Java 中 Comparator 与 Comparable区别

简介

比较Comparator 与 Comparable接口。

示例

创建一个Player 类，并加入ArrayList

public class TestLearnJava {
    public static void main(String[] args) {
        List<Player> footballTeam = new ArrayList<>();
        Player player1 = new Player(59, "John", 20);
        Player player2 = new Player(67, "Roger", 22);
        Player player3 = new Player(45, "Steven", 24);
        footballTeam.add(player1);
        footballTeam.add(player2);
        footballTeam.add(player3);

        System.out.println("Before Sorting : " + footballTeam);
        Collections.sort(footballTeam);
        System.out.println("After Sorting : " + footballTeam);
    }
}

class Player {
    private int ranking;
    private String name;
    private int age;

    public Player(int ranking, String name, int age) {
        this.ranking = ranking;
        this.name = name;
        this.age = age;
    }

    public int getRanking() {
        return ranking;
    }

    public void setRanking(int ranking) {
        this.ranking = ranking;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

输出

java: 对于sort(java.util.List<com.xx.learn.Player>), 找不到合适的方法
    方法 java.util.Collections.<T>sort(java.util.List<T>)不适用
      (推论变量 T 具有不兼容的上限
        等式约束条件：com.xx.learn.Player
        下限：java.lang.Comparable<? super T>)
    方法 java.util.Collections.<T>sort(java.util.List<T>,java.util.Comparator<? super T>)不适用
      (无法推断类型变量 T
        (实际参数列表和形式参数列表长度不同))

Comparable

正如命名所描述，Comparable是一个接口，定义一个策略用来比较相同的对象。这个被称为【自然排序】(natural ordering)。

所以，为了排序必须将类Player实现Comparable接口。

public class TestLearnJava {
    public static void main(String[] args) {
        List<Player> footballTeam = new ArrayList<>();
        Player player1 = new Player(59, "John", 20);
        Player player2 = new Player(67, "Roger", 22);
        Player player3 = new Player(45, "Steven", 24);
        footballTeam.add(player1);
        footballTeam.add(player2);
        footballTeam.add(player3);

        System.out.println("Before Sorting : " + footballTeam);
        Collections.sort(footballTeam);
        System.out.println("After Sorting : " + footballTeam);
    }
}

class Player implements Comparable<Player> {
    @Override
    public int compareTo(Player otherPlayer) {
        return Integer.compare(getRanking(), otherPlayer.getRanking());
    }

    private int ranking;
    private String name;
    private int age;

    public Player(int ranking, String name, int age) {
        this.ranking = ranking;
        this.name = name;
        this.age = age;
    }

    public int getRanking() {
        return ranking;
    }

    public void setRanking(int ranking) {
        this.ranking = ranking;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

输出

Before Sorting : [com.xx.learn.Player@6d311334, com.xx.learn.Player@682a0b20, com.xx.learn.Player@3d075dc0]
After Sorting : [com.xx.learn.Player@3d075dc0, com.xx.learn.Player@6d311334, com.xx.learn.Player@682a0b20]

Comparator

Comparator接口定义了一个compare(arg1, arg2)方法。

示例

public class TestLearnJava {
    public static void main(String[] args) {
        List<Player> footballTeam = new ArrayList<>();
        Player player1 = new Player(59, "John", 20);
        Player player2 = new Player(67, "Roger", 22);
        Player player3 = new Player(45, "Steven", 24);
        footballTeam.add(player1);
        footballTeam.add(player2);
        footballTeam.add(player3);

        System.out.println("Before Sorting : " + footballTeam);
        PlayerRankingComparator playerComparator = new PlayerRankingComparator();
        Collections.sort(footballTeam, playerComparator);
        System.out.println("After Sorting : " + footballTeam);
    }

class PlayerRankingComparator implements Comparator<Player> {

    @Override
    public int compare(Player firstPlayer, Player secondPlayer) {
        return Integer.compare(firstPlayer.getRanking(), secondPlayer.getRanking());
    }
}

class PlayerAgeComparator implements Comparator<Player> {
    @Override
    public int compare(Player firstPlayer, Player secondPlayer) {
        return Integer.compare(firstPlayer.getAge(), secondPlayer.getAge());
    }
}

输出

Before Sorting : [com.xx.learn.Player@6d311334, com.xx.learn.Player@682a0b20, com.xx.learn.Player@3d075dc0]
After Sorting : [com.xx.learn.Player@3d075dc0, com.xx.learn.Player@6d311334, com.xx.learn.Player@682a0b20]

Java 8 Comparator使用方式

Comparator<Player> byRanking = Comparator
                .comparing(Player::getRanking);

Comparator vs Comparable比较

当不能修改类的源代码时，又需要排序，则使用Comparator，否则使用comparable

当需要不同的比较策略时，可以使用Comparator

参考链接:https://www.baeldung.com/java-comparator-comparable