HashTable特性

  • HashTable是线程安全的集合，通过synchronized关键字实现；
  • HashTable的key和value均不允许为null，使用会产生空指针问题；

HashTable源码分析

  • table是一个数组，表示哈希桶；
  • count表示存储的元素个数；
  • loadFactor表示负载因子，默认值是0.75，这是时间和空间权衡后的最优值；
  • threshold表示元素存储的阈值，容量*负载因子可以得到该值；

HashTable初始化

  1.     /**
  2.      * Constructs a new, empty hashtable with the specified initial
  3.      * capacity and the specified load factor.
  4.      *
  5.      * @param      initialCapacity   the initial capacity of the hashtable.
  6.      * @param      loadFactor        the load factor of the hashtable.
  7.      * @throws     IllegalArgumentException  if the initial capacity is less
  8.      *             than zero, or if the load factor is nonpositive.
  9.      */
  10.     public Hashtable(int initialCapacity, float loadFactor) {
  11.         if (initialCapacity < 0)
  12.             throw new IllegalArgumentException("Illegal Capacity: "+
  13.                                                initialCapacity);
  14.         if (loadFactor <= 0 || Float.isNaN(loadFactor))
  15.             throw new IllegalArgumentException("Illegal Load: "+loadFactor);
  17.         if (initialCapacity==0)
  18.             initialCapacity = 1;
  19.         //初始化负载因子、数组和存储元素的阈值
  20.         this.loadFactor = loadFactor;
  21.         table = new Entry<?,?>[initialCapacity];
  22.         threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
  23.     }

put操作

  1.     /**
  2.      * Maps the specified {@code key} to the specified
  3.      * {@code value} in this hashtable. Neither the key nor the
  4.      * value can be {@code null}. <p>
  5.      *
  6.      * The value can be retrieved by calling the {@code get} method
  7.      * with a key that is equal to the original key.
  8.      *
  9.      * @param      key     the hashtable key
  10.      * @param      value   the value
  11.      * @return     the previous value of the specified key in this hashtable,
  12.      *             or {@code null} if it did not have one
  13.      * @throws     NullPointerException  if the key or value is
  14.      *               {@code null}
  15.      * @see     Object#equals(Object)
  16.      * @see     #get(Object)
  17.      */
  18.     public synchronized V put(K key, V value) {
  19.         // HashTable的key和value都不允许为null，否则会产生空指针
  20.         if (value == null) {
  21.             throw new NullPointerException();
  22.         }
  24.         // Makes sure the key is not already in the hashtable.
  25.         Entry<?,?> tab[] = table;
  26.         int hash = key.hashCode();
  27.         int index = (hash & 0x7FFFFFFF) % tab.length;
  28.         @SuppressWarnings("unchecked")
  29.         Entry<K,V> entry = (Entry<K,V>)tab[index];
  30.         //如果有相同的entry，替换对应的value
  31.         for(; entry != null ; entry = entry.next) {
  32.             if ((entry.hash == hash) && entry.key.equals(key)) {
  33.                 V old = entry.value;
  34.                 entry.value = value;
  35.                 return old;
  36.             }
  37.         }
  38.         //如果没有对应的entry，新增一个Entry，链表的插入使用头插法
  39.         addEntry(hash, key, value, index);
  40.         return null;
  41.     }

HashTable计算桶位置的公式：

  1. int index = (hash & 0x7FFFFFFF) % tab.length;

在此处做&操作的目的是获取到正整数。