JuniorTheoryCommon

What does the program print?

JAVA

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        P[] ps = {new P("A", 2), new P("C", 1), new P("B", 3)};
        Arrays.sort(ps);
        for (P p : ps) { System.out.print(p.name()); }
    }
}
record P(String name, int age) implements Comparable<P> {
    public int compareTo(P o) { return Integer.compare(age, o.age); }
}

CAB

Expecting the names in alphabetical order: Arrays.sort and Collections.sort follow compareTo, and here it compares age, so the youngest comes first.
Expecting the oldest first: compareTo returns a negative number when this goes first, and Integer.compare(age, o.age) is negative for a smaller age.

How do you change compareTo to sort from the oldest to the youngest?
What happens if P does not implement Comparable and you call Arrays.sort?

Back to questions Open topic in roadmap →Answer it yourself — Quiz

Back to questions

JuniorTheoryCommon

What does the program print?

JAVA

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        P[] ps = {new P("A", 2), new P("C", 1), new P("B", 3)};
        Arrays.sort(ps);
        for (P p : ps) { System.out.print(p.name()); }
    }
}
record P(String name, int age) implements Comparable<P> {
    public int compareTo(P o) { return Integer.compare(age, o.age); }
}

CAB

Expecting the names in alphabetical order: Arrays.sort and Collections.sort follow compareTo, and here it compares age, so the youngest comes first.
Expecting the oldest first: compareTo returns a negative number when this goes first, and Integer.compare(age, o.age) is negative for a smaller age.

How do you change compareTo to sort from the oldest to the youngest?
What happens if P does not implement Comparable and you call Arrays.sort?

Back to questions Open topic in roadmap →Answer it yourself — Quiz