Sorting a double value of an object within an arrayList

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).