HashTable和HashMap的区别

最新推荐文章于 2024-11-18 20:55:43 发布

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原文链接：http://www.cnblogs.com/softidea/p/5487973.html

本文详细对比了HashTable与HashMap的不同之处，包括它们继承的父类、初始数组长度、同步性、对null的支持、hash值计算算法及扩容策略等方面。

继承的父类不同：

 * @since JDK1.0
 */
public class Hashtable<K,V>
    extends Dictionary<K,V>
    implements Map<K,V>, Cloneable, java.io.Serializable {

 * @since   1.2
 */

public class HashMap<K,V>
    extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable
{

初始数组长度不同：

    /**
     * Constructs a new, empty hashtable with a default initial capacity (11)
     * and load factor (0.75).
     */
    public Hashtable() {
    this(11, 0.75f);
    }

   /**
     * Constructs an empty <tt>HashMap</tt> with the default initial capacity
     * (16) and the default load factor (0.75).
     */
    public HashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR;
        threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
        table = new Entry[DEFAULT_INITIAL_CAPACITY];
        init();
    }

方法时否是同步的：
key是否可以是null:HashMap可以，HashTable不可以
value是不可以是null,HashMap可以，HashTable不可以

   /**
     * 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;
    for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
        if ((e.hash == hash) && e.key.equals(key)) {
        V old = e.value;
        e.value = value;
        return old;
        }
    }

    modCount++;
    if (count >= threshold) {
        // Rehash the table if the threshold is exceeded
        rehash();

            tab = table;
            index = (hash & 0x7FFFFFFF) % tab.length;
    }

    // Creates the new entry.
    Entry<K,V> e = tab[index];
    tab[index] = new Entry<K,V>(hash, key, value, e);
    count++;
    return null;
    }

 HashMap:
 /**
     * Associates the specified value with the specified key in this map.
     * If the map previously contained a mapping for the key, the old
     * value is replaced.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
     *         (A <tt>null</tt> return can also indicate that the map
     *         previously associated <tt>null</tt> with <tt>key</tt>.)
     */
    public V put(K key, V value) {
        if (key == null)
            return putForNullKey(value);
        int hash = hash(key.hashCode());
        int i = indexFor(hash, table.length);
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }

        modCount++;
        addEntry(hash, key, value, i);
        return null;
    }

HashTable:

如果value是null，则会抛NullPointerException

    /**
     * Returns a string representation of this <tt>Hashtable</tt> object
     * in the form of a set of entries, enclosed in braces and separated
     * by the ASCII characters "<tt>,&nbsp;</tt>" (comma and space). Each
     * entry is rendered as the key, an equals sign <tt>=</tt>, and the
     * associated element, where the <tt>toString</tt> method is used to
     * convert the key and element to strings.
     *
     * @return  a string representation of this hashtable
     */
    public synchronized String toString() {
    int max = size() - 1;
    if (max == -1)
        return "{}";

    StringBuilder sb = new StringBuilder();
    Iterator<Map.Entry<K,V>> it = entrySet().iterator();

    sb.append('{');
    for (int i = 0; ; i++) {
        Map.Entry<K,V> e = it.next();
            K key = e.getKey();
            V value = e.getValue();
            sb.append(key   == this ? "(this Map)" : key.toString());
        sb.append('=');
        sb.append(value == this ? "(this Map)" : value.toString());

        if (i == max)
        return sb.append('}').toString();
        sb.append(", ");
    }
    }

put和get时计算hash值的算法不同:
HashTable直接使用了hashCode()方法的值;

HashMap:
  
  public V put(K key, V value) {
        if (key == null)
            return putForNullKey(value);
        int hash = hash(key.hashCode());

HashMap:
    /**
     * Applies a supplemental hash function to a given hashCode, which
     * defends against poor quality hash functions.  This is critical
     * because HashMap uses power-of-two length hash tables, that
     * otherwise encounter collisions for hashCodes that do not differ
     * in lower bits. Note: Null keys always map to hash 0, thus index 0.
     */
    static int hash(int h) {
        // This function ensures that hashCodes that differ only by
        // constant multiples at each bit position have a bounded
        // number of collisions (approximately 8 at default load factor).
        h ^= (h >>> 20) ^ (h >>> 12);
        return h ^ (h >>> 7) ^ (h >>> 4);
    }

HashTable:

    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;

扩容的大小不同：

HashTable 

 /**
     * Increases the capacity of and internally reorganizes this
     * hashtable, in order to accommodate and access its entries more
     * efficiently.  This method is called automatically when the
     * number of keys in the hashtable exceeds this hashtable's capacity
     * and load factor.
     */
    protected void rehash() {
    int oldCapacity = table.length;
    Entry[] oldMap = table;

    int newCapacity = oldCapacity * 2 + 1;
    Entry[] newMap = new Entry[newCapacity];

    modCount++;
    threshold = (int)(newCapacity * loadFactor);
    table = newMap;

    for (int i = oldCapacity ; i-- > 0 ;) {
        for (Entry<K,V> old = oldMap[i] ; old != null ; ) {
        Entry<K,V> e = old;
        old = old.next;

        int index = (e.hash & 0x7FFFFFFF) % newCapacity;
        e.next = newMap[index];
        newMap[index] = e;
        }
    }
    }

HashMap
    /**
     * Adds a new entry with the specified key, value and hash code to
     * the specified bucket.  It is the responsibility of this
     * method to resize the table if appropriate.
     *
     * Subclass overrides this to alter the behavior of put method.
     */
    void addEntry(int hash, K key, V value, int bucketIndex) {
    Entry<K,V> e = table[bucketIndex];
        table[bucketIndex] = new Entry<K,V>(hash, key, value, e);
        if (size++ >= threshold)
            resize(2 * table.length);
    }