继承、实现 | Hashtable extends Dictionaryimplements Map, Cloneable,Serializable | HashMap extends AbstractMap implements Map, Cloneable,Serializable |
对null的处理
| Hashtable table = new Hashtable(); table.put(null, "Null"); table.put("Null", null); table.contains(null); table.containsKey(null); table.containsValue(null); 后面的5句话在编译的时候不会有异常,可在运行的时候会报空指针异常具体原因可以查看源代码 public synchronized V put(K key, V value) { // Make sure the value is not null if (value == null) { throw new NullPointerException(); } | HashMap map = new HashMap(); map.put(null, "Null"); map.put("Null", null); map.containsKey(null); map.containsValue(null); 以上这5条语句无论在编译期,还是在运行期都是没有错误的. 在HashMap中,null可以作为键,这样的键只有一个;可以有一个或多个键所对应的值为null。当get()方法返回null值时,即可以表示 HashMap中没有该键,也可以表示该键所对应的值为null。因此,在HashMap中不能由get()方法来判断HashMap中是否存在某个键,而应该用containsKey()方法来判断。 |
增长率 | 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; } } }
| 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); }
|
哈希值的使用 | HashTable直接使用对象的hashCode,代码是这样的: public synchronized booleancontainsKey(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 true; } } return false; } | HashMap重新计算hash值,而且用与代替求模 public boolean containsKey(Object key) { Object k = maskNull(key); int hash = hash(k.hashCode()); int i = indexFor(hash, table.length); Entry e = table[i]; while (e != null) { if (e.hash == hash && eq(k, e.key)) return true; e = e.next; } return false; } |