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Java 遍历HashMap并修改(remove)

java容器 来源:E01014165 2次浏览

遍历HashMap的方法有多种,比如通过获取map的keySet, entrySet, iterator之后,都可以实现遍历,然而如果在遍历过程中对map进行读取之外的操作则需要注意使用的遍历方式和操作方法。

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());
        }
    }
}


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)

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。

1 2 3 4 5 6 7 8          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” ) ;              }          }

1 2 3 4 5 6 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 )

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。

1 2 3 4 5 6 7 8 9 10 11          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” ) ;                }          }

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 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

 

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

1 2 3 4 5 6 7      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

1 2 3 4 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

1 2 3 4 5 6 7 8 9 10 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。

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26      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 ;      }

 

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();
}

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


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