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

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8.2 Comparable — what it unlocks

Lesson

Зачем оно нужно

Implementing one method makes your class work with the whole Java library. Collections.sort(list), Collections.max(list), Collections.min(list) — all of them call your compareTo and need nothing else.

Без Comparable библиотека отказывается

Collections.sort(listOfPlainObjects);
// error: no suitable method found for sort(ArrayList<Plain>)

This is not a runtime failure — the compiler refuses. Collections.sort demands that the element type be Comparable, and a class without it does not qualify.

So the contract is real: implement the interface and a lot of code you never wrote starts working with your objects.

▶ 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) {
        ArrayList<Student> list = new ArrayList<Student>();
        list.add(new Student("Ada", 90));
        list.add(new Student("Sam", 55));
        list.add(new Student("Bo", 70));
        System.out.println(list);
        Collections.sort(list);
        System.out.println(list);
        System.out.println(Collections.max(list));
    }
}

Collections.sort orders your objects by...

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Sorting a list of objects whose class is not Comparable gives...

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Type exactly what this program prints.

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

class Item implements Comparable<Item> {
    private int price;

    public Item(int price) {
        this.price = price;
    }

    public int compareTo(Item other) {
        return price - other.price;
    }

    public String toString() {
        return "" + price;
    }
}

public class Main { public static void main(String[] a){
    ArrayList<Item> l = new ArrayList<Item>();
    l.add(new Item(30));
    l.add(new Item(10));
    l.add(new Item(20));
    Collections.sort(l);
    System.out.println(l);
}}

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Which of these needs Comparable?

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True or false?

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