Sorting a double value of an object within an arra

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

Answer 1

With Java SE8 you could use lambda expression like so:

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

Answer 2

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()); 

Answer 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]

Answer 4

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).

Answer 5

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.