Practice›Java Fundamentals›Module 8. Interfaces & Abstract Classes›8.2 Comparable

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8.2 Comparable — a different order with Comparator

Lesson

Один класс — один естественный порядок

Comparable gives a class one natural order: students by score. But sometimes you need another — the same students by name. Editing the class is wrong: the natural order is still by score, you just want a different sort this time.

That is what Comparator is for: a separate object holding an order, passed to sort as a second argument.

Worked example

Collections.sort(list, new Comparator<Student>() {
    public int compare(Student p, Student q) {
        return p.getName().compareTo(q.getName());
    }
});

Two interfaces, one job, different placement. Comparable is implemented by the class and answers «how do I compare to another?». Comparator is a separate object and answers «how do I compare these two?». One class can be sorted by any number of Comparators without being touched.

▶ Try it: run this, then change something and run it again.

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;

class Student implements Comparable<Student> {
    private String name;
    private int score;

    public Student(String name, int score) {
        this.name = name;
        this.score = score;
    }

    public String getName() {
        return name;
    }

    public int getScore() {
        return score;
    }

    public int compareTo(Student other) {
        return score - other.score;
    }

    public String toString() {
        return name + "(" + score + ")";
    }
}

public class Main {
    public static void main(String[] args) {
        ArrayList<Student> list = new ArrayList<Student>();
        list.add(new Student("Sam", 55));
        list.add(new Student("Ada", 90));
        list.add(new Student("Bo", 70));
        Collections.sort(list);
        System.out.println(list);
        Collections.sort(list, new Comparator<Student>() {
            public int compare(Student p, Student q) {
                return p.getName().compareTo(q.getName());
            }
        });
        System.out.println(list);
    }
}

Comparator differs from Comparable because it is...

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How many Comparators can one class have?

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To sort by an order other than the natural one, you pass...

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The method a Comparator must provide is called...

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True or false?

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