java之map集合存放null值的问题

	public int deleteHead() {
        while(s2.isEmpty()){
        //注意这里，我直接判断第二个栈空就加入第一个栈中的元素，没有判断第一个栈是否为空（当时想的是第一个栈为空不会加入元素或者抛出异常）
            s2.offerFirst(s1.pollFirst());
        }
        if(s1.isEmpty()&&s2.isEmpty()){
            return -1;
        }
        return s2.pollFirst();
    }

	public int deleteHead() {
        while(s1.isEmpty()&&s2.isEmpty()){
            return -1;
        }
        if(s2.isEmpty()){
            while(!s1.isEmpty()){
                s2.offerFirst(s1.pollFirst());
            }
        }
        return s2.pollFirst();
    }

	@Test
    public void testArrayList() {
        ArrayList<Integer> list = new ArrayList<>();
        list.add(null);
        list.add(null);
        System.out.println(list.size());//2
    }

    @Test
    public void testLinkedList() {
        LinkedList<Integer> list = new LinkedList<>();
        list.add(null);
        list.add(null);
        System.out.println(list.size());//2
    }

	@Test
    public void testHashMap() {
        HashMap<Integer, Integer> map = new HashMap<>();
        map.put(null, null);
        map.put(null, null);
        System.out.println(map.size());//1
    }
    @Test
    public void testTreeMap() {
        TreeMap<Integer, Integer> map = new TreeMap<>();
        map.put(null, null);
        System.out.println(map.size());//NullPointerException
    }

    @Test
    public void VectorTest(){
        Vector box = new Vector();
        box.add(null);
        box.add(null);
        Assert.assertEquals(2,box.size()); //ok
    }

    /**
    *此为Vector的add函数
    **/
    public synchronized boolean add(E e) {
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = e;
        return true;
    }

    @Test
    public void HashTableTest(){
        Hashtable table = new Hashtable();
        //table.put(new Object(),null); //Exception
        //table.put(null,new Object()); //Exception
        //table.put(null,null); //Exception
        Assert.assertEquals(1,table.size());
    }

    //此为hashTable#put函数源码
   public synchronized V put(K key, V value) {
        // Make sure the value is not null
        if (value == null) {   //value 需要判空，所以value不可为null
            throw new NullPointerException();
        }

        // Makes sure the key is not already in the hashtable.
        Entry<?,?> tab[] = table;
        int hash = key.hashCode();     //key需要拥有实例去调用hashCode方法，所以也不能为空
        int index = (hash & 0x7FFFFFFF) % tab.length;
        @SuppressWarnings("unchecked")
        Entry<K,V> entry = (Entry<K,V>)tab[index];
        for(; entry != null ; entry = entry.next) {
            if ((entry.hash == hash) && entry.key.equals(key)) {
                V old = entry.value;
                entry.value = value;
                return old;
            }
        }

        addEntry(hash, key, value, index);
        return null;
    }