HashTable和Vector为什么逐渐被废弃

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HashTable，不允许键值为null，还一个就是put方法使用sychronized方法进行线程同步，单线程无需同步，多线程可用concurren包的类型。 如编程思想里面说的作为工具类，封闭性做的不好没有一个方法变量是final的，作为java1.0/1.1时代的容器逐渐被HashMap取代。同样Vector也有同样的问题，作为普通集合类，添加数据的方法也是做了同步，效率受极大影响。

HashTable源码中的put，键和值都不允许为null 
   /**
     * Maps the specified key to the specified
     * value in this hashtable. Neither the key nor the
     * value can be null. 

     *
     * The value can be retrieved by calling the get method
     * with a key that is equal to the original key.
     *
     * @param      key     the hashtable key
     * @param      value   the value
     * @return     the previous value of the specified key in this hashtable,
     *             or null if it did not have one
     * @exception  NullPointerException  if the key or value is
     *               null
     * @see     Object#equals(Object)
     * @see     #get(Object)
     */
    public synchronized V put(K key, V value) {
        // Make sure the value is not null，值为null报空指针

        if (value == null) {
            throw new NullPointerException();
        }

        // Makes sure the key is not already in the hashtable.
        Entry tab[] = table;
        int hash = key.hashCode();//key为null，无法计算hashcode也会报空指针
        int index = (hash & 0x7FFFFFFF) % tab.length;
        @SuppressWarnings("unchecked")
        Entry entry = (Entry)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;
    }

 

 

转载于:https://my.oschina.net/u/1271447/blog/2988285