深入浅析HashMap key和value能否为null

static final int hash(Object key) {
    int h;
    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

public class Test {
	public static void main(String[] args) {
		Map<String, String> map = new HashMap<String, String>();//HashMap对象
		Map<String, String> tableMap = new Hashtable<String, String>();//HashTable对象
		map.put(null, null);
		System.out.println("hashMap的[key]和[value]均可以为null:" + map.get(null));
		try {
			tableMap.put(null, "3");
			System.out.println(tableMap.get(null));
		} catch (Exception e) {
			System.out.println("【ERROR】：hashTable的[key]不能为null");
		}
		try {
			tableMap.put("3", null);
			System.out.println(tableMap.get("3"));
		} catch (Exception e) {
			System.out.println("【ERROR】：hashTable的[value]不能为null");
		}
	}
}

import java.util.HashMap;
import java.util.Hashtable;
public class TestMap {
    public static void main(String[] args){
        HashMap<Integer,Integer> map = new HashMap<>();
        System.out.println(map.containsKey(null));
        System.out.println(map.get(null));
        //验证 HashMap的key和value都可以为null
        //当key为空时，key的hash值为0
        map.put(null,null);
        System.out.println(map.containsKey(null));
        System.out.println(map.get(null));
        //当key为空时，key的hash值为0，所以如果再设置一个值会对原有value进行覆盖
        map.put(null,123);
        System.out.println(map.containsKey(null));
        System.out.println(map.get(null));
        //验证 Hashtable
        Hashtable<Integer,Integer> hashtable = new Hashtable<>();
        System.out.println(hashtable.containsKey(null));
        System.out.println(hashtable.get(null));
        //HashTable是线程安全的，key和value都不可以为null
        //HashMap是线程不安全的，他的key和value都可以为null
        //验证 HashMap的key和value都可以为null
        //当key为空时，key的hash值为0
        hashtable.put(null,null);
        System.out.println(hashtable.containsKey(null));
        System.out.println(hashtable.get(null));
    }
}

    public synchronized V put(K key, V value) {
        // 确保value不为空。这句代码过滤掉了所有value为null的键值对。因此Hashtable不能
        // 存储value为null的键值对
        if (value == null) {
            throw new NullPointerException();
        }
        // 确保key在table数组中尚未存在。
        Entry<?,?> tab[] = table;
        int hash = key.hashCode(); //在此处计算key的hash值，如果此处key为null，则直接抛出空指针异常。
        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;
    }

public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }