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 <code>key</code> to the specified
* <code>value</code> in this hashtable. Neither the key nor the
* value can be <code>null</code>. <p>
*
* The value can be retrieved by calling the <code>get</code> 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 <code>null</code> if it did not have one
* @exception NullPointerException if the key or value is
* <code>null</code>
* @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<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;
}