ArrayList的subList方法

 參考博文使用java.util.List.subList時最好當心點

List接口中定義：

List<E> subList(int fromIndex, int toIndex);

英文註釋：

Returns a view of the portion of this list between the specified fromIndex, inclusive, and toIndex, exclusive. (If fromIndex and toIndex are equal, the 
returned list is empty.) The returned list is backed by this list, so non-structural changes in the returned list are reflected in this list, and vice-versa. 
The returned list supports all of the optional list operations supported by this list.

This method eliminates the need for explicit range operations (of the sort that commonly exist for arrays). Any operation that expects a list can be used as a 
range operation by passing a subList view instead of a whole list. For example, the following idiom removes a range of elements from a list: 
      list.subList(from, to).clear();
 
Similar idioms may be constructed for indexOf and lastIndexOf, and all of the algorithms in the Collections class can be applied to a subList.
The semantics of the list returned by this method become undefined if the backing list (i.e., this list) is structurally modified in any way other than via 
the returned list. (Structural modifications are those that change the size of this list, or otherwise perturb it in such a fashion that iterations in 
progress may yield incorrect results.)

根據註釋得知：

1，該方法返回的是父list的一個視圖，從fromIndex（包含），到toIndex（不包含）。fromIndex=toIndex 表示子list爲空

2，父子list作的非結構性修改（non-structural changes）都會影響到彼此：所謂的「非結構性修改」，是指不涉及到list的大小改變的修改。相反，結構性修改，指改變了list大小的修改。

3，對於結構性修改，子list的全部操做都會反映到父list上。但父list的修改將會致使返回的子list失效。

4，tips：如何刪除list中的某段數據：

list.subList(from, to).clear();

示例代碼：

來自【Java每日一題】20170105，就是看到這個題目才讓我知道list的這個方法我沒有接觸過

package ques;  
  
import java.util.ArrayList;  
import java.util.List;  
  
public class Ques0105 {  
  
    public static void main(String[] args) {  
        List<String> list = new ArrayList<String>();  
        list.add("a");  
  
        // 使用構造器建立一個包含list的列表list1  
        List<String> list1 = new ArrayList<String>(list);  
        // 使用subList生成與list相同的列表list2  
        List<String> list2 = list.subList(0, list.size());  
        list2.add("b");  
  
        System.out.println(list.equals(list1));  
        System.out.println(list.equals(list2));  
    }  
}

返回結果以下：

能夠發現，list2爲list的子list，當list2發生結構性修改（list2.add("b")）後，list也發生相應改變，因此返回結果爲false和true