HashMap中套map的作用

public static void main(String[] args) {
        HashMap<String, String> hashMap = new HashMap<>();
        hashMap.put("1", "11");
        hashMap.put("3","33");
        HashMap<String, String> hashMap1 = new HashMap<>(hashMap);
        hashMap1.put("2", "22");
        hashMap1.put("4","44");
        for (Map.Entry<String, String> entry : hashMap1.entrySet()) {
            String key = entry.getKey();
            String value = entry.getValue();
            System.out.println(key + ":" + value);
        }
    }