public void clear() {modCount++;// clear to let GC do its workfor (int i = 0; i < size; i++)elementData[i] = null;size = 0;}

public Iterator<E> iterator() {return new Itr();}

final void checkForComodification() {if (modCount != expectedModCount)throw new ConcurrentModificationException();}

@SuppressWarnings("unchecked")public E next() {checkForComodification();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];}

　　　　 ArrayList<Integer> al = new ArrayList<>();for(int i=0;i<10;i++)al.add(i);Iterator<Integer> it = al.iterator();while(it.hasNext()) {System.out.println(it.next());if(!it.hasNext())al.add(10);}    

        final ArrayList<Integer> al = new ArrayList<>();for(int i=0;i<10;i++)al.add(i);    new Thread(new Runnable() {            @Overridepublic void run() {Iterator<Integer> it = al.iterator();while(it.hasNext()) {System.out.print(it.next()+" ");}                }}).start();        new Thread(new Runnable() {                    @Overridepublic void run() {al.remove(6);        }                    }).start();

0 1 Exception in thread "Thread-0" java.util.ConcurrentModificationExceptionat java.util.ArrayList$Itr.checkForComodification(ArrayList.java:909)at java.util.ArrayList$Itr.next(ArrayList.java:859)at com.rw.importword.utils.Text$1.run(Text.java:19)at java.lang.Thread.run(Thread.java:748)