Java Custom Size String Comparator

Question

I have the following Strings:

50ml,100g,3.5ml,10g,0.4g,320ml and I need to order them as follows:

0.4g,10g,100g,3.5ml,50ml,320ml

The criteria is order the type of size measure alphabetically (g comes before m) and then the actual size ASCENDANT (0.4 10g 100g)

How can I achieve this using a Comparator?

protected int compareInstances(String instance1, String instance2) {
    return 0;
}

Answer 1

Probably the easiest (however not the prettiest) approach to take here is to simply split the values into prefix (actual value) and suffix (measurement), to then compare these seperately.

So I implemented a Comparator<String>, which splits the given values into prefix and suffix, then firstly compares the suffix alphabetically. (Disclaimer: Uppercase letters will be considered as "smaller" than the lowercase equivalent) If they are equal, the prefix values are compared.

Here is a small example with the described logic:

import java.util.Arrays;
import java.util.Comparator;

public class Test {

    public static void main(String[] args) {
        String input = "50ml,100g,3.5ml,10g,0.4g,320ml,32.3a,3.6ml";
        String[] splitInput = input.split(",");
        System.out.println("Before:\t" + Arrays.toString(splitInput));
        Arrays.sort(splitInput, new MyComparator());
        System.out.println("After:\t" + Arrays.toString(splitInput));
    }

    static class MyComparator implements Comparator<String> {

        @Override
        public int compare(String o1, String o2) {
            // extract the suffix by removing the digits and dots
            String suffix1 = o1.replaceAll("[\\d\\.]", "");
            String suffix2 = o2.replaceAll("[\\d\\.]", "");
            if (suffix1.compareTo(suffix2) != 0) {
                return suffix1.compareTo(suffix2); // String#compareTo
            }
            // extract the prefix by removing the characters
            double value1 = Double.parseDouble(o1.replaceAll("[A-Za-z]", ""));
            double value2 = Double.parseDouble(o2.replaceAll("[A-Za-z]", ""));
            // compare the double values
            return (int) Double.compare(value1, value2);
        }
    }

}

Output:

Before: [50ml, 100g, 3.5ml, 10g, 0.4g, 320ml, 32.3a, 3.6ml]
After:  [32.3a, 0.4g, 10g, 100g, 3.5ml, 3.6ml, 50ml, 320ml]