HashTable和HashMap的区别
说明：本文源码及源码注释基于jdk1.8.0_151
1.继承类不同
HashTable继承自Dictionary，而HashMap继承自AbstractMap。Dictionary已经被废弃。
/**
* Hash table based implementation of the <tt>Map</tt> interface. This
* implementation provides all of the optional map operations, and permits
* <tt>null</tt> values and the <tt>null</tt> key. (The <tt>HashMap</tt>
* class is roughly equivalent to <tt>Hashtable</tt>, except that it is
* unsynchronized and permits nulls.) This class makes no guarantees as to
* the order of the map; in particular, it does not guarantee that the order
* will remain constant over time.
**/
//HashMap大致相等于HashTable，除了不同步（不线程安全）和允许为null。
public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable {
...
}
public class Hashtable<K, V> extends Dictionary<K, V> implements Map<K, V>, Cloneable, Serializable {
...
}
继续看Dictionary和AbstractMap的源码，Dictionary就是一个抽象类，废弃，而AbstractMap是实现接口Map的抽象类。
/** <strong>NOTE: This class is obsolete. New implementations should
* implement the Map interface, rather than extending this class.</strong>
*/
//这个类已经过时了。新的实现应该实现MAP接口，而不是扩展这个类。
public abstract class Dictionary<K,V> {
...
}
public abstract class AbstractMap<K,V> implements Map<K,V> {
...
}
下面是他们的继承关系：
hashtable的继承图：
hashMap的继承图：
2.HashMap支持键值对为空，HashTable是同步的
从代码中看， Hashtable的put方法如果put一个null则抛出NullPointerException异常，而HashMap则是不会判断是否为null，可以插入null值。第二对于HashTable，方法前加入synchronized，是同步的。
public class Hashtable<K, V> extends Dictionary<K, V> implements Map<K, V>, Cloneable, Serializable {
...
public synchronized V put(K var1, V var2) {
if (var2 == null) {
throw new NullPointerException();
} else {
...
}
}
}
public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable {
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
/**
* Implements Map.put and related methods
*
* @param hash hash for key
* @param key the key
* @param value the value to put
* @param onlyIfAbsent if true, don't change existing value
* @param evict if false, the table is in creation mode.
* @return previous value, or null if none
*/
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
//判断tab是否为空，是否还有剩余空间
if ((tab = table) == null || (n = tab.length) == 0)
//扩容
n = (tab = resize()).length;
//判断tab是否为空
if ((p = tab[i = (n - 1) & hash]) == null)
//创建一个Node，可以传null值进去
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
//比较哈希值是否相同,比较key实例是否相同,比较key值是否相同
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//如果p是TreeNode（LinkedHashMap）
else if (p instanceof TreeNode)
//在红黑树中插入键值对
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
////如果链表的长度大于TREEIFY_THRESHOLD，转成红黑树
treeifyBin(tab, hash);
break;
}
//找到key，跳出循环
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
//为key赋值
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
//可以替换成null值
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
}