JDK源码阅读--Hashtable

public class Hashtable<K,V>
    extends Dictionary<K,V>
    implements Map<K,V>, Cloneable, java.io.Serializable {

Hashtable的默认初始容量是11，加载因子是0.75f。通过synchronized方法保证线程安全。
实现结构与HashMap基本一致。Hashtable的键和值都不能为空。

Hashtable的构造函数：

     /**
      * 使用指定的初始容量和装载因子构造一个空的Hashtable.
      *
      * @param      initialCapacity   hashtable的初始容量
      * @param      loadFactor        hashtable的装载因子
      * @exception  IllegalArgumentException  如果初始容量小于0，或装载因子是负数，则抛出IllegalArgumentException异常
      */
     public Hashtable(int initialCapacity, float loadFactor) {
         if (initialCapacity < 0)
             throw new IllegalArgumentException("Illegal Capacity: "+
                                                initialCapacity);
         if (loadFactor <= 0 || Float.isNaN(loadFactor))
             throw new IllegalArgumentException("Illegal Load: "+loadFactor);

         if (initialCapacity==0)
             initialCapacity = 1;
         this.loadFactor = loadFactor;
         table = new Entry<?,?>[initialCapacity];
         threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
     }

     /**
      * Constructs a new, empty hashtable with the specified initial capacity
      * and default load factor (0.75).
      *
      * @param     initialCapacity   the initial capacity of the hashtable.
      * @exception IllegalArgumentException if the initial capacity is less
      *              than zero.
      */
     public Hashtable(int initialCapacity) {
         this(initialCapacity, 0.75f);
     }

     /**
      * Constructs a new, empty hashtable with a default initial capacity (11)
      * and load factor (0.75).
      */
     public Hashtable() {
         this(11, 0.75f);
     }

     /**
      * Constructs a new hashtable with the same mappings as the given
      * Map.  The hashtable is created with an initial capacity sufficient to
      * hold the mappings in the given Map and a default load factor (0.75).
      *
      * @param t the map whose mappings are to be placed in this map.
      * @throws NullPointerException if the specified map is null.
      * @since   1.2
      */
     public Hashtable(Map<? extends K, ? extends V> t) {
         this(Math.max(2*t.size(), 11), 0.75f);
         putAll(t);
     }