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本文的目的并不是让你对 一、 类定义public class Hashtable<K,V> extends Dictionary<K,V> implements Map<K,V>, Cloneable, java.io.Serializable
可以看到它和 /**
* Maps the specified <code>key</code> to the specified
* <code>value</code> in this dictionary. Neither the key nor the
* value can be <code>null</code>.
*/abstract public V put(K key, V value);abstract public Enumeration<K> keys();abstract public Enumeration<V> elements();public interface Enumeration<E> { boolean hasMoreElements(); E nextElement();
}同
二、构造方法和成员变量private transient Entry<?,?>[] table; // 哈希槽private int threshold; // 阈值private float loadFactor; // 负载系数 以上三个应该就是 Map 中最重要的成员变量了，阈值和负载系数控制扩容时机，同 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);
}public Hashtable(int initialCapacity) { this(initialCapacity, 0.75f);
}public Hashtable() { this(11, 0.75f);
}public Hashtable(Map<? extends K, ? extends V> t) { this(Math.max(2*t.size(), 11), 0.75f);
putAll(t);
}如代码所示四个构造函数，主要就是为了初始化以上三个成员变量，但是注意
三、重要方法1. 哈希槽定位int index = (hash & 0x7FFFFFFF) % tab.length; 哈希表中最重要的方法肯定是哈希槽定位，如上面的原因 2. get 方法public synchronized V get(Object key) {
Entry<?,?> tab[] = table; int hash = key.hashCode(); int index = (hash & 0x7FFFFFFF) % tab.length; for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) { if ((e.hash == hash) && e.key.equals(key)) { return (V)e.value;
}
} return null;
}注意
private static class A {
String name;
public A(String name) {this.name = name;} @Override
public boolean equals(Object o) { ... } @Override
public int hashCode() { ... }
}private static void test01() {
Map<A, String> map = new Hashtable<>();
A a1 = new A("a");
A a2 = new A("a");
map.put(a1, "a");
map.put(a2, "a");
System.out.println(map.get(a1));
a1.name = "b";
System.out.println(map.get(a1));
}// 打印： 3. put 方法public synchronized V put(K key, V value) { // Make sure the value is not null
if (value == null) { throw new NullPointerException();
} // Makes sure the key is not already in the hashtable.
Entry<?,?> tab[] = table; int hash = key.hashCode(); 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;
}
4. addEntry 方法private void addEntry(int hash, K key, V value, int index) {
modCount ;
Entry<?,?> tab[] = table; if (count >= threshold) { // Rehash the table if the threshold is exceeded
rehash();
tab = table;
hash = key.hashCode();
index = (hash & 0x7FFFFFFF) % tab.length;
} // Creates the new entry.
@SuppressWarnings("unchecked")
Entry<K,V> e = (Entry<K,V>) tab[index];
tab[index] = new Entry<>(hash, key, value, e);
count ;
}private static class Entry<K,V> implements Map.Entry<K,V> { final int hash; final K key;
V value;
Entry<K,V> next; protected Entry(int hash, K key, V value, Entry<K,V> next) { this.hash = hash; this.key = key; this.value = value; this.next = next;
}
...
}这里添加元素的时候首先判断是否扩容，然后添加节点；值得注意的是添加的节点是直接放在哈希槽里的（ 5. rehash 方法protected void rehash() { int oldCapacity = table.length;
Entry<?,?>[] oldMap = table; // overflow-conscious code
int newCapacity = (oldCapacity << 1) 1; if (newCapacity - MAX_ARRAY_SIZE > 0) { if (oldCapacity == MAX_ARRAY_SIZE) // Keep running with MAX_ARRAY_SIZE buckets
return;
newCapacity = MAX_ARRAY_SIZE;
}
Entry<?,?>[] newMap = new Entry<?,?>[newCapacity];
modCount ;
threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE 1);
table = newMap; for (int i = oldCapacity ; i-- > 0 ;) { for (Entry<K,V> old = (Entry<K,V>)oldMap[i] ; old != null ; ) {
Entry<K,V> e = old;
old = old.next; int index = (e.hash & 0x7FFFFFFF) % newCapacity;
e.next = (Entry<K,V>)newMap[index];
newMap[index] = e;
}
}
}扩容的时候也是，先计算新容量，在得到一个新的哈希槽，然后将节点在依次放入；同添加节点一样是将节点直接放到哈希槽中，那么在扩容完毕之后，链表的相对顺序会反向； 总结
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