JDK源码 0923 HashSet HashTable

进了HashXXX家族，感觉好难啊
HashSet的底层实现竟然是HashMap,年少无知了。不过想想也是复用的典范了。
HashSet 实现了Set接口，实际是由HashMap 实例进行支持，那么同样它也不能保证迭代顺序。

public class HashSet<E>
    extends AbstractSet<E>
    implements Set<E>, Cloneable, java.io.Serializable
{
    static final long serialVersionUID = -5024744406713321676L;

    private transient HashMap<E,Object> map;
    /**
     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
     * default initial capacity (16) and load factor (0.75).
     */
    public HashSet() {
        map = new HashMap<>();
    }
 /**
     * Returns an iterator over the elements in this set.  The elements
     * are returned in no particular order.
     *
     * @return an Iterator over the elements in this set
     * @see ConcurrentModificationException
     */
    public Iterator<E> iterator() {
        return map.keySet().iterator();
    }

    /**
     * Returns the number of elements in this set (its cardinality).
     *
     * @return the number of elements in this set (its cardinality)
     */
    public int size() {
        return map.size();
    }

    /**
     * Returns <tt>true</tt> if this set contains no elements.
     *
     * @return <tt>true</tt> if this set contains no elements
     */
    public boolean isEmpty() {
        return map.isEmpty();
    }
}

可以看到Set接口中的方法实际都是调用HashMap中已经存在的方法。包含Set的遍历，是取的HashMap中的KeySet()进行遍历。

HashTable 作为与HashMap 一同提起的类，只是这个类是同步进行实现的，且效率慢于HashMap 与 ConcurrentHashMap。遍览全文，发现该类在多处方法调用时候，进行synchronized关键字处理。其实现应与JDK1.7的hashMap基本相同。

public class Hashtable<K,V>
    extends Dictionary<K,V>
    implements Map<K,V>, Cloneable, java.io.Serializable {
 /**
     * The hash table data.
     */
    private transient Entry<?,?>[] table;

    /**
     * The total number of entries in the hash table.
     */
    private transient int count;

    /**
     * The table is rehashed when its size exceeds this threshold.  (The
     * value of this field is (int)(capacity * loadFactor).)
     *
     * @serial
     */
    private int threshold;

    /**
     * The load factor for the hashtable.
     *
     * @serial
     */
    private float loadFactor;

    /**
     * The number of times this Hashtable has been structurally modified
     * Structural modifications are those that change the number of entries in
     * the Hashtable or otherwise modify its internal structure (e.g.,
     * rehash).  This field is used to make iterators on Collection-views of
     * the Hashtable fail-fast.  (See ConcurrentModificationException).
     */
    private transient int modCount = 0;

    /**
     * Returns the number of keys in this hashtable.
     *
     * @return  the number of keys in this hashtable.
     */
    public synchronized int size() {
        return count;
    }

    /**
     * Tests if this hashtable maps no keys to values.
     *
     * @return  <code>true</code> if this hashtable maps no keys to values;
     *          <code>false</code> otherwise.
     */
    public synchronized boolean isEmpty() {
        return count == 0;
    }
 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;
    }
  /**
     * Maps the specified <code>key</code> to the specified
     * <code>value</code> in this hashtable. Neither the key nor the
     * value can be <code>null</code>. <p>
     *
     * The value can be retrieved by calling the <code>get</code> method
     * with a key that is equal to the original key.
     *
     * @param      key     the hashtable key
     * @param      value   the value
     * @return     the previous value of the specified key in this hashtable,
     *             or <code>null</code> if it did not have one
     * @exception  NullPointerException  if the key or value is
     *               <code>null</code>
     * @see     Object#equals(Object)
     * @see     #get(Object)
     */
    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;
    }

    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++;
    }
}

值得一提的是，实现了Dictionary 抽象类，这是第一次看到这个接口
解释中表明了这个类为抽象父类，作为键值一对一映射，且此类已过时，新的实现应实现Map接口。