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).

With Java SE8 you could use lambda expression like so:

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