Sorting By Functions in Karel

Sometimes we’ll want to sort a collection by something other than its natural order. For example, suppose we wanted to sort strings by their length instead of alphabetically. Here’s an example of custom sorts in Java.

import java.util.*;

public class SortingByFunctions {
    public static void main(String[] args) {
        List<String> fruits = new ArrayList<>(Arrays.asList("peach", "banana", "kiwi"));

        // We implement a comparison function for string lengths using a Comparator.
        Comparator<String> lenCmp = (a, b) -> Integer.compare(a.length(), b.length());

        // Now we can call Collections.sort with this custom comparison function to sort fruits by name length.
        Collections.sort(fruits, lenCmp);
        System.out.println(fruits);

        // We can use the same technique to sort a list of values that aren't built-in types.
        static class Person {
            String name;
            int age;

            Person(String name, int age) {
                this.name = name;
                this.age = age;
            }

            @Override
            public String toString() {
                return "{" + name + " " + age + "}";
            }
        }

        List<Person> people = Arrays.asList(
            new Person("Jax", 37),
            new Person("TJ", 25),
            new Person("Alex", 72)
        );

        // Sort people by age using Collections.sort.
        //
        // Note: if the Person class is large,
        // you may want the list to contain Person references instead
        // and adjust the sorting function accordingly. If in
        // doubt, benchmark!
        Collections.sort(people, (a, b) -> Integer.compare(a.age, b.age));
        System.out.println(people);
    }
}

When you run this program, you’ll see:

[kiwi, peach, banana]
[{TJ 25}, {Jax 37}, {Alex 72}]

In this Java example, we use the Collections.sort method along with custom Comparator implementations to achieve custom sorting. The Comparator interface in Java serves a similar purpose to the comparison functions in the original example.

For sorting the list of Person objects, we use a lambda expression to create a Comparator that compares the age fields. This allows us to sort complex objects based on specific attributes.

Remember that unlike Go’s slices.SortFunc, which works on slices, Java’s Collections.sort works on List implementations. If you need to sort arrays, you can use Arrays.sort with similar Comparator arguments.

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Sorting By Functions in Karel