How to add items to ArrayList based on substring?

Question

I have a sorted ArrayList A based on substring(1,4). I want to add all the Strings with the same substring(1,4) to a new ArrayList.

ArrayList A: x111abc, x111acb, x111bca, x222abc, x222cba, x333abc, x333bca, x444abc

So I want x111abc, x111acb, x111bca all in one ArrayList, x222abc, x222cba in a different ArrayList, x333abc, x333bca in a different ArrayList, and x444abc in a different ArrayList.

I've tried this and a while loop (instead of the if statement) similar to this, but I know that it's going to create an error once I get to the last item in A. I feel like there has to be an easier way, but I can't figure it out.

for (int i = 0; i < A.size(); i++) {
    if (A.get(i).substring(1, 4).equals(A.get(i).substring(1, 4))) {
        //add it to an ArrayList
    }
}

Robby Cornelissen · Accepted Answer · 2021-02-18 06:04:54Z

1

You can use a stream with groupingBy():

import java.util.Arrays;
import java.util.Collection;
import java.util.List;

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

public class Test {
    public static void main(String[] args) {
        List<String> list = Arrays
                .asList("x111abc", "x111acb", "x111bca", "x222abc",
                        "x222cba", "x333abc", "x333bca", "x444abc");

        Collection<List<String>> grouped = list.stream()
                .collect(groupingBy(s -> s.substring(1, 4)))
                .values();

        System.out.println(grouped);
    }
}

answered Feb 18, 2021 at 6:04

Robby Cornelissen

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1 Comment

anon Over a year ago

How would I be able to loop through each group to perform basic operations without knowing specifically what substring(1,4) is? I have looked up examples, but everything I have found the developer knows that 111 is in the list for example. I have a large dataset and everything is already sorted by substring(1,4), but now I just need to loop through each group to perform operations. Thanks in advance!

V_K · Accepted Answer · 2021-02-18 06:14:00Z

0

If you want to use your logic then it will be like

String prevStr = origList.get(0).substring(1,4);
for (int i = 0; i < A.size(); i++) {
  if(prevStr.equals(origList.get(i).substring(1,4)) {
    //add it to an ArrayList
  }else{
    //create new list
  }
  prevStr = origList.get(i).substring(1,4);
}

answered Feb 18, 2021 at 6:14

V_K

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Comments

score 0 · Accepted Answer · 2021-03-09 12:12:48Z

You can collect a map of lists from this list:

List<String> listA = List.of("x111abc", "x111acb", "x111bca",
        "x222abc", "x222cba", "x333abc", "x333bca", "x444abc");

Map<String, List<String>> map = listA.stream().collect(Collectors.toMap(
        // key - substring with a number
        e -> e.substring(1, 4),
        // value - list of strings
        e -> new ArrayList<>(List.of(e)),
        // merge values
        (l1, l2) -> {
            l1.addAll(l2);
            return l1;
        },
        // sort by numbers
        () -> new TreeMap<String, List<String>>(Comparator.naturalOrder())));

// output
map.forEach((k, v) -> System.out.println(k + "=" + v));
//111=[x111abc, x111acb, x111bca]
//222=[x222abc, x222cba]
//333=[x333abc, x333bca]
//444=[x444abc]

^{See also: Split an array based on regex}

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