java.util.Map

    public static void main(String[] args) {
        Map<String, String> map = new HashMap<String, String>();
        map.put("hello", "world");
        map.put(null, "HelloWorld");
        System.out.println(map);
        System.out.println(map.get(null));
        
        System.out.println("-----");
        for (String key : map.keySet()) {
            System.out.println(key+":"+map.get(key));
        }
    }