import java.util.ArrayList;
import java.util.List;


public class TestList {
private List<String> list;
private List<List<String>> mainList;
public TestList(){
    list = new ArrayList<String>();
    mainList = new ArrayList<List<String>>();
}
public static void main(String arg[]){
    String[] mylist = {"sunday","monda","tuesday","wednesday","thurday","friday","saturday"};

    TestList tl1 = new TestList();

    for(String s : mylist)
        tl1.list.add(s);//这里为什么可以这样写？ list是私有变量 可以对象点么？不解
    
        tl1.mainList.add(tl1.list);
    //tl1.list.clear(); // here if clear then not getting the list
//    tl1.list = null; ///// but here to assign null still getting list
    tl1.list.clear();
    System.out.println("after clear");
    System.out.println("@@@@@@@@@======="+tl1.list);
    for(List<String> list : tl1.mainList){
        for(String s : list){
            System.out.println("*********"+s);
        }
    }
}
 }
=================================== clear 的时候结果为
after clear
@@@@@@@@@=======[]
===================================null 的时候结果为
after clear
@@@@@@@@@=======null
*********sunday
*********monda
*********tuesday
*********wednesday
*********thurday
*********friday
*********saturday