Sort a ArrayList<String> by number value

Question

I have an ArrayList of video resolutions that looks like this:

"1024x768", "800x600", "1280x1024", etc

I want to sort it based on numeric value in first part of string. Ie, the above would sort out to look like this:

"800x600","1024x768","1280x1024"

Is there a quick and dirty way to do this, by that I mean in less then 2-3 lines of code? If not, what would be the proper way? The values I get are from an object not my own. It does have a getWidth() and getHeight() methods that return ints.

Answer 1

The proper way is to write a Comparator implementation that operates on Strings, except that it parses up to the first non-numeric character. It then creates an int out of that and compares the ints.

You can then pass an instance of that Comparator into Collections.sort() along with your List.

Answer 2

Use a custom Comparator to sort the ArrayList via the Collections api.

Collections.sort(resolutionArrayList, new ResolutionComparator())

Answer 3

Using a Comparator will work, but will get slow if you have lots of values because it will parse each String more than once.

An alternative is to add each value to a TreeMap, with the number you want as the key, i.e., Integer.valueOf(s.substring(0,s.indexOf('x'))), then create a new ArrayList from the sorted values in treeMap.values().

Answer 4

If the objects in the array are Resolution instances with getWidth methods then you can use a Comparator to sort on those:

Collections.sort(resolutions, new Comparator {
    public int compare(Resolution r1, Resolution r2) {
        return Integer.valueOf(r1.getWidth()).compareTo(Integer.valueOf(r2.getWidth()));
    }
});

Answer 5

The solution suggested by Shadwell in his answer is correct and idiomatic.

But if you're looking for a more concise solution, then I'd advise you use lambdaj which will enable you to write code like:

List<Resolution> sortedResolutions = sort(resolutions, on(Resolution.class).getWidth());