Hashtable实际上就是一个线程安全的HashMap,不过它是个遗留下来的过气类,其性能也并比不少ConcurrentHashMap。


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//HashMap默认长度是1<<4 aka 16
public Hashtable() {
this(11, 0.75f);
}
//很明显,HashMap没有synchronized,并不线程安全。
public synchronized V put(K key, V value) {
// Make sure the value is not null
if (value == null) {
//HashMap的key允许一个null
//HashMap源码:return putForNullKey(value);
throw new NullPointerException();
}
// Makes sure the key is not already in the hashtable.
Entry tab[] = table;
int hash = hash(key);
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> 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;
hash = hash(key);
index = (hash & 0x7FFFFFFF) % tab.length;
}
// Creates the new entry.
Entry<K,V> e = tab[index];
tab[index] = new Entry<>(hash, key, value, e);
count++;
return null;
}
public synchronized V get(Object key) {
//if (key == null)
// return getForNullKey();HashMap的key有null值
Entry tab[] = table;
int hash = hash(key);
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
return e.value;
}
}
return null;
}