继承的父类不同：
* @since JDK1.0 */public class Hashtable extends Dictionary implements Map , Cloneable, java.io.Serializable {
* @since 1.2 */public class HashMap extends AbstractMap implements Map , 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 HashMap 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 key to the specified * value in this hashtable. Neither the key nor the * value can be null. * * The value can be retrieved by calling the get 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 null if it did not have one * @exception NullPointerException if the key or value is * null * @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
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 e = tab[index]; tab[index] = new Entry (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 key, or * null if there was no mapping for key. * (A null return can also indicate that the map * previously associated null with key.) */ 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 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 Hashtable object * in the form of a set of entries, enclosed in braces and separated * by the ASCII characters ", " (comma and space). Each * entry is rendered as the key, an equals sign =, and the * associated element, where the toString 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 > it = entrySet().iterator(); sb.append('{'); for (int i = 0; ; i++) { Map.Entry 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 old = oldMap[i] ; old != null ; ) { Entry 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 e = table[bucketIndex]; table[bucketIndex] = new Entry (hash, key, value, e); if (size++ >= threshold) resize(2 * table.length); }
http://my.oschina.net/placeholder/blog/180069
http://blog.csdn.net/chenssy/article/details/22896871