Java遍历HashMap并修改(remove)

public class MapIteratorTest {

    private static Map<Integer, String> map = new HashMap<Integer, String>();

    public static void main(String[] args) {
        //init
        for(int i = 0; i < 10; i++){
            map.put(i, "value" + i);
        }

        for(Map.Entry<Integer, String> entry : map.entrySet()){
             Integer key = entry.getKey();
             if(key % 2 == 0){
            	 System.out.println("To delete key " + key);
                 map.remove(key);
                 System.out.println("The key " + + key + " was deleted");
             }
        }

        System.out.println("map size = " + map.size());
        for(Map.Entry<Integer, String> entry : map.entrySet()){
            System.out.println( entry.getKey() +" = " + entry.getValue());
        }
    }
}

{

;

{

//init

{

;

}

{

;

{

;

}

;

{

;

}

上面代码的输出结果为

To delete key 0
The key 0 was deleted
Exception in thread "main" java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
	at java.util.HashMap$EntryIterator.next(HashMap.java:834)
	at java.util.HashMap$EntryIterator.next(HashMap.java:832)
	at com.gpzuestc.collection.MapIteratorTest.main(MapIteratorTest.java:60)

deleted

ConcurrentModificationException

)

通过上面的输出可以发现第一个偶数key元素已经被成功remove，异常的抛出位置是在迭代器遍历下一个元素的时候。

如果把上面高亮的遍历代码替换成keySet的方式，通过keySet的remove操作同样会在遍历下个元素时抛出异常，示例如下。

        Set<Integer> keySet = map.keySet();
        for(Integer key : keySet){
            if(key % 2 == 0){
                System.out.println("To delete key " + key);
                keySet.remove(key);
                System.out.println("The key " + + key + " was deleted");
             }
        }

;

{

;

}

To delete key 0
The key 0 was deleted
Exception in thread "main" java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
	at java.util.HashMap$KeyIterator.next(HashMap.java:828)
	at com.gpzuestc.collection.MapIteratorTest.main(MapIteratorTest.java:49)

deleted

ConcurrentModificationException

)

如果要实现遍历过程中进行remove操作，上面两种方式都不能使用，而是需要通过显示获取keySet或entrySet的iterator来实现。

        Iterator<Map.Entry<Integer, String>> it = map.entrySet().iterator();
        while(it.hasNext()){
            Map.Entry<Integer, String> entry = it.next();
            Integer key = entry.getKey();
            if(key % 2 == 0){
           	 System.out.println("To delete key " + key);
           	 it.remove();    
           	 System.out.println("The key " + + key + " was deleted");

            }
        }

;

{

;

{

;

}

To delete key 0
The key 0 was deleted
To delete key 2
The key 2 was deleted
To delete key 4
The key 4 was deleted
To delete key 6
The key 6 was deleted
To delete key 8
The key 8 was deleted
map size = 5
1 = value1
3 = value3
5 = value5
7 = value7
9 = value9

deleted

value1

value3

value5

value7

value9

分析原因

其实上面的三种遍历方式从根本上讲都是使用的迭代器，之所以出现不同的结果是由于remove操作的实现不同决定的。

首先前两种方法都在调用nextEntry方法的同一个地方抛出了异常

    final Entry<K,V> nextEntry() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            Entry<K,V> e = next;
            ...
            ...
     }

{

)

;

}

这里modCount是表示map中的元素被修改了几次(在移除，新加元素时此值都会自增)，而expectedModCount是表示期望的修改次数，在迭代器构造的时候这两个值是相等，如果在遍历过程中这两个值出现了不同步就会抛出ConcurrentModificationException异常。

1、HashMap的remove方法实现

public V remove(Object key) {
    Entry<K,V> e = removeEntryForKey(key);
    return (e == null ? null : e.value);
}

{

;

}

2、HashMap.KeySet的remove方法实现

public boolean remove(Object o) {
    return HashMap.this.removeEntryForKey(o) != null;
}

{

;

}

3、HashMap.HashIterator的remove方法实现

public void remove() {
   if (current == null)
        throw new IllegalStateException();
   if (modCount != expectedModCount)
        throw new ConcurrentModificationException();
   Object k = current.key;
   current = null;
   HashMap.this.removeEntryForKey(k);
   expectedModCount = modCount;
}

{

)

;

)

;

}

以上三种实现方式都通过调用HashMap.removeEntryForKey方法来实现删除key的操作。在removeEntryForKey方法内只要移除了key modCount就会执行一次自增操作，此时modCount就与expectedModCount不一致了，上面三种remove实现中，只有第三种iterator的remove方法在调用完removeEntryForKey方法后同步了expectedModCount值与modCount相同，所以在遍历下个元素调用nextEntry方法时，iterator方式不会抛异常。

    final Entry<K,V> removeEntryForKey(Object key) {
        int hash = (key == null) ? 0 : hash(key.hashCode());
        int i = indexFor(hash, table.length);
        Entry<K,V> prev = table[i];
        Entry<K,V> e = prev;

        while (e != null) {
            Entry<K,V> next = e.next;
            Object k;
            if (e.hash == hash &&
                ((k = e.key) == key || (key != null && key.equals(k)))) {
                modCount++;
                size--;
                if (prev == e)
                    table[i] = next;
                else
                    prev.next = next;
                e.recordRemoval(this);
                return e;
            }
            prev = e;
            e = next;
        }

        return e;
    }

{

;

{

;

{

;

)

;

else

;

}

;

}

;

}

发散

1、如果是遍历过程中增加或修改数据呢？
增加或修改数据只能通过Map的put方法实现，在遍历过程中修改数据可以，但如果增加新key就会在下次循环时抛异常，因为在添加新key时modCount也会自增。

2、有些集合类也有同样的遍历问题，如ArrayList，通过Iterator方式可正确遍历完成remove操作，直接调用list的remove方法就会抛异常。

//会抛ConcurrentModificationException异常
for(String str : list){
	list.remove(str);
}

//正确遍历移除方式
Iterator<String> it = list.iterator();
while(it.hasNext()){
	it.next();
	it.remove();
}

//会抛ConcurrentModificationException异常

{

;

}

//正确遍历移除方式

;

{

;

}

3、jdk为什么这样设计，只允许通过iterator进行remove操作？
HashMap和keySet的remove方法都可以通过传递key参数删除任意的元素，而iterator只能删除当前元素(current)，一旦删除的元素是iterator对象中next所正在引用的，如果没有通过modCount、 expectedModCount的比较实现快速失败抛出异常，下次循环该元素将成为current指向，此时iterator就遍历了一个已移除的过期数据。