Java How can ArrayList<String> be sorted/ordered by index?

Question

So i have this ArrayList:

list.get(0) == "love"  
list.get(1) == "foo"  
list.get(2) == "make"  
list.get(3) == "links"

What i want is:

list.get(0) == "links"  
list.get(1) == "make"  
list.get(2) == "foo"  
list.get(3) == "love"

I have tried this but not working:

public static void orderDescending(final ArrayList<String> list){
    Collections.sort(list, new Comparator<String>() {
        public int compare(String s1, String s2) {
            Integer i1 = list.indexOf(s1);
            Integer i2 = list.indexOf(s2);
            return i1.compareTo(i2);
        }
    });
 }

what you're showing here is not sorting, but reversing... did you want to reverse the list? — Hristo
– Hristo, Commented Apr 11, 2012 at 6:35
If your intention is to reverse the elements then use this Collections.reverse(list). — Phani
– Phani, Commented Apr 11, 2012 at 6:38
yeah, i agree with @Hristo. What you want is reversing the arraylist. — Agung Pratama
– Agung Pratama, Commented Apr 11, 2012 at 6:38
Although I have to say, I respect the audacity of sorting with a comparator based on the index of the item in the list. — Richante
– Richante, Commented Apr 11, 2012 at 6:39

Ted Hopp · Accepted Answer · 2012-04-11 06:38:02Z

7

You can do this:

Collections.reverse(list);

answered Apr 11, 2012 at 6:38

Ted Hopp

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user370305 · Accepted Answer · 2012-04-11 06:38:35Z

1

Simply,

Collections.reverse(list);

answered Apr 11, 2012 at 6:38

user370305

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Comments

Hristo · Accepted Answer · 2012-04-11 07:06:25Z

0

Here's a simple list reversal algorithm. If you don't care about the algorithm, use Collections.reverse().

public static void reverse(List<String> list) {

    if (list == null) {
        return;
    }

    int size = list.size();
    int mid = list.size() / 2;

    ListIterator<String> fwd = list.listIterator();
    ListIterator<String> rev = list.listIterator(size);

    for (int i = 0; i < mid; i++) {
        String tmp = fwd.next();
        fwd.set(rev.previous());
        rev.set(tmp);
    }
}

edited Apr 11, 2012 at 7:06

answered Apr 11, 2012 at 6:38

Hristo

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Comments

Amitabha Roy · Accepted Answer · 2013-04-27 06:57:11Z

You did a small mistake otherwise you have almost done it.

You should have tried "return i2.compareTo(i1);"

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;

public class Reverse {

    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<String>();
        list.add("love");
        list.add("foo");
        list.add("make");
        list.add("links");
        System.out.println(list);
        orderDescending(list);
        System.out.println(list);
    }

    public static void orderDescending(final ArrayList<String> list) {
        Collections.sort(list, new Comparator<String>() {
            public int compare(String s1, String s2) {
                Integer i1 = list.indexOf(s1);
                Integer i2 = list.indexOf(s2);
                // return i1.compareTo(i2);
                return i2.compareTo(i1);
            }
        });
    }
}

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