I'm trying to add ArrayList twice(with different content) to 2-D ArrayList, but 2-D ArrayList got same content?

Asked Sep 11 '22 at 12:03

Active Sep 11 '22 at 12:14

Viewed 16 times

Below is my java code, but eventually I'm going to create 2-D ArrayList in for-loop, that's why I test it first to use same ArrayList(sList).

package fc;

import java.util.ArrayList;

public class AppendList03 {

    public static void main(String[] args) {
        
        ArrayList<ArrayList<String>> wList = new ArrayList<ArrayList<String>>();
        ArrayList<String> sList = new ArrayList<String>();
        sList.add("1");
        sList.add("2");     
        System.out.println(sList); 
        
        wList.add(sList);
        System.out.println(wList);
        
        sList.clear();
        sList.add("3");
        wList.add(sList);
        System.out.println(wList);
        
    }

}

edited Sep 11 '22 at 12:14

DevilsHnd - 退職した

8,739
2
19
22

asked Sep 11 '22 at 12:03

Frankie Chiu

This is the result I got! – Frankie Chiu Sep 11 '22 at 12:06
[1, 2] [[1, 2]] [[3], [3]] – Frankie Chiu Sep 11 '22 at 12:06
You've added an object to the wList ArrayList, and then *modify* the same object added. Your result is not surprising in this light. If you want a new ArrayList, you must first create a ***new*** ArrayList. So not, `sList.clear();` but rather `sList = new ArrayList();` – Hovercraft Full Of Eels Sep 11 '22 at 12:12
It works, Thanks a lot! Below is my new code. – Frankie Chiu Sep 11 '22 at 13:00
public static void main(String[] args) { ArrayList> wList = new ArrayList>(); ArrayList sList; for (int i=0; i<10; i++) { sList = new ArrayList(); sList.add(String.valueOf(i)); wList.add(sList); } System.out.println(wList); } – Frankie Chiu Sep 11 '22 at 13:04
[[0], [1], [2], [3], [4], [5], [6], [7], [8], [9]] – Frankie Chiu Sep 11 '22 at 13:08

I'm trying to add ArrayList twice(with different content) to 2-D ArrayList, but 2-D ArrayList got same content?

0 Answers0