sorting with collection class in Java not working

Question

public static void main(String args[]) {
   int g=0;int x=0;int y=0;
    List<Integer> temporaryOpenList = new ArrayList<>();
    List<ArrayList> listToSort = new ArrayList<>();
    for(int i=3 ; i>0 ; i--) {
        g = g + i;
        x = x + i;
        y = y + i;
        temporaryOpenList.add(g);
        temporaryOpenList.add(x);
        temporaryOpenList.add(y);
        ArrayList<Integer> openList = new ArrayList<Integer>(temporaryOpenList);
        temporaryOpenList.clear();
        listToSort.add(openList);
        System.out.println(listToSort);
   }

The output will be :

[[3, 3, 3]]  
[[3, 3, 3], [5, 5, 5]]  
[[3, 3, 3], [5, 5, 5], [6, 6, 6]]

Now I want to sort listToSort based upon g value means the 1st value of the value triplet [g,x,y].But collections.sort(listToSort) command doesn't work

I'm sorry, but where did you get the idea that standard Java sorting supports sorting list of lists based on a first value of a triplet??? — TheJavaGuy-Ivan Milosavljević
– TheJavaGuy-Ivan Milosavljević, Commented Feb 14, 2018 at 17:55
I suggest revising your title to focus on your task (sorting triplets within list) rather than complaining about Collections.sort. — Basil Bourque
– Basil Bourque, Commented Feb 14, 2018 at 18:33

Lothar · Accepted Answer · 2018-02-14 17:55:40Z

2

You have to provide your own Comparator:

Collections.sort(list1, (elem1, elem2) -> {
    return elem1.get(0).compareTo(elem2.get(0));
});

To make this compilable you have to change the declaration of list1 to

ArrayList<ArrayList<Integer>> list1 = new ArrayList<>();

I wrote that directly into the text editor in my browser, so if you find a typo, you can keep it ;-)

answered Feb 14, 2018 at 17:55

Lothar

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Comments

Nomade · Accepted Answer · 2018-02-14 18:22:13Z

2

Try to use a clean comparator to sort your elements:

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

public class MyClass {
public static void main(String args[]) {
   int g=0;int x=0;int y=0;
    List<Integer> temporaryOpenList = new ArrayList<>();
    List<List<Integer>> listToSort = new ArrayList<>();
    for(int i=3 ; i>0 ; i--) {
        g = g + i + 8; // i added 8 just for change 'g' value, to use it for sort
        x = x + i;
        y = y + i;
        temporaryOpenList.add(g);
        temporaryOpenList.add(x);
        temporaryOpenList.add(y);
        ArrayList<Integer> openList = new ArrayList<Integer>(temporaryOpenList);
        temporaryOpenList.clear();
        listToSort.add(openList);

   }

   Collections.sort(listToSort, new MyComparator());
   System.out.println(listToSort);
}
}




class MyComparator
    implements Comparator<List<Integer>>
{
    public int compare(List<Integer> list1, List<Integer> list2) {
        return list1.get(0).compareTo( list2.get(0));
    }
}

That works! but you should know that comparators are some of good things to use when you try to sort or compare objects.

edited Feb 14, 2018 at 18:22

answered Feb 14, 2018 at 18:01

Nomade

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5 Comments

Rima Sur Over a year ago

Thank you . I subtract 3 from g for better understanding. It show that sorting is in ascending order. If I want to sort it in descending order what will I changed in MyComparator class. Also this Comparator function is new to me. I don't understand the code.Can you suggest me a link other than the Java documentation from where I can learn it.

Nomade Over a year ago

If you want to sort it in descending order you only need to change code to return list2.get(0).compareTo( list1.get(0)); . For more details about Java Comparator you can see this

Lothar Over a year ago

@Nomade alternatively just negate the result of compareTo, i.e. return -list1.get(0).compareTo( list2.get(0));

Nomade Over a year ago

@Lothar, yes it is an other solution to get such result, but i would prefer to do it clean by using a function instead of - operator : return negate(list1.get(0).compareTo( list2.get(0))); Where negate is : public int negate(value) { return (value = -value); }

Lothar Over a year ago

@Nomade If clarity is your goal you can use Collections.sort(listToSort, Collections.reverseOrder(new MyComparator()));

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