Sorting List of String array by 3rd element

Question

I am trying to sort a List of String arrays by its 3rd element. The 3rd element is a date value. I've been trying to solve this for hours now by swapping but still can't get this solved. For my current List, I don't believe Arrays.sort() would be ideal that's why I am sorting this via loop.

List<String[]> Record = new ArrayList<>();
Record.add(new String[] { "0001047166", "11047161", "20191223", "20200916" });
Record.add(new String[] { "0001047166", "11047162", "20191223", "20200916" });
Record.add(new String[] { "0001047166", "11047163", "20191222", "20200916" });
Record.add(new String[] { "0001047166", "11047166", "20191218", "20200916" });
Record.add(new String[] { "0001047359", "11047359", "20191217", "20200917" });

int size = Record.size();
int swapIndex = 0;
for (int i = 0; i < size-1; i++) {
    String[] currentArray = Record.get(i); //get array on current index
    String date = currentArray[2]; //get the date on 3rd position, index 2 of current array
    swapIndex = i;
    for (int j = i + 1; j < Record.size(); j++) {
    String[] nextArray = Record.get(j);  //get the next array (i + 1)
        String otherDate = nextArray[2]; //get the date on 3rd position, index 2 of next arrray
        if (date.compareTo(otherDate) > 0) {
            String[] currArray = currentArray; //save the current array to temp
        //swap 
            Record.set(swapIndex, nextArray); 
            Record.set(j, currArray);
            swapIndex++;
        }
    }           
}

//Display result
for (String[] a : Record) {
    System.out.println(a[2]);
}

Output I get is

20191222 (wrong)
20191217
20191218
20191223
20191223

Desired output:

I'd appreciate any ideas or suggestions.

Thank you.

Why are you doing swapIndex++;?

Naetmul
– Naetmul

2020-06-16 06:12:43 +00:00
Commented Jun 16, 2020 at 6:12 — Naetmul
– Naetmul, Commented Jun 16, 2020 at 6:12

Tim Biegeleisen · Accepted Answer · 2020-06-16 06:12:40Z

3

You may sort using a lambda with a custom comparator which uses the third (string) date:

Record.sort((String[] sa1, String[] sa2) -> sa1[2].compareTo(sa2[2]));
Record.stream().forEach(sa -> System.out.println(sa[2]));

This prints:

Note that ideally you should be comparing some sort of bona-fide date types, and not date strings. That being said, given that your date strings appear to be in an ISO format, sorting as text should work. Also, Java naming convention holds that variable names should not begin with uppercase letters. So, ideally your list should be called record and not Record.

edited Jun 16, 2020 at 6:12

answered Jun 16, 2020 at 6:09

Tim Biegeleisen

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1 Comment

heisenberg Over a year ago

Thanks! Tim. I appreciate the help. That one-liner lambda which I haven't used in the past is really neat and efficient. I'll research on lambda and I'm sure it will help in future developments. You solved my problem!

ernest_k · Accepted Answer · 2020-06-16 06:13:31Z

3

Collections.sort will work here (or List.sort):

Collections.sort(Record, Comparator.comparing(array -> array[2]));
Record.sort(Comparator.comparing(array -> array[2]));

And if your Java is older than 8, you still can use Collections.sort:

Collections.sort(Record, new Comparator<String[]>() {
    @Override
    public int compare(String[] o1, String[] o2) {
        return o1[2].compareTo(o2[2]);
    }
});

You do not need to parse into a Date object before comparing, because the format yyyyMMdd allows for String comparison to be consistent with Date's.

answered Jun 16, 2020 at 6:13

ernest_k

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1 Comment

heisenberg Over a year ago

Thanks! Ernest. That one is very useful. The compiler I have is actually using Java 7.

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