Finding the max of a list of arrays using custom comparator

Question

I have a Breakfast class that looks like this:

class Breakfast {

     String[] fruits;
     ...

     // Getter and Setter here
     ...
}

The fruits field will always be a size two array containing one of three possible values: {"apple", "pear"} , {"apple", "grape"}, {"pear", "grape"}

I have designed a custom order for the three values, like this:

    String[] orderOne = {"apple", "pear"};
    String[] orderTwo = {"apple", "grape"};
    String[] orderThree = {"pear", "grape"};

And I wrote my own custom comparator:

    List<String[]> breakfastOrder = Arrays.asList(orderOne, orderTwo, orderThree);

    Comparator<Breakfast> orderComparator = Comparator.comparing(b -> breakfastOrder.indexOf(new String[] {breakfast.getFruits()[0], breakfast.getFruits()[1]});

When working with a list of Breakfast objects, I am hoping to find the "Max" fruit combination.

In other words, if {"pear", "grape"} is found, {"pear", "grape"} would be the "Max". If {"pear", "grape"} is not found, but {"apple", "grape"} is found, {"apple", "grape"} would be the max.

How do I find the "Max" when I have a list of Breakfast objects? The stream has a max function, could I use it with my custom comparator?

I was thinking something like this:

List<Breakfast> bList = //initialize the list

String[] max = bList.stream.max(orderComparator).get().getFruits();

Please let me know if any part has changed in Java 11. Also, please let me know if there anything wrong with my code or if my logic/implementation is flawed.

Answer 1

If you can override equals/hashCode for Breakfast , simplified here (don't write equals like this):

    @Override
    public int hashCode() {
        return Arrays.hashCode(fruits);
    }

    @Override
    public boolean equals(Object other) {
        Breakfast b = (Breakfast) other;
        return Arrays.equals(b.getFruits(), getFruits());
    }

You could create a Map and keep indexes (you can think about it as a Comparator strength if you want):

 Map<Breakfast, Integer> MAP = ImmutableMap.of(
        new Breakfast(new String[]{"pear", "grape"}), 1,
        new Breakfast(new String[]{"apple", "grape"}), 2,
        new Breakfast(new String[]{"apple", "pear"}), 3);

And sorting them via:

Breakfast max = Collections.max(
        yourListOfGrapes,
        Comparator.comparingInt(b -> Optional.ofNullable(MAP.get(b)).orElse(0))
                  .reversed());

Answer 2

Because you are manually defining the order/priority of the pair. You can make your job easier by adding order# in the String array to make it of 3 elements size. Now you can sort/max/min with the 3rd member of the array.

String[] orderOne = { "apple", "pear", "1" };
    String[] orderTwo = { "apple", "grape", "2" };
    String[] orderThree = { "pear", "grape", "3" };

    List<String[]> bList = Arrays.asList(orderOne, orderTwo, orderThree);
    String[] max = bList.stream().max((a, b) -> a[2].compareTo(b[2])).get();
    System.out.println(Arrays.toString(max));