Hashtable与HashMap的区别（源码级）

首先他们的相同点都实现了Map接口，不同点有很多： 
1. Hashtable 继续了抽象类Dictionary,所有Hashtable是线程安全的，而HashMap不是 

public class Hashtable<K,V>
    extends Dictionary<K,V>
implements Map<K,V>, Cloneable, java.io.Serializable {

2. Hashtable不能存放key为null的对象，它会抛出异常，而HashMap是可以的。 

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;
    }
}

3. 初始化数组大小不一样，HashMap是16，而Hashtable是11 

public Hashtable() {
    this(11, 0.75f);
}

4. 扩容机制也不同，HashMap是旧的大小×2，而Hashtable是旧的大小×2+1 
  

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;
    }
}

5. 计算数组下标index的方法不同： 
Hashtable的如下 
      

int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;

HashMap的如下 
   

/**
     * 计算hashCode的方法
     */
    static int hash(int h) {

        h ^= (h >>> 20) ^ (h >>> 12);
        return h ^ (h >>> 7) ^ (h >>> 4);
    }

    /**
     * 根据hashcode和数组的大小来计算数组下标的方法
     */
    static int indexFor(int h, int length) {
        return h & (length-1);
    }

评论

1 楼 xiangzi21 2011-05-10  

不错,简单易懂