Sorting a double value of an object within an arrayList

Question

I'm trying to sort my custom class chromosome by the value of their score attribute which is a double. These chromosomes are stored within an ArrayList. I know I have to use a comparator but I've read so many differing opinions online in the last hour that I'm utterly confused.

Attached is my code, if someone could point me in the right direction I would be much appreciated.

public class Chromosome
{

    public Gene[] genes;
    public double score;

    public Chromosome(int l)
    {
        genes = new Gene[l]; 
    }

    public int getLength()
    {
        return genes.length;
    }

    public void printChromo()
    {
        for(int i=0;i<this.genes.length;i++)
        {
            System.out.println(""+this.genes[i].teacher+","+
                this.genes[i].lecture+","+
                this.genes[i].room+","+
                this.genes[i].time+"");
        }   
    }

    public void setScore(double score)
    {
        this.score=score;
    }

    public double getScore()
    {
        return this.score;
    }
}

Don't know this make a difference but the score can only be a double between and including 0.0 to 1.0

found this question while google "java double sort" for my genetic alghorithm — sweetrenard
– sweetrenard, Commented Dec 24, 2016 at 19:10

arshajii · Accepted Answer · 2013-12-06 15:58:51Z

79

To use a Comparator:

Collections.sort(myList, new Comparator<Chromosome>() {
    @Override
    public int compare(Chromosome c1, Chromosome c2) {
        return Double.compare(c1.getScore(), c2.getScore());
    }
});

If you plan on sorting numerous Lists in this way I would suggest having Chromosome implement the Comparable interface (in which case you could simply call Collections.sort(myList), without the need of specifying an explicit Comparator).

edited Dec 6, 2013 at 15:58

answered Nov 17, 2012 at 21:55

arshajii

130k26 gold badges246 silver badges293 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Melo1991 Over a year ago

How would I get it an ascending list?

arshajii Over a year ago

@Melo1991 There is a Collections.reverse method if that's what you're after.

Machisuji Over a year ago

Or just c2.compareTo(c1) instead of c1.compareTo(c2).

ivanjermakov · Accepted Answer · 2020-06-07 07:57:21Z

12

With Java SE8 you could use lambda expression like so:

list.sort((o1, o2) -> Double.compare(o2.doubleField, o1.doubleField));

edited Jun 7, 2020 at 7:57

answered Apr 13, 2018 at 16:26

ivanjermakov

1,35015 silver badges24 bronze badges

Comments

matsev · Accepted Answer · 2012-11-17 22:12:59Z

3

Why not use a PriorityQueue with a Comparator like this:

// your code
PriorityQueue<Chromosome> entries = new PriorityQueue<Chromosome>(1, new Comparator<Chromosome> () {
    @Override
    public int compare(Chromosome arg0, Chromosome arg1) {
        return (Double)(arg1.getScore()).compareTo((Double)arg0.getScore());
    }
});
entries.addAll(arrayListOfChromosomes);
// your code

The priority queue will then keep your data structure in sorted order.

edited Nov 17, 2012 at 22:12

matsev

34k18 gold badges128 silver badges169 bronze badges

answered Nov 17, 2012 at 19:50

hd1

34.9k5 gold badges83 silver badges95 bronze badges

2 Comments

Melo1991 Over a year ago

Would I have this piece of code within my main class where I create my actual arrayList?

hd1 Over a year ago

You could, or instead of the entire ArrayList, you call .add on every Chromosome.

matsev · Accepted Answer · 2012-11-17 22:15:37Z

3

I would implement the interface Comparable:

public class Chromosome implements Comparable<Chromosome>{

    private double score;

    public Chromosome(double score){
        this.score = score;
    }
    @Override
    public int compareTo(Chromosome o) {
        return new Double(score).compareTo( o.score);
    }
    @Override
    public String toString() {
        return String.valueOf(score);
    }
}

Note that i moved score inside the Class..

Now you can use any Collection that is Sorted (like a TreeSet)

If you insist on using the Arraylist you can use:

ArrayList<Chromosome> out = new ArrayList<Chromosome>();
out.add(new Chromosome(20));
out.add(new Chromosome(15));
System.out.println(out);
Collections.sort(out);
System.out.println(out);

Result:

[0.2, 0.15]
[0.15, 0.2]

edited Nov 17, 2012 at 22:15

matsev

34k18 gold badges128 silver badges169 bronze badges

answered Nov 17, 2012 at 19:47

Frank

15.7k6 gold badges38 silver badges50 bronze badges

6 Comments

Melo1991 Over a year ago

When using this and I want to sort the arrayList I call the collections.sortmethod yes? Could you please tell me how to do that? @Frank

Robin Over a year ago

Or if you do not want to implement the Comparable interface, you can use the Collections.sort( List<T>, Comparator<? super T> ) method

Melo1991 Over a year ago

I've tried using this code @Frank and it still won't sort it :-(

Frank Over a year ago

I added a proof of concept, note that i also refined the compareTo method.

Melo1991 Over a year ago

See the input in the Chromosome constructor is setting the amount of genes of the chromosome. Score is the fitness function score of the variables within the genes. I want to sort my list to have the best scores at the top and the worst at the bottom. It's not doing that, im just so confused here!

|

Beno · Accepted Answer · 2018-03-02 07:33:20Z

3

Since java 8 you can sort list of Double elements very simple.

list.sort(Comparator.comparingDouble(Chromosome::getScore));

or

Collections.sort(list, omparator.comparingDouble(Chromosome::getScore));

If you wanna get sorted list but you don't want to change your beginning list you can do it as following:

List<Chromosome> sortedList = list.stream()
     .sorted(Comparator.comparingDouble(A::getScore))
     .collect(Collectors.toList());

edited Mar 2, 2018 at 7:33

answered Mar 2, 2018 at 7:26

Beno

1,02713 silver badges25 bronze badges

Collectives™ on Stack Overflow

Sorting a double value of an object within an arrayList

5 Answers 5

3 Comments

Comments

2 Comments

6 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

3 Comments

Comments

2 Comments

6 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related