时间角度： Hashtable * @since JDK1.0 ; HashMap* @since   1.2

基类与接口角度：

public class Hashtable<K,V>

extends Dictionary<K,V>

implements Map<K,V>, Cloneable, java.io.Serializable

public class HashMap<K,V>

extends AbstractMap<K,V>

implements Map<K,V>, Cloneable, Serializable

实现了一样的接口，继承自不同的基类（字典）Dictionary<K,V>与（抽象映射）AbstractMap<K,V>。

线程安全角度：

HashMap.java ：

* <p><strong>Note that this implementation is not synchronized.</strong>

* If multiple threads access a hash map concurrently, and at least one of

* the threads modifies the map structurally, it <i>must</i> be

* synchronized externally. (A structural modification is any operation

* that adds or deletes one or more mappings; merely changing the value

* associated with a key that an instance already contains is not a

* structural modification.) This is typically accomplished by

* synchronizing on some object that naturally encapsulates the map.

Hashtable.java ： 

* Java Collections Framework</a>. Unlike the new collection

* implementations, {@code Hashtable} is synchronized. If a

* thread-safe implementation is not needed, it is recommended to use

* {@link HashMap} in place of {@code Hashtable}. If a thread-safe

* highly-concurrent implementation is desired, then it is recommended

* to use {@link java.util.concurrent.ConcurrentHashMap} in place of

* {@code Hashtable}.