

private static Map<String, Object> convertToFlatMap(String prefix, Object obj, Map<String, Object> map) {
        if (obj == null) {
            return map;
        }
        if (obj instanceof Map) {
            Map<?, ?> objMap = (Map<?, ?>) obj;
            objMap.forEach((key, value) -> {
                String fullKey = prefix.isEmpty() ? key.toString() : prefix + "." + key.toString();
                convertToFlatMap(fullKey, value, map);
            });
        } else if (obj.getClass().isArray()) {
            Object[] array = (Object[]) obj;
            for (int i = 0; i < array.length; i++) {
                String fullKey = prefix + "[" + i + "]";
                convertToFlatMap(fullKey, array[i], map);
            }
        } else if (obj instanceof Iterable) {
            Iterable<?> iterable = (Iterable<?>) obj;
            int i = 0;
            for (Object item : iterable) {
                String fullKey = prefix + "[" + i + "]";
                convertToFlatMap(fullKey, item, map);
                i++;
            }
        } else if (obj.getClass().isPrimitive() || obj instanceof String) {
            map.put(prefix, obj);
        } else {
            // recursively flatten nested object properties
            Map<String, Object> nestedMap = new HashMap<>();
            for (java.lang.reflect.Field field : obj.getClass().getDeclaredFields()) {
                field.setAccessible(true);
                String fullKey = prefix.isEmpty() ? field.getName() : prefix + "." + field.getName();
                try {
                    Object fieldValue = field.get(obj);
                    convertToFlatMap(fullKey, fieldValue, nestedMap);
                } catch (IllegalAccessException e) {
                    throw new RuntimeException("Error accessing field " + field.getName() + " of object " + obj.getClass().getName());
                }
            }
            map.putAll(nestedMap);
        }
        return map;
    }