HashMap和Hashtable

https://www.cnblogs.com/williamjie/p/9099141.html

HashMap的key和value都可以为null

Hashtable的key和value都不能为null

    @Test
    public void test1() {
        Map<Object, Object> map = new HashMap<>();
        map.put("a", "aaa");
        map.put("b", "bbb");
        map.put("c", "ccc");
        map.put("d", null);
        map.put(null, "eee");
        System.out.println(map.get("d")); //null 值为null
        System.out.println(map.get("e")); //null 没有key
        System.out.println(map.get(null)); //null 没有key
        System.out.println(map.containsKey(null));
    }

    @Test
    public void test2() {
        Hashtable<Object, Object> table = new Hashtable<>();
        table.put("a", "aaa");
        table.put("b", "bbb");
        table.put("c", "ccc");
//        table.put("d", null);       //NullPointerException
//        table.put(null, "eee");     //NullPointerException
        System.out.println(table.get("a")); //null 值为null
        System.out.println(table.get("b")); //null 没有key
        System.out.println(table.contains("aaa"));
        System.out.println(table.containsValue("aaa"));
    }