每个循环工作的Java

Now I just want to know what is happening behind the every for each
loop quoted above

1. for (String output : splitted) 
    {
        mylist.add(output);
    }

这会将分割数组中的每个输出字符串添加到mylist列表中.

2. for (String output : mylist) 
{
      System.out.println(output);
      mylist = new ArrayList<String>(); //It worked 
      mylist.add(output);
}

for语句由以下产品管理：

for ( FormalParameter : Expression )
            Statement

其中Expression必须是java.lang.Iterable或数组的实例.所以这个：每个循环都等同于：

Iterator<String> iterator = mylist.iterator();
while (iterator.hasNext()) {
    System.out.println(output);
    mylist = new ArrayList<String>(); //It worked 
    mylist.add(output);
}

这里mylist.iterator()将返回Iterator类型的新实例：

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

因此,即使您正在创建新的ArrayList实例并在每次迭代时将它们分配给mylist,从原始mylist获取的迭代器仍将具有对原始mylist的引用,并将继续迭代原始mylist的元素.迭代器保持对其创建的列表的引用.赋值mylist = new ArrayList< String>()对迭代器处理的数据没有影响,因为它更改了变量mylist而不是列表本身.

3. for (String output : mylist) 
    {
        System.out.println(output);             
        mylist.add(output); // After this line it threw exception java.util.ConcurrentModificationException
    }

以下语句解释了这种行为.它是从Arraylist doc复制的：

The iterators returned by this class’s iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created,in any way except through the iterator’s own remove or add methods,the iterator will throw a ConcurrentModificationException. Thus,in the face of concurrent modification,the iterator fails quickly and cleanly,rather than risking arbitrary,non-deterministic behavior at an undetermined time in the future.

4. for (Iterator iterator2 = mylist.iterator(); iterator2.hasNext();)
{
    String string = (String) iterator2.next();
    System.out.println(string);
    iterator2.remove(); //It worked but if I used the same thing to remove element from original list it threw exception.
}

上面的语句还解释了这个for循环的行为：迭代遍历列表时,迭代器自己的remove或add方法可以在结构上修改列表.