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:

    public synchronized V put(K key, V value) {//线程安全的
        // Make sure the value is not null
        if (value == null) {
            throw new NullPointerException();
        }

        // Makes sure the key is not already in the hashtable.
        Entry<?,?> tab[] = table;
        int hash = key.hashCode();//key为null,会抛出空指针异常
        int index = (hash & 0x7FFFFFFF) % tab.length;
        @SuppressWarnings("unchecked")
        Entry<K,V> entry = (Entry<K,V>)tab[index];
        for(; entry != null ; entry = entry.next) {
            if ((entry.hash == hash) && entry.key.equals(key)) {
                V old = entry.value;
                entry.value = value;
                return old;
            }
        }

        addEntry(hash, key, value, index);
        return null;
    }

HashMap:

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

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

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