Sorting a List in descending order using reverseOrder() with custom Comparator?

Question

I have the following list of objects of type Supplier and I want to sort them using the reverseOrder() method (so they will be in descending order). However, after reading whole day on the internet, I still can't get this working. I'm pretty sure that this is something really small that I am missing here. The natural order in ascending order works just fine.

Here is my Supplier class:

public class Supplier  {
    private String supplierName = "";
    private String representative = "";
    private String representativesPhoneNumber = "";

    private Map<Drug, Integer> listOfDrugs = new HashMap<Drug, Integer>();

    Supplier(String n, String rep, String repPhoneNum, String drugName, double drugPrice, int stock) {
        this.supplierName = n;
        this.representative = rep;
        this.representativesPhoneNumber = repPhoneNum;
        listOfDrugs.put(new Drug(drugName, drugPrice), stock);
    }

    public Map<Drug, Integer> getListOfDrugs() {
        return this.listOfDrugs;
    }

    public static Integer getKeyExtractor(Supplier supplier, Drug drug) {
        return Optional.ofNullable(Optional.ofNullable(supplier.getListOfDrugs())
                                   .orElseThrow(() -> new IllegalArgumentException("drugs is null")).get(drug))
                       .orElseThrow(() -> new IllegalArgumentException("the drug couldn't be found"));
    }
}

It has a Map if objects <Drug, Integer> . Here is my Drug class:

public class Drug {
    private String name = "";
    private double price = 0.0;

    Drug(String n, double p) {
        this.name = n;
        this.price = p;
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        long temp;
        temp = Double.doubleToLongBits(price);
        result = prime * result + (int) (temp ^ (temp >>> 32));
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Drug other = (Drug) obj;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        if (Double.doubleToLongBits(price) != Double.doubleToLongBits(other.price))
            return false;
        return true;
    }
}

Most of the code is trimmed, for the sake of spam. :)

And my Orders class, where I actually do the sorting:

public class Orders {
    private Map <Drug, Integer> orderedDrugs = new HashMap <Drug, Integer>();
    private Vector<Supplier> suppliers = new Vector <Supplier>();   

    public void sort(Drug drug, List<Supplier> sortedSuppliers) {
        Collections.sort(suppliers, Comparator.comparing(s -> Supplier.getKeyExtractor(s, drug), Comparator.reverseOrder()));   
    }

    public List<Supplier> getSortedSuppliersByQuantity(Drug drug) {
        List <Supplier> sortedSuppliers = new ArrayList <Supplier>();
        for(Supplier s : suppliers) {
            for(Entry<Drug, Integer> entry : s.getListOfDrugs().entrySet()) {
                if(entry.getKey().getDrugsName().equals(drug.getDrugsName()));
                    sortedSuppliers.add(s);
            }
        }
        sort(drug, sortedSuppliers);
        return sortedSuppliers;
    }
}

The code is trimmed again, only displaying the needed methods for the actual problem.

So I've tried so far with:

1. Collections.sort(suppliers, Comparator.comparing(s -> Supplier.getKeyExtractor(s, drug), Comparator.reverseOrder()));

2. Collections.sort(suppliers, Collections.reverseOrder(Comparator.comparing(s -> Supplier.getKeyExtractor(s, drug))));

But both don't work. Do I need to implement compareTo() somewhere or am I missing some method? Since ascending order is working, but not descending.

Going with Collections.sort(suppliers, Comparator.comparing(s -> Supplier.getKeyExtractor(s, drug))); sorts them in ascending order and works.

Thank you for the help in advance and I'm sorry for the long post!

UPDATE:

I have also tried to implement compareTo in the Supplier class, but I get a NPE. :/

public int compareTo(Supplier a) {
    for(Entry<Drug, Integer> entry : listOfDrugs.entrySet()) {
        int result = listOfDrugs.get(entry.getKey()).compareTo(a.listOfDrugs.get(entry.getKey()));
        if(result != 0)
            return result;
    }
    return 0;
}

Answer 1

Try

Collections.sort(suppliers, 
                 Comparator.comparing((Supplier s) -> Supplier.getKeyExtractor(s, drug)).reversed());

I built a simplified version and this worked. I did not try it with you Supplier , etc classes.

Answer 2

OK, I've found a workaround to the problem, since everything else didn't work for me. After I sort the list in ascending order, using the sort method, I just call: Collections.reverse(myList); and I get the sorted list in descending order. I know that this is probably lame, but it works for me.

Answer 3

Yes you need to. Implement Comparator (and tweak it as required), and invoke the sort method.

Update : For doing it using lambda expressions, try as shown here .

Update #2:

Below is what I came up with. Hope it helps :

    /**
Input:[9, 9, 5, 1, 6, 3, 9, 4, 7, 1]
Reversed:[1, 7, 4, 9, 3, 6, 1, 5, 9, 9]
ReverseOrdered:[9, 9, 9, 7, 6, 5, 4, 3, 1, 1]
     */
    private static void testCollectionsSort() {

        List<Integer> integerList = new ArrayList<>();
        int size=10;
        Random random = new Random();
        for(int i=0;i<size;i++) {
            integerList.add(random.nextInt(size));
        }

        System.out.println("Input:"+integerList);
        List<Integer> integerListTwo = new ArrayList<>(integerList);
        Collections.reverse(integerListTwo);
        System.out.println("Reversed:"+integerListTwo);
        Comparator<Integer> integerComparator = (Integer a, Integer b) -> b.compareTo(a); // 'b' is compared to 'a' to enable reverse
        Collections.sort(integerList, integerComparator);

        System.out.println("ReverseOrdered:"+integerList);
    }