public class test {
    public static void main(String[] args) {
        testForHashMap();
    }
    private static void testForHashMap() {
        HashMap<String,String> hashMap =new LinkedHashMap<>();
        hashMap.put("1","a");
        hashMap.put("2","b");
        hashMap.put("3","c");
        Iterator<Map.Entry<String,String>> iterator=hashMap.entrySet().iterator();
        while (iterator.hasNext()) {
            hashMap.put("bloom","bloom");
            System.out.println(iterator.next());
        }
    }
}

public class test {   
    public static void main(String[] args) {
        testForHashTable();
    }
    private static void testForHashTable() {
        Hashtable<String,String> hashtable =new Hashtable();
        hashtable.put("4","d");
        hashtable.put("5","e");
        hashtable.put("6","f");
        Enumeration<String> iterator1=hashtable.elements();
        while (iterator1.hasMoreElements()) {
            hashtable.put("bloom","bloom");
            System.out.println(iterator1.nextElement());
        }
    }
}

public static void main(String[] args) {
Hashtable<String, String> table = new Hashtable<String, String>();
table.put("a", "aa");
table.put("b", "bb");
table.put("c", "cc");
table.remove("c");
Iterator<Entry<String, String>> iterator = table.entrySet().iterator();
while (iterator.hasNext()) {
System.out.println(iterator.next().getValue());
//采用iterator直接進(jìn)行修改 程序正常
iterator.remove();
//直接從hashtable增刪數(shù)據(jù)就會(huì)報(bào)錯(cuò)
table.put("d", "dd");
//直接從hashtable增刪數(shù)據(jù)就會(huì)報(bào)錯(cuò)，hashtable，hashmap等非并發(fā)集合，如果在迭代過(guò)程中增減了數(shù)據(jù)，就是快速失敗
table.remove("c");
}
System.out.println("-----------");
Lock lock = new ReentrantLock();
//即使加上lock，還是會(huì)跑出ConcurrentModificationException異常
lock.lock();
HashMap<String, String> hashmap = new HashMap<String, String>();
hashmap.put("a", "aa");
hashmap.put("b", "bb");
hashmap.put("c", "cc");
Iterator<Entry<String, String>> iterators = hashmap.entrySet().iterator();
while (iterators.hasNext()) {
System.out.println(iterators.next().getValue());
// 正常
iterators.remove();
//直接從hashtable增刪數(shù)據(jù)就會(huì)報(bào)錯(cuò)。
//hashtable，hashmap等非并發(fā)集合，如果在迭代過(guò)程中增減了數(shù)據(jù)，會(huì)快速失敗 (一檢測(cè)到修改，馬上拋異常) 
//java.util.ConcurrentModificationException
hashmap.remove("c");
}
System.out.println("-----------");
lock.unlock();
ConcurrentHashMap<String, String> map = new ConcurrentHashMap<String, String>();
map.put("a", "aa");
map.put("b", "bb");
map.put("c", "cc");
Iterator<Entry<String, String>> mapiterator = map.entrySet().iterator();
while (mapiterator.hasNext()) {
System.out.println(mapiterator.next().getValue());
map.remove("c");// 正常 并發(fā)集合不存在快速失敗問(wèn)題
map.put("c", "cc");// 正常 并發(fā)集合不存在快速失敗問(wèn)題
}
System.out.println("-----------");
}

final Node<K,V> nextNode() {
            Node<K,V>[] t;
            Node<K,V> e = next;
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            if (e == null)
                throw new NoSuchElementException();
            if ((next = (current = e).next) == null && (t = table) != null) {
                do {} while (index < t.length && (next = t[index++]) == null);
            }
            return e;
        }

public final Map.Entry<K,V> next() {
            Node<K,V> p;
            if ((p = next) == null)
                throw new NoSuchElementException();
            K k = p.key;
            V v = p.val;
            lastReturned = p;
            advance();
            return new MapEntry<K,V>(k, v, map);
        }
/**
         * Advances if possible, returning next valid node, or null if none.
         */
        final Node<K,V> advance() {
            Node<K,V> e;
            if ((e = next) != null)
                e = e.next;
            for (;;) {
                Node<K,V>[] t; int i, n;  // must use locals in checks
                if (e != null)
                    return next = e;
                if (baseIndex >= baseLimit || (t = tab) == null ||
                    (n = t.length) <= (i = index) || i < 0)
                    return next = null;
                if ((e = tabAt(t, i)) != null && e.hash < 0) {
                    if (e instanceof ForwardingNode) {
                        tab = ((ForwardingNode<K,V>)e).nextTable;
                        e = null;
                        pushState(t, i, n);
                        continue;
                    }
                    else if (e instanceof TreeBin)
                        e = ((TreeBin<K,V>)e).first;
                    else
                        e = null;
                }
                if (stack != null)
                    recoverState(n);
                else if ((index = i + baseSize) >= n)
                    index = ++baseIndex; // visit upper slots if present
            }
        }