java需要关注的知识点---Hashtable

在concurrentHashMap出来之前，hashtable一直是作为线程安全的map使用。
hashtable默认的初始大小是:11.
1.Hashtable中的put方法。

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

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

    1.1 不同于HashMap,Hashtable不允许出现空的key.
    1.2 不同于HashMap,下标计算方式
           HashMap : h & (length-1)
           Hashtable: (hash & 0x7FFFFFFF) % tab.length
2 Hashtable中的rehash方法：

   protected void rehash() {
	int oldCapacity = table.length;
	Entry[] oldMap = table;

	int newCapacity = oldCapacity * 2 + 1;
	Entry[] newMap = new Entry[newCapacity];

	modCount++;
	threshold = (int)(newCapacity * loadFactor);
	table = newMap;

	for (int i = oldCapacity ; i-- > 0 ;) {
	    for (Entry<K,V> old = oldMap[i] ; old != null ; ) {
		Entry<K,V> e = old;
		old = old.next;

		int index = (e.hash & 0x7FFFFFFF) % newCapacity;
		e.next = newMap[index];
		newMap[index] = e;
	    }
	}
    }

      Hashtable的增长方式是按照原有长度的2倍再加上1的方式进行增长的，不同于hashMap，hashMap是按照原有长度的2倍。