Java底层类和源码分析系列-HashTable底层架构和源码分析

public class Hashtable<K,V>
    extends Dictionary<K,V>
    implements Map<K,V>, Cloneable, java.io.Serializable

  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);
    }

    public Hashtable(Map<? extends K, ? extends V> t) {
        this(Math.max(2*t.size(), 11), 0.75f);
        putAll(t);
    }

public static void main(String[] args) {
         Hashtable<Integer, String> tb = new Hashtable<Integer,String>();
         tb.put(1, "BUPT");
         tb.put(2, "PKU");
         tb.put(3, "THU");
         Iterator<Entry<Integer, String>> iter = tb.entrySet().iterator();
         while(iter.hasNext()){
             Entry<?, ?> entry = (Entry<?, ?>) iter.next(); //此处会抛出异常
             System.out.println(entry.getValue());
             if("THU".equals(entry.getValue())){
                 tb.remove(entry.getKey());
             }
         }
    }
/* 输出结果如下：
THU
Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.Hashtable$Enumerator.next(Hashtable.java:1367)
    at ali.Main.main(Main.java:16) */

　　　　　public T next() {
            //首先判断modCount和expectedModCount是否相等
            //由于在主程序中Hashtable对象通过tb.remove()方法修改了modCount的值，使得expectedModCount和modCount不相等而抛出异常
            //解决办法就是将tb.remove()方法替换为iter.remove()方法
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            return nextElement();
        }

        //该方法在remove元素的同时修改了modCount和expectedModCount的值
        public void remove() {
            if (!iterator)
                throw new UnsupportedOperationException();
            if (lastReturned == null)
                throw new IllegalStateException("Hashtable Enumerator");
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();

            synchronized(Hashtable.this) {
                Entry<?,?>[] tab = Hashtable.this.table;
                int index = (lastReturned.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 == lastReturned) {
                        modCount++;
                        expectedModCount++;
                        if (prev == null)
                            tab[index] = e.next;
                        else
                            prev.next = e.next;
                        count--;
                        lastReturned = null;
                        return;
                    }
                }
                throw new ConcurrentModificationException();
            }
        }

public synchronized V put(K key, V value) {
        //值不允许为null
        if (value == null) {
            throw new NullPointerException();
        }

        // Makes sure the key is not already in the hashtable.
        Entry<?,?> tab[] = table;
        //得到键的hash
        int hash = key.hashCode();
        //得到对应hash在数组中的桶索引
        int index = (hash & 0x7FFFFFFF) % tab.length;
        @SuppressWarnings("unchecked")
        //得到桶中链表头节点
        Entry<K,V> entry = (Entry<K,V>)tab[index];
        //从头开始遍历
        for(; entry != null ; entry = entry.next) {
            //一旦hash值相等并且键相等，替换旧值
            if ((entry.hash == hash) && entry.key.equals(key)) {
                V old = entry.value;
                entry.value = value;
                return old;
            }
        }
        //如果没有找到相同键，那么添加新节点
        addEntry(hash, key, value, index);
        return null;
    }

private void addEntry(int hash, K key, V value, int index) {
        modCount++;

        Entry<?,?> tab[] = table;
        //如果尺寸超过了阈值，进行rehash
        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);
        count++;
    }

public synchronized V get(Object key) {
        Entry<?,?> tab[] = table;
        int hash = key.hashCode();
        int index = (hash & 0x7FFFFFFF) % tab.length;
    //遍历所有元素，哈希值和key一致，则返回
        for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) {
            if ((e.hash == hash) && e.key.equals(key)) {
                return (V)e.value;
            }
        }
        return null;
    }