java 中hash表的用法（集合篇-HashtableJAVA）

Hashtable也是Map接口的一种实现，也是key-value形式存在，不过key和value都不能为null，且key必须实现hashCode方法；在性能和空间考虑，和HashMap类一样用加载因子和初始容量。且是线程安全的；底层是用数组 链表实现。

1、new Hashtable<>();

public Hashtable() { this(11, 0.75f); }

//initialCapacity = 11，loadFactor = 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; //定义了数组 table = new Entry<?,?>[initialCapacity]; //threshold = 11 * 0.75 = 8.25 threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE 1); }

2、java.util.Hashtable#put

// 为啥说Hashtable是线程安全的，主要是因为使用synchronized关键字 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; //这里就说明了为啥key需要实现hashCode方法 int hash = key.hashCode(); // hash值算出数组下标 int index = (hash & 0x7FFFFFFF) % tab.length; @SuppressWarnings("unchecked") Entry<K,V> entry = (Entry<K,V>)tab[index]; //这里就说明是链表 for(; entry != null ; entry = entry.next) { if ((entry.hash == hash) && entry.key.equals(key)) { V old = entry.value; entry.value = value; return old; } } // 如果找不到值就添加entry条目 addEntry(hash, key, value, index); return null; }

//添加条目 private void addEntry(int hash, K key, V value, int index) { Entry<?,?> tab[] = table; //如果count>=阈值后，就需要扩容 if (count >= threshold) { // Rehash the table if the threshold is exceeded //重新hash且扩容 rehash(); 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]; //这里形成链表 tab[index] = new Entry<>(hash, key, value, e); count ; modCount ; }

//重新hash且扩容 protected void rehash() { int oldCapacity = table.length; Entry<?,?>[] oldMap = table; // overflow-conscious code // newCapacity = 2 * oldCapacity 1 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); table = newMap; //将旧的数组数据重新根据新的数组长度hash算出来的数组下标放到新的数组中 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; e.next = (Entry<K,V>)newMap[index]; newMap[index] = e; } } }

3、java.util.Hashtable#remove(java.lang.Object)

// 根据key移除元素 public synchronized V remove(Object key) { Entry<?,?> tab[] = table; int hash = key.hashCode(); int index = (hash & 0x7FFFFFFF) % tab.length; //hash找到数组下标 @SuppressWarnings("unchecked") Entry<K,V> e = (Entry<K,V>)tab[index]; //遍历查找元素。 for(Entry<K,V> prev = null ; e != null ; prev = e, e = e.next) { if ((e.hash == hash) && e.key.equals(key)) { //查到元素 if (prev != null) { prev.next = e.next; } else { tab[index] = e.next; } modCount ; count--; V oldValue = e.value; //帮助GC回收 e.value = null; return oldValue; } } return null; }

其实Hashtable的底层实现挺简单的；线程安全从上面的源码分析中可以知道，通过synchronized关键字来保证线程安全的。且实现是通过数组 链表。

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