Nested List In One List

Question

How to convert a nested list into a single list in java?

List list = new ArrayList();
List newList = new ArrayList();
List<Integer> list1 = new ArrayList<Integer>();

list1.add(2);

list1.add(4);

list.add(5);
list.addAll(list1);
list.add(6);

How can i add the elements of list to newList so that when i print newList

it prints

[5,2,4,6]

Are you trying to figure out how to [add all](http://docs.oracle.com/javase/7/docs/api/java/util/List.html#addAll(int,%20java.util.Collection)) of the elements into a specific point in the list? — Floegipoky, Oct 01 '14 at 16:50
By the way, there is no nested list in the code you've included. A nested list would be a list of lists eg `List> nestedList;` — Floegipoky, Oct 01 '14 at 16:53
I rolled back your edit.... you can't go and change the core concepts of your question after there are 3 answers to your original question. — rolfl, Oct 01 '14 at 17:00
yeah i know but if you see the code you can think what i asked ?okay — Shashank Pathak, Oct 01 '14 at 17:01
as I said before, that you should ask a different question, if you have a different question. (but first search for duplicates), like [this one](http://stackoverflow.com/q/20144826/1305253) and [this one](http://stackoverflow.com/q/7431006/1305253) — rolfl, Oct 01 '14 at 17:05

score 2 · Answer 1 · answered Oct 01 '14 at 16:46

2

Unless there is something you're not telling us, it is simply a repeat of what you have already done:

newList.addAll(list);

As has been said in the comments though, you really should be using full generic typing for your list variables:

 List<Integer> list = new ArrayList<>();
 List<Integer> newList = new ArrayList<>();
 List<Integer> list1 = new ArrayList<>();

answered Oct 01 '14 at 16:46

rolfl

17,539
7
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it will not work as when you iterate over the list list the frist element is 5 which is integer and the second element of this list i mean list is a list [2,4] this is the problem – Shashank Pathak Oct 01 '14 at 16:49
@user3098769 - it will work.... because the second element is not a list, because you added the content with `addAll` and not `add` – rolfl Oct 01 '14 at 16:50
suppose if i used add instead of addAll to add the list1 to list then can you give me a solution – Shashank Pathak Oct 01 '14 at 16:54
If you use add, instead of addAll, it would be a different question then, wouldn't it? – rolfl Oct 01 '14 at 16:55
yeah can you give me a solution if i used add instead of addAll – Shashank Pathak Oct 01 '14 at 16:57

score 0 · Answer 2 · answered Oct 01 '14 at 16:50

You shouldn't be using raw types, and you can call newList.addAll(list); -

public static void main(String[] args) {
    List<Integer> list = new ArrayList<>(); // <-- diamond operator, Java 7 and up.
    List<Integer> newList = new ArrayList<>();
    List<Integer> list1 = new ArrayList<>();
    list1.add(2);
    list1.add(4);
    list.add(5);
    list.addAll(list1);
    list.add(6);
    newList.addAll(list); // <-- here
    System.out.println(newList);
}

Output is the requested

[5, 2, 4, 6]

score 0 · Answer 3 · answered Oct 01 '14 at 16:52

You already used a possible way with

list.addAll(list1)

Another way would be walking through the list resp. using it's iterator to fetch and put all items into the newList.

Have a look at the API for more detailed info: http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html#addAll(java.util.Collection)

score 0 · Answer 4 · edited Oct 17 '14 at 12:19

0

  for(int i=0;i<list.size();i++){
      if(list.get(i) instanceof Integer){ 
           newlist.add(list.get(i));
      }else{
           newlist.addAll((ArrayList)list.get(i));
      }
  }

edited Oct 17 '14 at 12:19

aphex

3,372
2
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56

answered Oct 17 '14 at 11:55

Shashank Pathak

1
1
8

score 0 · Answer 5 · answered Sep 16 '20 at 09:56

The list that has to hold another list should be defined properly, rest working will be same. Refer below:

import java.util.List;
import java.util.ArrayList;
public class demo2 {
    public static void main(String[] args) {
        List<List<Integer>> nestedList = new ArrayList<>();
        List<Integer> list = new ArrayList<>();
        list.add(1);
        nestedList.add(list);
        System.out.println(nestedList);
    }
}

Nested List In One List

5 Answers5