HashMap

key，value都可以为null

    static final int hash(Object key) {

        int h;

        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);

    }

key只能有一个为null，多个key=null的会覆盖，value可以多个为null

        HashMap<Integer, Integer> map = new HashMap<>();

        map.put(1, null);

        map.put(2, null);

        map.put(null, 1);

        map.put(null, 2);

        System.out.println(map);//{null=2, 1=null, 2=null}

Hashtable

key，value都不能为null

        // Make sure the value is not null

        if (value == null) {

            throw new NullPointerException();

        }

        // Makes sure the key is not already in the hashtable.

        Entry<?,?> tab[] = table;

        int hash = key.hashCode();

直接调用key.hashcode方法，所以key不能为null
value为null，抛出空指针

ConcurrentHashMap

key，value都不能为null

        if (key == null || value == null) throw new NullPointerException();

        int hash = spread(key.hashCode());

key或value为null，抛出空指针
key调用hashcode方法后，用spread方法二次hash

TreeMap

key不能为null，value可以为null

            if (key == null)

                throw new NullPointerException();

key为null，抛出空指针