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

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8.2 Comparable — the ordering contract

Lesson

Интерфейс, который вы уже встречали

Comparable is the interface Java itself defines, and it has exactly one method: compareTo. Implementing it means promising «objects of my class know how to compare themselves to each other».

You have used it already without noticing: String and Integer implement Comparable — that is why "apple".compareTo("banana") works.

Как реализовать

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

    public int compareTo(Student other) {
        return score - other.score;   // negative / 0 / positive
    }
}

The contract is about the sign, not the number: negative means «I come first», 0 means equal, positive means «the other comes first». Subtracting is the shortest way for numbers. (For very large values the subtraction can overflow; Integer.compare(a, b) is the safe version.)

Notice: your class decides its own natural order. For students by score, for books by year, for cards by rank.

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

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

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) {
        Student x = new Student("X", 50);
        Student y = new Student("Y", 80);
        System.out.println(x.compareTo(y));
        System.out.println(x.compareTo(y) < 0);
        System.out.println(y.compareTo(x) > 0);
    }
}

How many methods does Comparable require?

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compareTo must return a negative number when...

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Type exactly what this program prints.

class Box implements Comparable<Box> {
    private int size;

    public Box(int size) {
        this.size = size;
    }

    public int compareTo(Box other) {
        return size - other.size;
    }
}

public class Main { public static void main(String[] a){
    Box small = new Box(2);
    Box big = new Box(9);
    System.out.println(small.compareTo(big));
    System.out.println(big.compareTo(small) > 0);
}}

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Why do String and Integer already have compareTo?

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True or false?

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