how to sort an arraylist that contains string arrays?

Question

List<String[]> allWordList = new ArrayList<>();

I would like to sort the "allWordList" list based on the first element in string array alphabetically.

I have a list that contains string array which are of size 2. So basically i want to sort this list by comparing the first element of the string array.

Collection.sort();

does not work as it is used to sort......

List<String>

and not

List<String[]>

to be clear i do not want to sort the individual string[] elements. I would like to sort the entire list based on the very first element of the string array.

Possible duplicate of Sorting arraylist in alphabetical order (case insensitive) — Arthur
– Arthur, Commented Aug 20, 2016 at 22:09
@Arrthur: Not a duplicate I have string[] inside a list whereas the other question has just strings — vivek verma
– vivek verma, Commented Aug 20, 2016 at 22:32

Sergey Kalinichenko · Accepted Answer · 2016-08-20 22:27:49Z

6

A simple custom comparator should do the trick.

The only tricky thing is making sure that you are not indexing into an empty array:

Collections.sort(allWordList, new Comparator<String[]>() {
    public int compare(String[] o1, String[] o2) {
        if (o1.length == 0) {
            return o2.length == 0 ? 0 : -1;
        }
        if (o2.length == 0) {
            return 1;
        }
        return o2[0].compareTo(o1[0]);
    }
});

edited Aug 20, 2016 at 22:27

answered Aug 20, 2016 at 22:11

Sergey Kalinichenko

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4 Comments

vivek verma Over a year ago

does not work says the first argument is wrong>>Found : List<String[]>, Required: String[][]

Sergey Kalinichenko Over a year ago

@vivekverma You are right, this should be Collections.sort. Thank you!

Jimmy T. Over a year ago

Why are you checking the array size and tolerate empty arrays? The arrays should have a size of 2.

Andreas Over a year ago

"sort this list by comparing the first element of the string array". Without further description, it is an error for the array to be null or empty, same as it is an error for the first element to be null. Both null and empty means "missing" and should fail! If you want to invent sorting rule for missing element (empty array), you might as well invent sorting rules for null too.

msumera · Accepted Answer · 2016-08-20 22:10:40Z

2

You can provide your own comparator, for example as a lambda expression:

allWordList.sort((o1, o2) -> o1[0].compareTo(o2[0]));

answered Aug 20, 2016 at 22:10

msumera

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1 Comment

Andreas Over a year ago

Since values are string arrays, maybe s1 (for "string") or a1 (for "array") is better than o1 (for "object").

pamcevoy · Accepted Answer · 2016-08-20 23:53:03Z

I'd go with a variation of @dasblinkenlight 's answer where I'd don't think comparing array length's is correct behavior.

private void sort() {
    List<String[]> allWordList = new ArrayList<String[]>();
    // fill the list...
    Collections.sort(allWordList, new FirstElementComparator());
    // and so on...
}

private static class FirstElementComparator implements Comparator<String[]>
{
    @Override
    public int compare(String[] sa1, String[] sa2) {
        String str1 = sa1[0];
        String str2 = sa2[0];

        int result = 0;
        if (str1 == null) {
            result = (str2 == null) ? 0 : -1;
        }
        else {
            result = str1.compareTo(str2);
        }
        return result;
    }
}

xehpuk · Accepted Answer · 2016-08-21 00:19:06Z

0

You don't need to write your own Comparator:

allWordList
    .stream()
    .sorted(Comparator.comparing(a -> a[0], String.CASE_INSENSITIVE_ORDER))
    .collect(Collectors.toList());

You can omit the String.CASE_INSENSITIVE_ORDER if the ordering should be case-sensitive.

Or if you need locale-dependent ordering, take a look at Collator.

edited Aug 21, 2016 at 0:19

answered Aug 21, 2016 at 0:02

xehpuk

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Comments

Community · Accepted Answer · 2017-05-23 12:00:46Z

-3

Check This :

Collections.sort(list, new Comparator<String>() {
        @Override
        public int compare(String s1, String s2) {
            return s1.compareToIgnoreCase(s2);
        }
    });

Source from here

Option 2 :

Collections.sort(teamsName.subList(1, teamsName.size()));

Source from here

edited May 23, 2017 at 12:00

CommunityBot

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answered Aug 20, 2016 at 22:10

Phoenix

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1 Comment

Marvin Over a year ago

You realize OP has a String array?

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