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：
1 /**
2 * 使用指定的初始容量和装载因子构造一个空的Hashtable.
3 *
4 * @param initialCapacity hashtable的初始容量
5 * @param loadFactor hashtable的装载因子
6 * @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
7 */
8 public Hashtable(int initialCapacity, float loadFactor) {
9 if (initialCapacity < 0)
10 throw new IllegalArgumentException("Illegal Capacity: "+
11 initialCapacity);
12 if (loadFactor <= 0 || Float.isNaN(loadFactor))
13 throw new IllegalArgumentException("Illegal Load: "+loadFactor);
14
15 if (initialCapacity==0)
16 initialCapacity = 1;
17 this.loadFactor = loadFactor;
18 table = new Entry<?,?>[initialCapacity];
19 threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
20 }
21
22 /**
23 * Constructs a new, empty hashtable with the specified initial capacity
24 * and default load factor (0.75).
25 *
26 * @param initialCapacity the initial capacity of the hashtable.
27 * @exception IllegalArgumentException if the initial capacity is less
28 * than zero.
29 */
30 public Hashtable(int initialCapacity) {
31 this(initialCapacity, 0.75f);
32 }
33
34 /**
35 * Constructs a new, empty hashtable with a default initial capacity (11)
36 * and load factor (0.75).
37 */
38 public Hashtable() {
39 this(11, 0.75f);
40 }
41
42 /**
43 * Constructs a new hashtable with the same mappings as the given
44 * Map. The hashtable is created with an initial capacity sufficient to
45 * hold the mappings in the given Map and a default load factor (0.75).
46 *
47 * @param t the map whose mappings are to be placed in this map.
48 * @throws NullPointerException if the specified map is null.
49 * @since 1.2
50 */
51 public Hashtable(Map<? extends K, ? extends V> t) {
52 this(Math.max(2*t.size(), 11), 0.75f);
53 putAll(t);
54 }