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
