Sorting an array of classes based only on field name

Question

I have an application where a user provides me with the name of a field, e.g name or costInCents, and I have to sort by that field. I have ways of guaranteeing that the field name will be correct. This application causes the complication that I simply cannot make my class Comparable and implement a specific compareTo(), since with a custom implementation of compareTo() I need to know which fields / methods to use at implementation time.

So to achieve this goal, I am trying to use reflection in order to match the field to its accessor. Here's a MWE of what I want to do.

Class Product is a simple POJO class whose instances I want to pairwise compare:

public class Product
{

    final String name;
    final Integer quantity;
    final Long costInCents;

    public Product(final String name, final Integer quantity, final Long costInCents)
    {
        this.name = name;
        this.quantity = quantity;
        this.costInCents = costInCents;
    }

    public String getName()
    {
        return name;
    }
    public Integer getQuantity()
    {
        return quantity;
    }
    public Long getCostInCents()
    {
        return costInCents;
    }
}

And my Main class, which is currently incomplete:

public class Main {

    public static void main(String[] args) {
        final Product[] productArray =
                {
                    new Product("Clorox wipes", 50, 700L),
                    new Product("Desk chair", 10, 12000L),
                    new Product("TV", 5, 30000L),
                    new Product("Bookcase", 5, 12000L),
                    new Product("Water bottle", 20, 700L),
                };

        // The following void methods are supposed to sort in-place with something like Arrays.sort() or Collections.sort(),
        // but I am also open to solutions involving stuff like Stream::sorted() or similar ones, which return a sorted array.
        sortByField(productArray, "costInCents");
        sortByField(productArray, "name");
    }

    private void sortByField(final Product[] productArray, final String sorterFieldName)
    {
        final Field sorterField = getSorterField(sorterFieldName, LiteProduct.class); // Gets the Field somehow
        final Method sorterAccessor = getSorterAccessor(sorterField, LiteProduct.class);    // Given the Field, this is easy
        Arrays.sort((Product p1, Product p2)->((Comparable<?>)sorterAccessor.invoke(p1)).compareTo(sorterAccessor.invoke(p2)) > 0); // Capture of ? instead of Object
    }
}

Unfortunately, the Arrays.sort() line results in a compile-time error with message Capture of ? instead of Object. I have tried casting the second argument to Comparable<?>, Comparable<? super sorterField.getType(), etc, with no luck. Ideas?

Answer 1

Possibly the best way - with sorting strategies. No need for reflection, compatible with more complex sorting logic:

Map<String, Comparator<Product>> sortingStrategies = new HashMap<>(){
    {
        put("costInCents", Comparator.comparingLong(p->p.costInCents));
        put("quantity", Comparator.comparingLong(p->p.quantity));
        put("name", Comparator.comparing(p->p.name));
    }
};

private void sortByField(final Product[] productArray, final String sorterFieldName)
{
    Arrays.sort(productArray, sortingStrategies.get(sorterFieldName));
}

Answer 2

You could write a Comparator for each field and use it by name via a Map:

public class Product
{
    private final static Map<String,Comparator<Product>> COMPARATORS;

    static {
        COMPARATORS = new HashMap<>();
        COMPARATORS.put("name", new NameComparator());
        COMPARATORS.put("costInCents", new CostInCentsComparator());
    }

    final String name;
    final Integer quantity;
    final Long costInCents;

    public Product(final String name, final Integer quantity, final Long costInCents)
    {
        this.name = name;
        this.quantity = quantity;
        this.costInCents = costInCents;
    }

    public String getName()
    {
        return name;
    }
    public Integer getQuantity()
    {
        return quantity;
    }
    public Long getCostInCents()
    {
        return costInCents;
    }

    static class NameComparator implements Comparator<Product> {

        @Override
        public int compare(Product o1, Product o2) {
            return o1.getName().compareTo(o2.getName());
        }

    }

    static class CostInCentsComparator implements Comparator<Product> {

        @Override
        public int compare(Product o1, Product o2) {
            return o1.getCostInCents().compareTo(o2.getCostInCents());
        }

    }

    static Comparator<Product> getComparator(String name) {
        return COMPARATORS.get(name);
    }

}

and use that in the main class

public class Main {

    public static void main(String[] args) {
        final Product[] productArray =
                {
                    new Product("Clorox wipes", 50, 700L),
                    new Product("Desk chair", 10, 12000L),
                    new Product("TV", 5, 30000L),
                    new Product("Bookcase", 5, 12000L),
                    new Product("Water bottle", 20, 700L),
                };

        // The following void methods are supposed to sort in-place with something like Arrays.sort() or Collections.sort(),
        // but I am also open to solutions involving stuff like Stream::sorted() or similar ones, which return a sorted array.
        sortByField(productArray, "costInCents");
        sortByField(productArray, "name");
    }

    private static void sortByField(final Product[] productArray, final String sorterFieldName)
    {
        Arrays.sort(productArray, Product.getComparator(sorterFieldName));
    }
}

You may have to do minor changes like makeing it null safe or something

Answer 3

you can try this:

public class Test{
  public static void main(String arg[]){
  final Product[] productArray =
            {
                new Product("Clorox wipes", 50, 700L),
                new Product("Desk chair", 10, 12000L),
                new Product("TV", 5, 30000L),
                new Product("Bookcase", 5, 12000L),
                new Product("Water bottle", 20, 700L),
            };
            
      Arrays.sort(productArray,(p1, p2) -> p1.getName().compareTo(p2.getName()));
 
     for(Product p: productArray){
        System.out.println(p.getName());
     }
    }
   }

in this case Comparator is a functional interface so I used lambad expression. but you can do this too