从源码看hashmap与hashtable区别

先看看hashmap与hashtable中的数据结构

 static class Entry<K,V> implements Map.Entry<K,V> {
        final K key;
        V value;
        Entry<K,V> next;//存储冲突中链表下一个元素
        final int hash;//依靠hash来索引map

从构造器看看两者不同
hashmap默认初始化容量为16；当使用非默认构造器时，其初始容量并非为所设置的容量initialCapacity，而是使用capacity，这里对初始容量做了个优化，保证初始容量为2的倍数，可以自己研究下原因。
还有在使用构造器初始化的时候最终hashmap都会调到init()；

 /**
     * Initialization hook for subclasses. This method is called
     * in all constructors and pseudo-constructors (clone, readObject)
     * after HashMap has been initialized but before any entries have
     * been inserted.  (In the absence of this method, readObject would
     * require explicit knowledge of subclasses.)
     */
    void init() {
    }

  public HashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR;
        threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
        table = new Entry[DEFAULT_INITIAL_CAPACITY];
        init();
    }

public HashMap(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " +
                                               initialCapacity);
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal load factor: " +
                                               loadFactor);

        // Find a power of 2 >= initialCapacity
        int capacity = 1;
       //看看这里所使用的初始容量
       while (capacity < initialCapacity)
            capacity <<= 1;       
        this.loadFactor = loadFactor;
        threshold = (int)(capacity * loadFactor);
        table = new Entry[capacity];
        init();
    }

hashtable默认初始化容量为11，并且需要调用Hashtable(int initialCapacity, float loadFactor)；

    public Hashtable() {
	this(11, 0.75f);
    }
 public Hashtable(int initialCapacity, float loadFactor) {
	if (initialCapacity < 0)
	    throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal Load: "+loadFactor);

        if (initialCapacity==0)
            initialCapacity = 1;
	this.loadFactor = loadFactor;
	table = new Entry[initialCapacity];
	threshold = (int)(initialCapacity * loadFactor);
    }

看完初始化我们看下起最主要的不同方法PUT；
hashmap中，支持key和value都为空的情况，使用的hash为重新计算后的hash值，使用计算位置的方式也不一样 h & (length-1);当hashmap中的容量超过临界值时是将容量扩大为原来的1倍2 * table.length；
hashtable（同步）中当key为空的时候就直接报空指针，hash使用的是类自身的hashcode方法，计算位置使用的(hash & 0x7FFFFFFF) % tab.length;当hashtable中的容量超过临界值时是将容量扩大为oldCapacity * 2 + 1；

还可以看下两者在重新分配空间时候的做法。

 public V put(K key, V value) {
    	//处理key为空的特殊情况
        if (key == null)
            return putForNullKey(value);
        //使用key的hashcode再hash
        int hash = hash(key.hashCode());
        //i为放在hashmap中的位置
        int i = indexFor(hash, table.length);
        //当相同位置有多个值时候，循环查找。
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
            //如果key为一样则返回原来的旧值，把新值插入到原key中
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }

        modCount++;
        //把对应的新map添加到i位置中
        addEntry(hash, key, value, i);
        return null;
    }

static int hash(int h) {
        h ^= (h >>> 20) ^ (h >>> 12);
        return h ^ (h >>> 7) ^ (h >>> 4);
    }
 static int indexFor(int h, int length) {
        return h & (length-1);
    }

 void addEntry(int hash, K key, V value, int bucketIndex) {
	Entry<K,V> e = table[bucketIndex];
	//把每个冲突节点的旧链表放入Entry的next中
        table[bucketIndex] = new Entry<K,V>(hash, key, value, e);        
        if (size++ >= threshold)
        	//如果map中内容超出threshold则将容量扩大1倍
            resize(2 * table.length);
    }

 void resize(int newCapacity) {
        Entry[] oldTable = table;
        int oldCapacity = oldTable.length;
        if (oldCapacity == MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return;
        }
        //构造新的map空间
        Entry[] newTable = new Entry[newCapacity];
        //将原有map中内容重新hash到新的位置
        transfer(newTable);
        table = newTable;
        threshold = (int)(newCapacity * loadFactor);
    }
 void transfer(Entry[] newTable) {
        Entry[] src = table;
        int newCapacity = newTable.length;
        for (int j = 0; j < src.length; j++) {
            Entry<K,V> e = src[j];
            if (e != null) {
            	//释放原有table在内存中的位置
                src[j] = null;
                //如果当前节点里面有多个值，需要对每个值重新分配位置
                do {
                    Entry<K,V> next = e.next;
                    int i = indexFor(e.hash, newCapacity);
                    e.next = newTable[i];
                    newTable[i] = e;
                    e = next;
                } while (e != null);
            }
        }
    }

 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;
	    }
	}

	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<K,V> e = tab[index];
	tab[index] = new Entry<K,V>(hash, key, value, e);
	count++;
	return null;
    }
 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;
	    }
	}
    }

其他都有的方法基本类似

参考
http://blog.csdn.net/romans1981/archive/2005/03/07/313232.aspx
http://tieba.baidu.com/f?kz=890094718