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);
    }