HashMap和HashTable到底哪不同？

 以下代码及注释来自java.util.HashTable  * @author  Arthur van Hoff * @author  Josh Bloch * @author  Neal Gafter  

 以下代码及注释来自java.util.HashMap   * @author  Doug Lea  * @author  Josh Bloch  * @author  Arthur van Hoff  * @author  Neal Gafter  

 以下代码及注释来自java.util.Dictionary   * <strong>NOTE: This class is obsolete.  New implementations should  * implement the Map interface, rather than extending this class.</strong>  

 以下代码及注释来自java.util.HashTable   public synchronized boolean contains(Object value) {      if (value == null) {          throw new NullPointerException();      }       Entry tab[] = table;      for (int i = tab.length ; i-- > 0 ;) {          for (Entry<K,V> e = tab[i] ; e != null ; e = e.next) {              if (e.value.equals(value)) {                  return true;              }          }      }      return false;  }   public boolean containsValue(Object value) {      return contains(value);  }  

 以下代码及注释来自java.util.HashTable  public synchronized V put(K key, V value) {      // 如果value为null，抛出NullPointerException     if (value == null) {         throw new NullPointerException();     }      // 如果key为null，在调用key.hashCode()时抛出NullPointerException      // ... }   以下代码及注释来自java.util.HasMap  public V put(K key, V value) {     if (table == EMPTY_TABLE) {         inflateTable(threshold);     }     // 当key为null时，调用putForNullKey特殊处理     if (key == null)         return putForNullKey(value);     // ... }  private V putForNullKey(V value) {     // key为null时，放到table[0]也就是第0个bucket中     for (Entry<K,V> e = table[0]; e != null; e = e.next) {         if (e.key == null) {             V oldValue = e.value;             e.value = value;             e.recordAccess(this);             return oldValue;         }     }     modCount++;     addEntry(0, null, value, 0);     return null; }  

 以下代码及注释来自java.util.HashTable  /**  * The hash table data.  */ private transient Entry<K,V>[] table;   以下代码及注释来自java.util.HashMap  /**  * The table, resized as necessary. Length MUST Always be a power of two.  */ transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;  

 以下代码及注释来自java.util.HashTable  // 哈希表默认初始大小为11 public Hashtable() {     this(11, 0.75f); }  protected void rehash() {     int oldCapacity = table.length;     Entry<K,V>[] oldMap = table;      // 每次扩容为原来的2n+1     int newCapacity = (oldCapacity << 1) + 1;     // ... }   以下代码及注释来自java.util.HashMap  // 哈希表默认初始大小为2^4=16 static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16  void addEntry(int hash, K key, V value, int bucketIndex) {     // 每次扩充为原来的2n      if ((size >= threshold) && (null != table[bucketIndex])) {        resize(2 * table.length); }  

 以下代码及注释来自java.util.HashTable  // hash 不能超过Integer.MAX_VALUE 所以要取其最小的31个bit int hash = hash(key); int index = (hash & 0x7FFFFFFF) % tab.length;  // 直接计算key.hashCode() private int hash(Object k) {     // hashSeed will be zero if alternative hashing is disabled.     return hashSeed ^ k.hashCode(); }   以下代码及注释来自java.util.HashMap int hash = hash(key); int i = indexFor(hash, table.length);  // 在计算了key.hashCode()之后，做了一些位运算来减少哈希冲突 final int hash(Object k) {     int h = hashSeed;     if (0 != h && k instanceof String) {         return sun.misc.Hashing.stringHash32((String) k);     }      h ^= k.hashCode();      // 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); }  // 取模不再需要做除法 static int indexFor(int h, int length) {     // assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";     return h & (length-1); }  

 以下代码及注释来自java.util.HashTable  public synchronized V get(Object key) {     Entry tab[] = table;     int hash = hash(key);     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)) {             return e.value;         }     }     return null; }  public Set<K> keySet() {     if (keySet == null)         keySet = Collections.synchronizedSet(new KeySet(), this);     return keySet; }  

 以下代码及注释来自java.util.HashTable  /**  * A hashtable enumerator class.  This class implements both the  * Enumeration and Iterator interfaces, but individual instances  * can be created with the Iterator methods disabled.  This is necessary  * to avoid unintentionally increasing the capabilities granted a user  * by passing an Enumeration.  */ private class Enumerator<T> implements Enumeration<T>, Iterator<T> {     Entry[] table = Hashtable.this.table;     int index = table.length;     Entry<K,V> entry = null;     Entry<K,V> lastReturned = null;     int type;      /**      * Indicates whether this Enumerator is serving as an Iterator      * or an Enumeration.  (true -> Iterator).      */     boolean iterator;      /**      * The modCount value that the iterator believes that the backing      * Hashtable should have.  If this expectation is violated, the iterator      * has detected concurrent modification.      */     protected int expectedModCount = modCount;      Enumerator(int type, boolean iterator) {         this.type = type;         this.iterator = iterator;     }          //...  }  

If a thread-safe implementation is not needed, it is recommended to use HashMap in place of Hashtable. If a thread-safe highly-concurrent implementation is desired, then it is recommended to use java.util.concurrent.ConcurrentHashMap in place of Hashtable.