[移动开发]java集合的遍历删除

java集合的遍历删除。

案例

1. 使用普通for循环遍历集合

   //以下代码不会有任何问题   
   	List<Integer> list = new ArrayList();
       list .add(1);
   	list.add(2);
       list.add(3);
       for(int i = 0 ; i < list.size();i++){
           list.remove(list.get(i));//或者 list.remove(i);或者使用iterator.remove();
       }
   

2. 当你使用集合的forEach()遍历集合时，

   //这里会报异常： java.util.ConcurrentModificationException   
   	List<Integer> list = new ArrayList();
       list .add(1);
   	list.add(2);
       list.add(3);
   	for (Integer integer : list) {
            list.remove(integer);
       }
   

   这是因为forEach底层用的是iterator迭代器

3. 当使用iterator迭代器遍历集合时

           List<Integer> integerList=new ArrayList<>();
           Iterator<Integer> iterator = integerList.iterator();
           while (iterator.hasNext()){
               Integer integer=iterator.next();
               integerList.remove(integer);//报异常java.util.ConcurrentModificationException  
               iterator.remove(); //没有问题
           }
   

   原因：

   这里查看ArrayList实现iterator的源码，注意iterator的next()和remove()方法。
   你会发现，iterator的hasnext()和remove()方法，都调用了checkForComodification()，当ArrayList的modCount （修改次数）!= expectedModCount（迭代器修改次数）时，会抛异常，当你调用iterator的remove()方法，会同步两个修改次数，而ArrayList不会同步，所以会抛异常

     private class Itr implements Iterator<E> {
           int cursor;       // index of next element to return
           int lastRet = -1; // index of last element returned; -1 if no such
           int expectedModCount = modCount;
   
           Itr() {}
   
           public boolean hasNext() {
               return cursor != size;
           }
   
           @SuppressWarnings("unchecked")
           public E next() {
               checkForComodification();//检查集合修改次数
               int i = cursor;
               if (i >= size)
                   throw new NoSuchElementException();
               Object[] elementData = ArrayList.this.elementData;
               if (i >= elementData.length)
                   throw new ConcurrentModificationException();
               cursor = i + 1;
               return (E) elementData[lastRet = i];
           }
   
           public void remove() {
               if (lastRet < 0)
                   throw new IllegalStateException();
               checkForComodification();//检查集合修改次数
   
               try {
                   ArrayList.this.remove(lastRet);
                   cursor = lastRet;
                   lastRet = -1;
                   expectedModCount = modCount;//这里同步了两个修改次数
               } catch (IndexOutOfBoundsException ex) {
                   throw new ConcurrentModificationException();
               }
           }
   
           @Override
           @SuppressWarnings("unchecked")
           public void forEachRemaining(Consumer<? super E> consumer) {
               Objects.requireNonNull(consumer);
               final int size = ArrayList.this.size;
               int i = cursor;
               if (i >= size) {
                   return;
               }
               final Object[] elementData = ArrayList.this.elementData;
               if (i >= elementData.length) {
                   throw new ConcurrentModificationException();
               }
               while (i != size && modCount == expectedModCount) {
                   consumer.accept((E) elementData[i++]);
               }
               // update once at end of iteration to reduce heap write traffic
               cursor = i;
               lastRet = i - 1;
               checkForComodification();
           }
   
           final void checkForComodification() {
               if (modCount != expectedModCount)
                   throw new ConcurrentModificationException();
           }
       }