How to sort Alphanumeric String

Question

I have a problem with sorting strings which include integers. If I use the below code I get sorting like: 1some, 2some, 20some, 21some, 3some, some

However I want it sorted like: 1some, 2some, 3some, 20some, 21some, some

How can I do this?

Thanks!

Collections.sort(selectedNodes,
    new Comparator<DefaultMutableTreeNode>() {
    @Override
    public int compare(DefaultMutableTreeNode o1,
        DefaultMutableTreeNode o2) {
        return o1.getUserObject().toString()
            .compareTo(o2.getUserObject().toString());
    }
    });

Answer 1

Here is a self-contained example on how to do this (not particularly optimized):

final Pattern p = Pattern.compile("^\\d+");
String[] examples = { 
   "1some", "2some", "20some", "21some", "3some", "some", "1abc", "abc"
};
Comparator<String> c = new Comparator<String>() {
    @Override
    public int compare(String object1, String object2) {
        Matcher m = p.matcher(object1);
        Integer number1 = null;
        if (!m.find()) {
            return object1.compareTo(object2);
        }
        else {
            Integer number2 = null;
            number1 = Integer.parseInt(m.group());
            m = p.matcher(object2);
            if (!m.find()) {
                return object1.compareTo(object2);
            }
            else {
                number2 = Integer.parseInt(m.group());
                int comparison = number1.compareTo(number2);
                if (comparison != 0) {
                    return comparison;
                }
                else {
                    return object1.compareTo(object2);
                }
            }
        }
    }
};
List<String> examplesList = new ArrayList<String>(Arrays.asList(examples));
Collections.sort(examplesList, c);
System.out.println(examplesList);

Output

[1abc, 1some, 2some, 3some, 20some, 21some, abc, some]

Explanation

• The example uses a constant Pattern to infer whether a number is in the String's starting position.
• If not present in the first String, it compares it as is to the second.
• If present indeed in the first, it checks the second.
• If not present in the second, it compares the two Strings as is, again
• If present in both, it compares the Integers instead of the whole Strings, hence resulting in a numerical comparison rather than a lexicographical one
• If the number compare identical, it goes back to lexicographic comparison of the whole Strings (thanks MihaiC for spotting this one)

Answer 2

First make an alphanumerical comparator splitting the string in String or Integer parts.

public class AlphaNumericalComparator implements Comparator<String> {
    @Override
    public int compare(String o1, String o2) {
        List<Object> parts1 = partsOf(o1);
        List<Object> parts2 = partsOf(o2);
        while (!parts1.isEmpty() && !parts2.isEmpty()) {
            Object part1 = parts1.remove(0);
            Object part2 = parts2.remove(0);
            int cmp = 0;
            if (part1 instanceof Integer && part2 instanceof Integer) {
                cmp = Integer.compare((Integer)part1, (Integer)part2);
            } else if (part1 instanceof String && part2 instanceof String) {
                cmp = ((String) part1).compareTo((String) part2);
            } else {
                cmp = part1 instanceof String ? 1 : -1; // XXXa > XXX1
            }
            if (cmp != 0) {
                return cmp;
            }
        }
        if (parts1.isEmpty() && parts2.isEmpty()) {
            return 0;
        }
        return parts1.isEmpty() ? -1 : 1;
    }

    private List<Object> partsOf(String s) {
        List<Object> parts = new LinkedList<>();
        int pos0 = 0;
        int pos = 0;
        boolean wasDigit = false;
        while (true) {
            if (pos >= s.length()
                    || Character.isDigit(s.charAt(pos)) != wasDigit) {
                if (pos > pos0) {
                    String part = s.substring(pos0, pos);
                    parts.add(wasDigit? Integer.valueOf(part) : part);
                    pos0 = pos;
                }
                if (pos >= s.length()) {
                    break;
                }
                wasDigit = !wasDigit;
            }
            ++pos;
        }
        return parts;
    }
};

Then use this comparator in your own one, in Java 8 you may simply use Comparator's static methods.

Answer 3

Your solution lies in The Alphanum Algorithm and you can implement like this