Hashtable 和 HashMap的区别

我们先看2个类的定义

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public class Hashtable        
        extends Dictionary        
        implements Map, Cloneable, java.io.Serializable    

public class Hashtable
        extends Dictionary
        implements Map, Cloneable, java.io.Serializable
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public class HashMap        
        extends AbstractMap        
        implements Map, Cloneable, Serializable    

public class HashMap
        extends AbstractMap
        implements Map, Cloneable, Serializable可见Hashtable 继承自 Dictiionary 而 HashMap继承自AbstractMap

Hashtable的put方法如下

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public synchronized V put(K key, V value) {    //###### 注意这里1        
    // Make sure the value is not null        
    if (value == null) { //###### 注意这里 2        
        throw new NullPointerException();        
    }        
    // Makes sure the key is not already in the hashtable.        
    Entry tab[] = table;        
    int hash = key.hashCode(); //###### 注意这里 3        
    int index = (hash & 0x7FFFFFFF) % tab.length;        
    for (Entry e = tab[index]; e != null; e = e.next) {        
        if ((e.hash == hash) && e.key.equals(key)) {        
            V old = e.value;        
            e.value = value;        
            return old;        
        }        
    }        
    modCount++;        
    if (count >= threshold) {        
        // Rehash the table if the threshold is exceeded        
        rehash();        
        tab = table;        
        index = (hash & 0x7FFFFFFF) % tab.length;        
    }        
    // Creates the new entry.        
    Entry e = tab[index];        
    tab[index] = new Entry(hash, key, value, e);        
    count++;        
    return null;        
}    

    public synchronized V put(K key, V value) {    //###### 注意这里1
        // Make sure the value is not null
        if (value == null) { //###### 注意这里 2
            throw new NullPointerException();
        }
        // Makes sure the key is not already in the hashtable.
        Entry tab[] = table;
        int hash = key.hashCode(); //###### 注意这里 3
        int index = (hash & 0x7FFFFFFF) % tab.length;
        for (Entry e = tab[index]; e != null; e = e.next) {
            if ((e.hash == hash) && e.key.equals(key)) {
                V old = e.value;
                e.value = value;
                return old;
            }
        }
        modCount++;
        if (count >= threshold) {
            // Rehash the table if the threshold is exceeded
            rehash();
            tab = table;
            index = (hash & 0x7FFFFFFF) % tab.length;
        }
        // Creates the new entry.
        Entry e = tab[index];
        tab[index] = new Entry(hash, key, value, e);
        count++;
        return null;
    }注意1 方法是同步的
注意2 方法不允许value==null
注意3 方法调用了key的hashCode方法，如果key==null,会抛出空指针异常 HashMap的put方法如下
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public V put(K key, V value) { //###### 注意这里 1        
    if (key == null)    //###### 注意这里 2        
        return putForNullKey(value);        
    int hash = hash(key.hashCode());        
    int i = indexFor(hash, table.length);        
    for (Entry e = table[i]; e != null; e = e.next) {        
        Object k;        
        if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {        
            V oldValue = e.value;        
            e.value = value;        
            e.recordAccess(this);        
            return oldValue;        
        }        
    }        
    modCount++;        
    addEntry(hash, key, value, i);    //###### 注意这里        
    return null;        
}    

    public V put(K key, V value) { //###### 注意这里 1
        if (key == null)    //###### 注意这里 2
            return putForNullKey(value);
        int hash = hash(key.hashCode());
        int i = indexFor(hash, table.length);
        for (Entry e = table[i]; e != null; e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }
        modCount++;
        addEntry(hash, key, value, i);    //###### 注意这里    
        return null;
    }注意1 方法是非同步的
注意2 方法允许key==null
注意3 方法并没有对value进行任何调用，所以允许为null    

补充：    
Hashtable 有一个 contains方法，容易引起误会，所以在HashMap里面已经去掉了
当然，2个类都用containsKey和containsValue方法。    

结论： ashMap 在大多数情况下是优先选择的。