HashTable源码解析
1、存储结构
private static class Entry<K,V> implements Map.Entry<K,V> { final int hash; final K key; V value; Entry<K,V> next; protected Entry(int hash, K key, V value, Entry<K,V> next) { this.hash = hash; this.key = key; this.value = value; this.next = next; } }
2、属性
// Hashtable保存数据的数组 private transient Entry<?,?>[] table; // hashtable的容量 private transient int count; // 阈值 private int threshold; // 负载因子 private float loadFactor; // 结构性修改 private transient int modCount = 0;
3、构造方法
//无参构造方法 public Hashtable() { //默认容量是11,默认装载因子是0.75 this(11, 0.75f); }
public Hashtable(int initialCapacity) { this(initialCapacity, 0.75f); }
public Hashtable(int initialCapacity, float loadFactor) { if (initialCapacity < 0) throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity); if (loadFactor <= 0 || Float.isNaN(loadFactor)) throw new IllegalArgumentException("Illegal Load: "+loadFactor); if (initialCapacity==0) initialCapacity = 1; this.loadFactor = loadFactor; table = new Entry<?,?>[initialCapacity]; threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1); }
3、添加(修改)
public synchronized V put(K key, V value) { // 确保value不能是null if (value == null) { throw new NullPointerException(); } // Makes sure the key is not already in the hashtable. Entry<?,?> tab[] = table; //根据key生成hash值 int hash = key.hashCode(); //通过hash值,找到存储的位置 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; } } //如果table中不存在指定的key-value对,那么插入到链表首部 addEntry(hash, key, value, index); return null; }
//将key-value对插入到链表首部 private void addEntry(int hash, K key, V value, int index) { modCount++; Entry<?,?> tab[] = table; //如果当前容量超过了阙值,则进行扩容 if (count >= threshold) { // Rehash the table if the threshold is exceeded rehash(); //因为扩容后,里面的位置全部改变了,所以需要获取新的值 tab = table; hash = key.hashCode(); index = (hash & 0x7FFFFFFF) % tab.length; } // Creates the new entry. @SuppressWarnings("unchecked") Entry<K,V> e = (Entry<K,V>) tab[index]; //插入到链表首部 tab[index] = new Entry<>(hash, key, value, e); //容量加一 count++; }
扩容:
protected void rehash() { //记录旧的容量 int oldCapacity = table.length; //记录旧的table Entry<?,?>[] oldMap = table; //扩容2倍加一,2N+1 int newCapacity = (oldCapacity << 1) + 1; //不能让新table的容量超过了约定的最大值 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++; //获取阙值 threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1); table = newMap; //将旧table中的值转移到新table中 for (int i = oldCapacity ; i-- > 0 ;) { for (Entry<K,V> old = (Entry<K,V>)oldMap[i] ; old != null ; ) { Entry<K,V> e = old; old = old.next; int index = (e.hash & 0x7FFFFFFF) % newCapacity; //使用链表头插法 e.next = (Entry<K,V>)newMap[index]; newMap[index] = e; } } }
4、删除
public synchronized V remove(Object key) { Entry<?,?> tab[] = table; int hash = key.hashCode(); int index = (hash & 0x7FFFFFFF) % tab.length; @SuppressWarnings("unchecked") Entry<K,V> e = (Entry<K,V>)tab[index]; for(Entry<K,V> prev = null ; e != null ; prev = e, e = e.next) { if ((e.hash == hash) && e.key.equals(key)) { modCount++; if (prev != null) { prev.next = e.next; } else { tab[index] = e.next; } count--; V oldValue = e.value; e.value = null; return oldValue; } } return null; }
5、查找
public synchronized V get(Object key) { Entry<?,?> tab[] = table; int hash = key.hashCode(); 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 (V)e.value; } } return null; }