public static void main(String[] args) {
        Map<Object, Object> hashMap = new HashMap<>();
        hashMap.put("a", 1);
        hashMap.put("b", 2);
        hashMap.put(null, "");
        hashMap.forEach((k, v) -> {
            System.out.println("key=" + k);
            System.out.println("value=" + v);
        });
    }