HashMap HashTable ConcurrentHashMap key和value是否可以null的問題 源碼分析

我們都知道結論是：

HashMap可以允許插入null key和null value

HashTable和ConcurrentHashMap都不可以插入null key和null value

具體原因可以看下面的源碼：

首先是HashMap的put源碼：

 

public V put(K key, V value) {
       if (table == EMPTY_TABLE) {
           inflateTable(threshold);
       }
     if (key == null)
           return putForNullKey(value);
       int hash = hash(key);
       int i = indexFor(hash, table.length);
       for (Entry<K,V> e = table[i]; e != null; e = e.next) {
           Object k;
           if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
               V oldValue = e.value;
               e.value = value;
               e.recordAccess(this);
               return oldValue;
           }
       }

       modCount++;
       addEntry(hash, key, value, i);
       return null;
   }

null key的情況，調用putForNullKey方法，之前我在博客上已經寫過關於HashMap插入null key後的具體執行過程和源碼分析了。

下面是HashTable的源碼：

public synchronized V put(K key, V value) {
       // 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 = hash(key);
       int index = (hash & 0x7FFFFFFF) % tab.length;
       for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
           if ((e.hash == hash) && e.key.equals(key)) {
               V old = e.value;
               e.value = value;
               return old;
           }
       }

       modCount++;
       if (count >= threshold) {
           // Rehash the table if the threshold is exceeded
           rehash();

           tab = table;
           hash = hash(key);
           index = (hash & 0x7FFFFFFF) % tab.length;
       }

       // Creates the new entry.
       Entry<K,V> e = tab[index];
       tab[index] = new Entry<>(hash, key, value, e);
       count++;
       return null;
   }

源碼中判斷了null value的情況，null value就拋出空指針異常異常

但是我們並沒有看到判斷null key然後拋出異常的語句，那麼繼續看看hash方法的源碼就明白了：

private int hash(Object k) {
        // hashSeed will be zero if alternative hashing is disabled.
        return hashSeed ^ k.hashCode();
    }

k.hashCode()，當k出入爲null key 就會報錯 空指針異常。

下面是ConcurrentHashMap的源碼：

public V put(K key, V value) {
        Segment<K,V> s;
      if (value == null)
            throw new NullPointerException();
      int hash = hash(key);
        int j = (hash >>> segmentShift) & segmentMask;
        if ((s = (Segment<K,V>)UNSAFE.getObject          // nonvolatile; recheck
             (segments, (j << SSHIFT) + SBASE)) == null) //  in ensureSegment
            s = ensureSegment(j);
        return s.put(key, hash, value, false);
    }

null value和null key 會報錯的問題和HashTable原因是一樣的，hash方法：

private int hash(Object k) {
       int h = hashSeed;

       if ((0 != h) && (k instanceof String)) {
           return sun.misc.Hashing.stringHash32((String) k);
       }

       h ^= k.hashCode();

       // Spread bits to regularize both segment and index locations,
       // using variant of single-word Wang/Jenkins hash.
       h += (h <<  15) ^ 0xffffcd7d;
       h ^= (h >>> 10);
       h += (h <<   3);
       h ^= (h >>>  6);
       h += (h <<   2) + (h << 14);
       return h ^ (h >>> 16);
   }