So how do you serialize NULL ?
NULL would typically mean that the attribute is omitted from the json, but what if you WANT the NULL to be there, to symbolize an attribute that should be REMOVED.

    @AllArgsConstructor
    @NoArgsConstructor
    @Getter
    @Setter
    @ToString
    public static class Product {
        public String model;
        public String color;
        public String shirtSize;
    }

    @Test
    void howToSerializeNULL() {
        Product bossShirt = new Product("super slim", "red", "xl");
        System.out.println("Product : " + JSONUtils.stringify(bossShirt));
        bossShirt.setShirtSize(null);
        System.out.println("Product : " + JSONUtils.stringify(bossShirt));
        Map<String, Object> obj = new HashMap<>();
        obj.put("model", "super slim");
        obj.put("color", "red");
        obj.put("shirtSize", JSONObject.NULL);
        System.out.println("Product : " + JSONUtils.stringify(obj));
        GsonBuilder builder = new GsonBuilder();
        builder.serializeNulls();
        Gson gson = builder.create();
        System.out.println("Product : " + gson.toJson(obj));
        System.out.println("Product : " + gson.toJson(bossShirt));
    }