Java的JDK下Hashtable与HashMap的区别

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}.