java数据结构源码解读——Hashtable

概述：

hashtable 类似hashTable 的数据结构。

重要方法分析：

public class Hashtable<K,V>
    extends Dictionary<K,V>
    implements Map<K,V>, Cloneable, java.io.Serializable {
   private transient Entry<?,?>[] table;
private transient int count;
private int threshold;
private float loadFactor;
 private transient int modCount = 0;
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
//methods.....
}

能看出Hashtable并不存在树化优化：

public abstract
class Dictionary<K,V> {
 
    public Dictionary() {
    }
 
 
    public abstract int size();
 
    public abstract boolean isEmpty();
 
 
    public abstract Enumeration<K> keys();
 
    public abstract Enumeration<V> elements();
 
    public abstract V get(Object key);
 
  
 
    public abstract V put(K key, V value);
 
 
    public abstract V remove(Object key);
}

直接看构造器！

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

1、默认的容量是11，而HashMap是16。

2、数组表是一旦创建就构造，属于饿汉模式，而HashMap是在第一次put时的resize构造。

    public synchronized V put(K key, V value) {
        // Make sure the value is not null
        if (value == null) {//不允许值是null的
            throw new NullPointerException();
        }
 
        // Makes sure the key is not already in the hashtable.
        Entry<?,?> tab[] = table;
        int hash = key.hashCode();
        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;
            }
        }
 
        addEntry(hash, key, value, index);
        return null;
    }
    private void addEntry(int hash, K key, V value, int index) {
        Entry<?,?> tab[] = table;
        if (count >= threshold) {
            // Rehash the table if the threshold is exceeded
            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];
        tab[index] = new Entry<>(hash, key, value, e);//头插
        count++;
        modCount++;
    }

1、HashMap采用尾插法，就是先扫描过去然后插到尾部；而Hashtable采用头插法，先扫描确认新节点不存在于链表中，然后通过重新构造，把新节点插到头部

2、线程安全？仅仅是put方法前面带有synchronize！

3、键值均不允许是null的，这个前面已经说过。

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);
        table = newMap;
 
        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;
            }
        }
    }

特性分析：

然后并没有采用高位1分割链表算法（因为也不是偶数，不适用）。

而且迭代遍历是借助 Enumerator——枚举器来遍历

Hashtable扩容公式是newCap=2*oldCap+1：