public synchronized V put(K key, V value) { // 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 = hash(key); 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; hash = hash(key); index = (hash & 0x7FFFFFFF) % tab.length; } // Creates the new entry. Entry e = tab[index]; tab[index] = new Entry<>(hash, key, value, e); count++; return null; } protected void rehash() { int oldCapacity = table.length; Entry [] oldMap = table; // overflow-conscious code int newCapacity = (oldCapacity << 1) + 1; if (newCapacity - MAX_ARRAY_SIZE > 0) { if (oldCapacity == MAX_ARRAY_SIZE) // Keep running with MAX_ARRAY_SIZE buckets return; newCapacity = MAX_ARRAY_SIZE; } Entry [] newMap = new Entry[newCapacity]; modCount++; threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1); boolean rehash = initHashSeedAsNeeded(newCapacity); table = newMap; for (int i = oldCapacity ; i-- > 0 ;) { for (Entry old = oldMap[i] ; old != null ; ) { Entry e = old; old = old.next; if (rehash) { e.hash = hash(e.key); } int index = (e.hash & 0x7FFFFFFF) % newCapacity; e.next = newMap[index]; newMap[index] = e; } } }
此方法带有synchronized关键字，表示此方法必须同步访问以此达到线程安全的目的；
新增Entry的key和value不能为null；
hash运算与HashMap有所区别，这里直接是自身hashcode与hash种子与运算；
获取Entry数组索引与HashMap不一样；
HashTable的扩容策略为原先容量的2倍加1，最大不超过默认最大值；
get方法
public synchronized V get(Object key) { Entry tab[] = table; int hash = hash(key); int index = (hash & 0x7FFFFFFF) % tab.length; for (Entry e = tab[index] ; e != null ; e = e.next) { if ((e.hash == hash) && e.key.equals(key)) { return e.value; } } return null; }
private void expungeStaleEntries() { for (Object x; (x = queue.poll()) != null; ) { synchronized (queue) { @SuppressWarnings("unchecked") Entry e = (Entry ) x; int i = indexFor(e.hash, table.length); Entry prev = table[i]; Entry p = prev; while (p != null) { Entry next = p.next; if (p == e) { if (prev == e) table[i] = next; else prev.next = next; // Must not null out e.next; // stale entries may be in use by a HashIterator e.value = null; // Help GC size--; break; } prev = p; p = next; } } } }
public V put(K key, V value) { Object k = maskNull(key); int h = hash(k); //getTable方法会调用上述的expungeStaleEntries方法； Entry [] tab = getTable(); int i = indexFor(h, tab.length); for (Entry e = tab[i]; e != null; e = e.next) { if (h == e.hash && eq(k, e.get())) { V oldValue = e.value; if (value != oldValue) e.value = value; return oldValue; } } modCount++; Entry e = tab[i]; tab[i] = new Entry<>(k, value, queue, h, e); if (++size >= threshold) resize(tab.length * 2); return null; }
public V get(Object key) { Object k = maskNull(key); int h = hash(k); //getTable方法会调用上述的expungeStaleEntries方法； Entry [] tab = getTable(); int index = indexFor(h, tab.length); Entry e = tab[index]; while (e != null) { if (e.hash == h && eq(k, e.get())) return e.value; e = e.next; } return null; } public V remove(Object key) { Object k = maskNull(key); int h = hash(k); Entry [] tab = getTable(); int i = indexFor(h, tab.length); Entry prev = tab[i]; Entry e = prev; while (e != null) { Entry next = e.next; if (h == e.hash && eq(k, e.get())) { modCount++; size--; if (prev == e) tab[i] = next; else prev.next = next; return e.value; } prev = e; e = next; } return null; }