How to sort String[] in List by timestamp?

Question

List<String[]> data = Importer.readCsv("test.csv");

A String[] item looks like this:

Position: name (String)
Position: 2017-03-14 18:22:44.149 (String)

What is a good approach in order to sort the list by the timestamp descending? I search for a solution without creating new Objects.

I could, but later I have to use functions which are using the Array. So later a have to transform the object back — lidox, Aug 29 '17 at 07:53
Possible duplicate of [How to sort an ArrayList in Java](https://stackoverflow.com/questions/18441846/how-to-sort-an-arraylist-in-java) — AxelH, Aug 29 '17 at 07:54
You just have to implement your `Comparator` to what you want — AxelH, Aug 29 '17 at 07:54
`Collections.sort(data, new Comparator() { @Override public int compare(String[] o1, String[] o2) { return o1[1].compareTo(o2[1]); } });` this should work as needed — XtremeBaumer, Aug 29 '17 at 07:56
Have you read about customer comparators, i.e. `Comparator` ? — vikingsteve, Aug 29 '17 at 07:56
To sort it, you have to treat the string array as an object so you will have to trasnform it 2 times, it doesn't make a big difference — Marco Salerno, Aug 29 '17 at 07:56

score 4 · Accepted Answer · edited Aug 29 '17 at 08:18

4

Use Collection.sort() and Comparator.comparing(), I find those easy to read when coming back to it later:

data.sort(Comparator.comparing((String[] o) -> o[1]).reversed());

You can compare the timestamp without conversion, as the format is "correctly" ordering the fields.

If you are confined to Java 7, there will be more boilerplate:

Collections.sort(data, new Comparator<String[]>()
{
    @Override
    public int compare(String[] o1, String[] o2)
    {
        return o2[1].compareTo(o1[1]);
        // return -o1[1].compareTo(o2[1]);
    }
});

Note that I need to compare o2 to o1 in this order or negate the result of comparing o1 to o2 to order the data descending. This becomes much more obvious in the first approach.

edited Aug 29 '17 at 08:18

shmosel

49,289
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answered Aug 29 '17 at 08:01

Malte Hartwig

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I guess it's a Java 8 solution? – lidox Aug 29 '17 at 08:08
Yes, all those methods were introduced in Java 8. I'll add a Java 7 solution. – Malte Hartwig Aug 29 '17 at 08:12

score 1 · Answer 2 · edited Aug 29 '17 at 08:19

You could use a custom Comparator that takes the second element in each array and converts it to a Timestamp. Note that a Comparator's function can't throw an exception, so you'd have to catch the potential ParseException and deal with it:

Function<String[], Date> parser = s -> {
    try {
        return new SimpleDateFormat("yyyy-MM-dd hh:mm:ss.SSS").parse(s[1]);
    } catch (ParseException e) {
        return null;
    }
};

list.sort(Comparator.comparing(parser));

score 1 · Answer 3 · answered Aug 29 '17 at 08:18

Well, thank you very much for your suggestions and impulses. Special Thanks goes to XtremeBaumer! Following code does the job too:

List<String[]> data = importer.readCsv(context, csvFile);


Collections.sort(data, new Comparator<String[]>() {
    @Override
    public int compare(String[] item1, String[] item2) {

        String dateString1 = item1[1];
        String dateString2 = item2[1];

        SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");

        Date date1 = null;
        Date date2 = null;

        try {
            date1 = format.parse(dateString1);
            date2 = format.parse(dateString2);
        } catch (ParseException e) {
            e.printStackTrace();
            return 0;
        }

        // dateString1 is an earlier date than dateString2
        return date1.compareTo(date2);
    }
});

How to sort String[] in List by timestamp?

3 Answers3