Ordering & iteration: interview questions & scenarios

Model answer

Implement Comparable<T> when the type has one obvious natural order, such as String, Integer, or a value object ordered by its stable ID. Pass a Comparator<T> when the order belongs to a use case: sort orders by total on one screen, by customer on another, and by creation time in an export.

compareTo and compare return a sign, not a distance. Negative means left comes first, zero means same ordering slot, positive means left comes after. For sorted collections, zero is a big deal: TreeSet treats compare result 0 as "already present", and TreeMap treats it as the same key.

Model answer

import java.util.ArrayList;
import java.util.List;

public class BadComparator {
    public static void main(String[] args) {
        List<Integer> values = new ArrayList<>(List.of(5, Integer.MIN_VALUE, 3));
        values.sort((a, b) -> a - b);
        System.out.println(values); // [3, 5, -2147483648]
    }
}

The comparator subtracts, and subtraction can overflow. Integer.MIN_VALUE - 1 wraps around to a positive number, so the comparator claims the smallest possible integer is larger than 1.

Use Integer.compare(a, b), Long.compare(a, b), or comparator factories such as Comparator.comparingInt.