HashTable和HashMap

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HashTable：

1. JDK1.0添加；
2. 继承Dictionary<K,V>，实现了Map<K,V>, Cloneable, java.io.Serializable；
3. 初始大小11，最大容量MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8；这个size同时也是桶的数量，entry最大数量为threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
4. 每次扩容时方法为newCapacity = (oldCapacity << 1) + 1;
5. 由于容量不规律，分桶的方法为index = (e.hash & 0x7FFFFFFF) % newCapacity;
6. 不允许null的key和value，否则抛出NullPointerException；
7. 桶中存的数据类型为链表；

    1     /**
    2      * Hashtable collision list.
    3      */
    4 private static class Entry<K,V> implements Map.Entry<K,V> {
    5     int hash;
    6     K key;
    7     V value;
    8     Entry<K,V> next;
    9 
   10     protected Entry(int hash, K key, V value, Entry<K,V> next) {
   11         this.hash = hash;
   12         this.key = key;
   13         this.value = value;
   14         this.next = next;
   15     }
   16 
   17     protected Object clone() {
   18         return new Entry<K,V>(hash, key, value,
   19                   (next==null ? null : (Entry<K,V>) next.clone()));
   20     }
   21 
   22     // Map.Entry Ops
   23 
   24     public K getKey() {
   25         return key;
   26     }
   27 
   28     public V getValue() {
   29         return value;
   30     }
   31 
   32     public V setValue(V value) {
   33         if (value == null)
   34         throw new NullPointerException();
   35 
   36         V oldValue = this.value;
   37         this.value = value;
   38         return oldValue;
   39     }
   40 
   41     public boolean equals(Object o) {
   42         if (!(o instanceof Map.Entry))
   43         return false;
   44         Map.Entry e = (Map.Entry)o;
   45 
   46         return (key==null ? e.getKey()==null : key.equals(e.getKey())) &&
   47            (value==null ? e.getValue()==null : value.equals(e.getValue()));
   48     }
   49 
   50     public int hashCode() {
   51         return hash ^ (value==null ? 0 : value.hashCode());
   52     }
   53 
   54     public String toString() {
   55         return key.toString()+"="+value.toString();
   56     }
   57 }

    

8. public 方法均用synchronized进行修饰，实现了线程安全；

HashMap:

1. JDK1.2添加；
2. 继承AbstractMap<K,V>，实现Map<K,V>, Cloneable, Serializable；
3. 初始大小16，最大容量2^30;threshold = (int)(capacity * loadFactor);
4. 每次扩容时方法为newCapacity = oldCapacity << 1;
5. 分桶方式为先对key.hashCode()做第二次hash

    1     /**
    2      * Applies a supplemental hash function to a given hashCode, which
    3      * defends against poor quality hash functions.  This is critical
    4      * because HashMap uses power-of-two length hash tables, that
    5      * otherwise encounter collisions for hashCodes that do not differ
    6      * in lower bits. Note: Null keys always map to hash 0, thus index 0.
    7      */
    8     static int hash(int h) {
    9         // This function ensures that hashCodes that differ only by
   10         // constant multiples at each bit position have a bounded
   11         // number of collisions (approximately 8 at default load factor).
   12         h ^= (h >>> 20) ^ (h >>> 12);
   13         return h ^ (h >>> 7) ^ (h >>> 4);
   14     }

   然后处理hash值，其中h = hash(key.hashCode()), length = table.length();

   1     /**
   2      * Returns index for hash code h.
   3      */
   4     static int indexFor(int h, int length) {
   5         return h & (length-1);
   6     }

    

6. 可以有null为key,用方法getForNullKey()处理；可以有null为value;
7. 桶中存的数据类型为链表；当单个桶中数据多于阈值时，将链表桶转为树形结构桶，用红黑树来实现，方便查询；

HashTable和HashMap

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原文地址：http://www.cnblogs.com/ibuki/p/4832391.html