How to convert List of List String into List of List Integer in java

Question

I have a List of List string, now I want to convert it into List of List Integer. Suggest some way, How to proceed for it?

here is my code:

public class convert {

    public static void main(String[] args) {
        try {

            List<List<String>> outerList = new ArrayList<List<String>>();
            outerList.add(new ArrayList<String>(asList("11","2")));
            outerList.add(new ArrayList<String>(asList("2","1")));
            outerList.add(new ArrayList<String>(asList("11","3")));

            System.out.println(outerList);

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Marko Topolnik · Accepted Answer · 2014-08-27 07:32:23Z

4

I woud suggest using the Streams API for this:

import static java.util.stream.Collectors.toList;

...

integerList = outerList.stream()
   .map(innerList->innerList.stream().map(Integer::valueOf).collect(toList()))
   .collect(toList());

answered Aug 27, 2014 at 7:32

Marko Topolnik

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3 Comments

Anoop Dobhal Over a year ago

This also works, can you tell how your appraoch differ from rest of the replies I got. I mean, can use of map add some difference in term of performance.

Narmer Over a year ago

Main difference is that this is Java 8, while other answers also work with previous versions of Java

Marko Topolnik Over a year ago

The approach differs from others in that it is more concise and more readable because it states the intent rather than going through the mechanics of achieving it. In terms of performance you can expect a very marginal overhead, which is very probably dwarfed by the computation going on in Integer::valueOf.

Rahul Tripathi · Accepted Answer · 2014-08-27 07:38:18Z

3

You simply try like this:

for(String s : yourStringList) 
{
  intList.add(Integer.valueOf(s));
}

EDIT

for (List<String> s : yourStringList) {
    List<Integer> x = new ArrayList<Integer>();
    for (String str: s) {
        x.add(Integer.parseInt(str));
    }
    intList.add(x);
}

edited Aug 27, 2014 at 7:38

answered Aug 27, 2014 at 7:29

Rahul Tripathi

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1 Comment

christopher Over a year ago

+1. Maybe enclose in { } and add a little validation in there?

score 1 · Accepted Answer · 2014-08-27 07:50:02Z

1

You will have to iterate over each subItem of each item.

List<List<String>> stringList = new ArrayList<List<String>>(); // Input
List<List<Integer>> intList = new ArrayList<List<Integer>>(); // Output
for (List<String> item : stringList) {
    List<Integer> temp = new ArrayList<Integer>();
    for (String subItem : item) {
        temp.add(Integer.parseInt(subItem));
    }
    intList.add(temp);
}

edited Aug 27, 2014 at 7:50

answered Aug 27, 2014 at 7:36

user3453226

Comments

user1935024 · Accepted Answer · 2014-08-27 07:30:25Z

1

res is new arrayList contains lists of integers.

       List<List<Integer>> res = new ArrayList<List<Integer>>();

        for(List<String> l : outerList){
            ArrayList<Integer> al = new ArrayList<Integer>();
            for(String s: l){
                al.add(Integer.valueOf(s));
            }
            res.add(al);
        }

answered Aug 27, 2014 at 7:30

user1935024

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