How to sort a list in ascending order of string index in java

Question

I've created a list in java as following :

1   bbbb london
1.1 aaaa nyc
1.10 cccc jaipur
...
1.2 test test
1.11 test1 test1

I need to sort it based on index 1,1.1,1.2 etc! These are string values.

Like:

1   bbbb london
1.1 aaaa nyc
1.2 test test
...
1.10 cccc jaipur
1.11 test1 test1

How can i do that?

Initially the index was float but in-order to get 1.10 in list i changed index to string so Collection.sort(list) doesnt give output as expected.

My purpose is like to create a numbered bullets like

1 Helo
 1.1 helo1
 1.2 helo2
 1.3 hello3
2 Test
 2.1 Test1
 2.2 Test2
3 World

Any help please?

I recommend having separate fields for the major and minor indices. — Tim Biegeleisen
– Tim Biegeleisen, Commented Dec 23, 2018 at 6:59
@NicholasK . It can only be 1 - 1.1 - 1.2 ... 1.10 - 1.11 - 2 - 2.1 - 2.2 blah blah. Its like adding intermediate points between two main points. Like numbered bullets — Dillz
– Dillz, Commented Dec 23, 2018 at 7:03
@TimBiegeleisen. But if i create another indices with float value the 1.10 will change to 1.1 and their will be two 1.1 and sorting will get wrong right? — Dillz
– Dillz, Commented Dec 23, 2018 at 7:05
You might have a working answer below. From a data model point of view, it is generally bad to store numbers as strings. — Tim Biegeleisen
– Tim Biegeleisen, Commented Dec 23, 2018 at 7:12
Check out this answer: stackoverflow.com/questions/44617214/… — Yousef Gamal
– Yousef Gamal, Commented Dec 23, 2018 at 7:13

Nicholas K · Accepted Answer · 2018-12-23 07:17:56Z

5

Given the list :

List<String> list = Arrays.asList("1.32   bbbb london", "21.1 aaaa nyc", 
                                  "100.10 cccc jaipur", "1.2 test test",
                                  "1.11 test1 test1");

You can write your own custom comparator as shown below :

Collections.sort(list, new Comparator<String>() {
    public int compare(String o1, String o2) {
        return Float.compare(Float.parseFloat(o1.split(" ")[0]),
                             Float.parseFloat(o2.split(" ")[0]));
    }
});

Here, we split the String by " " and fetch the floating part of the string which happens to be the first index of the array. Now we parse it to a float and compare it with the second string.

If you are on java-8 you could do it one line :

Collections.sort(list, (o1, o2) -> 
      Float.compare(Float.parseFloat(o1.split(" ")[0]), Float.parseFloat(o2.split(" ")[0])));

edited Dec 23, 2018 at 7:17

answered Dec 23, 2018 at 7:10

Nicholas K

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8 Comments

Dillz Over a year ago

Collections.sort(mRoute.getWayPointList(), new Comparator<WayPoint>() {             public int compare(WayPoint o1, WayPoint o2) {                 return Float.compare(Float.parseFloat(o1.getId().split(" ")[0]),                         Float.parseFloat(o2.getId().split(" ")[0]));             }         });

Actualy i tried this still no it is not sorting! elements in List is added through a model Waypoint! I Could get the index(id) using that model.getId

Nicholas K Over a year ago

Is the String stored in the field id? If possible do share the class Waypoint

Prasad Karunagoda Over a year ago

This answer does not work. Yet it has 4 votes. My answer works and it has -1 votes. It's funny :)

Nicholas K Over a year ago

@PrasadKarunagoda : This solution does work. Please go ahead and try it. As mentioned in the comments by OP, probably he is not using it correctly as there is a model class Waypoint. That's the reason I've asked him to share the file if possible. Meanwhile you could try out the answer for yourself.

Prasad Karunagoda Over a year ago

@NicholasK, I tried your code on the input data provided in the question. And it does not give the answer expected in the question. I think the point you are missing is, Dillz's requirement is not for a "natural" ordering. That's why it does not work as String nor float.

|

Prasad Karunagoda · Accepted Answer · 2018-12-23 07:27:42Z

Try this. This should work. Main logic is in compareTo method.

import java.util.ArrayList;
import java.util.Collections;

public class Sort
{
  public static void main(String[] args)
  {
    ArrayList<Entry> entries = new ArrayList<>();
    entries.add(new Entry("1", "bbbb london"));
    entries.add(new Entry("1.1", "aaaa nyc"));
    entries.add(new Entry("1.10", "cccc jaipur"));
    entries.add(new Entry("1.2", "test test"));
    entries.add(new Entry("1.11", "test1 test1"));

    Collections.sort(entries);
    for (Entry e : entries)
    {
      System.out.println(e);
    }
  }
}

class Entry implements Comparable<Entry>
{
  String index;
  String value;
  int major;
  int minor;

  Entry(String index, String value)
  {
    this.index = index;
    this.value = value;

    String[] array = index.split("\\.");
    if (array.length == 2)
    {
      major = Integer.valueOf(array[0]);
      minor = Integer.valueOf(array[1]);
    }
    else if (array.length == 1)
    {
      major = Integer.valueOf(array[0]);
    }
    else
    {
      throw new IllegalArgumentException("Invalid index : " + index);
    }
  }

  @Override
  public int compareTo(Entry otherEntry)
  {
    if (this.major < otherEntry.major)
    {
      return -1;
    }
    else if (this.major > otherEntry.major)
    {
      return 1;
    }
    else
    {
      if (this.minor < otherEntry.minor)
      {
        return -1;
      }
      else if (this.minor > otherEntry.minor)
      {
        return 1;
      }
      else
      {
        return 0;
      }
    }
  }

  @Override
  public String toString()
  {
    return index + " " + value;
  }
}

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