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 nullif (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