HashMap、HashTable

http://www.cnblogs.com/xinzhao/p/5644175.html

附上同行写的博客,写的很好

平常还是要多看源码

这两个双列集合有什么不同呢?

做个总结:

1.HashTable产生于JDK 1.1，而HashMap产生于JDK 1.2。HashMap要比HashTable出现得晚一些

2.贴源码

HashTable:

 1     public synchronized V put(K key, V value) {//线程安全的
 2         // Make sure the value is not null
 3         if (value == null) {
 4             throw new NullPointerException();
 5         }
 6 
 7         // Makes sure the key is not already in the hashtable.
 8         Entry<?,?> tab[] = table;
 9         int hash = key.hashCode();//key为null,会抛出空指针异常
10         int index = (hash & 0x7FFFFFFF) % tab.length;
11         @SuppressWarnings("unchecked")
12         Entry<K,V> entry = (Entry<K,V>)tab[index];
13         for(; entry != null ; entry = entry.next) {
14             if ((entry.hash == hash) && entry.key.equals(key)) {
15                 V old = entry.value;
16                 entry.value = value;
17                 return old;
18             }
19         }
20 
21         addEntry(hash, key, value, index);
22         return null;
23     }

HashMap:

1     public V put(K key, V value) {
2         return putVal(hash(key), key, value, false, true);
3     }

1     static final int hash(Object key) {
2         int h;
3         return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);//key可以为null
4     }

HashTable:不能存null键和null值,线程是安全的

HashMap:可以存null键和null值,线程是不安全的

HashTable已经被淘汰了,不要在代码中使用

看看作者怎么说的:

If a thread-safe implementation is not needed, it is recommended to use HashMap in place of Hashtable. If a thread-safe highly-concurrent implementation is desired, then it is recommended to use java.util.concurrent.ConcurrentHashMap in place of Hashtable.

翻译:就是不需要线程安全的时候使用HashMap,需要线程安全的时候使用ConcurrentHashMap,HashTable已经被淘汰了

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