Java Comparator接口学习笔记

Comparator是一个泛型函数式接口，T表示待比较对象的类型：

@FunctionalInterface
public interface Comparator<T> {
}

1. 抽象方法

唯一的非Object抽象方法： compare

int compare(T o1, T o2)

根据o1, o2的大小关系返回：负数、0、正数

其比较逻辑依赖于赋值给Comparator类型变量的Lambda表达式、Comparator对象等：

Comparator<String> comparator = Comparator.comparingInt(String::length); // 比较String类对象的length
System.out.println(comparator.compare("Tom", "Jerry")); // -1

2. 常用静态/默认方法

① 静态方法comparing

// 源码1
public static <T, U extends Comparable<? super U>> Comparator<T> comparing(
    Function<? super T, ? extends U> keyExtractor)
{
    Objects.requireNonNull(keyExtractor);
    return (Comparator<T> & Serializable)
        (c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));
}

• 返回值：comparing返回一个T类型的Comparator对象（也就是用于比较T类型对象的Comparator）

• 参数：comparing的参数是一个Function类型，它接收T类型及T的超类型， 返回U类型及U的子类型。keyExtractor用Lambda表达式实现

  例如，输入String，提取其长度：

  Comparator<String> keyComparator = Comparator.comparing(String::length);
  keyComparator.compare("Tom", "Jerry");

• 函数体：
  • 首先，确保keyExtractor不是null
  • 然后，返回一个Lambda表达式，该表达式对c1和c2提取出来的key进行比较 （用的是c1和c2类实现的compareTo()进行比较）

public static <T, U> Comparator<T> comparing(
    Function<? super T, ? extends U> keyExtractor, Comparator<? super U> keyComparator)
{
    Objects.requireNonNull(keyExtractor);
    Objects.requireNonNull(keyComparator);
    return (Comparator<T> & Serializable)
        (c1, c2) -> keyComparator.compare(keyExtractor.apply(c1),
                                          keyExtractor.apply(c2));
}

与上面类似，只不过将key的比较逻辑由compareTo()替换成了自定义的比较逻辑

实例：

package Lambda.ComparatorDemo;

import java.util.Arrays;
import java.util.Comparator;

class Person {
    private String fname;
    private String lname;

    public Person(String fname, String lname) {
        this.fname = fname;
        this.lname = lname;
    }

    public String getFname() {
        return fname;
    }

    public String getLname() {
        return lname;
    }

    @Override
    public String toString() {
        return fname + " " + lname;
    }
}

public class ComparingDemo {
    public static void main(String[] args) {
        Person p1 = new Person("Tom", "Kenn");
        Person p2 = new Person("Alice", "Zed");
        Person[] persons1 = {p1, p2};
        Person[] persons2 = persons1.clone();

        // 依据First Name排序（用的是String类的compareTo的比较逻辑）
        Arrays.sort(persons1, Comparator.comparing(Person::getFname));
        System.out.println("Person1 (Sorted by first name): ");
        for (Person p : persons1) {
            System.out.println(p);
        }

        System.out.println();

        // 依据First Name的长度排序
        Arrays.sort(persons2, Comparator.comparing(Person::getFname, Comparator.comparingInt(String::length)));
        System.out.println("Person2 (Sorted by length of first name):");
        for (Person p : persons2) {
            System.out.println(p);
        }
    }
}

② 静态方法comparingXXX()

• comparingInt()
• comparingDouble()
• comparingLong()

与comparing是一样的思路，只不过comparingXXX()将comparing中的提取出来的key类型固定为了XXX：

public static <T> Comparator<T> comparingInt(ToIntFunction<? super T> keyExtractor) {
        Objects.requireNonNull(keyExtractor);
        return (Comparator<T> & Serializable)
            (c1, c2) -> Integer.compare(keyExtractor.applyAsInt(c1),                                       keyExtractor.applyAsInt(c2));
    }

③ 默认方法thenComparing()

thenComparing是默认方法，需要对具体的Comparator对象调用，它用于将多个Comparator结合起来实现“链式的比较”：先根据XXX比较，再根据XXX比较，...

// 源码
default Comparator<T> thenComparing(Comparator<? super T> other) {
    Objects.requireNonNull(other);
    return (Comparator<T> & Serializable) (c1, c2) -> {
        int res = compare(c1, c2);
        return (res != 0) ? res : other.compare(c1, c2);
    };
}

public class thenComparingDemo {
    public static void main(String[] args) {
        Person p1 = new Person("Alice", "Jessy");
        Person p2 = new Person("Alice", "Ann");
        Person p3 = new Person("Tom", "Smith");

        Person[] persons = {p1, p2, p3};

        // 先根据first name比较，再根据last name的长度比较
        Comparator<Person> chainedComparator = Comparator.comparing(Person::getFname)
                .thenComparing((x, y) -> (x.getLname().length() - y.getLname().length()));
        
         // 先根据first name排序，再根据last name的长度排序
        Arrays.sort(persons, chainedComparator);
        for (Person p : persons) {
            System.out.println(p);
        }
    }
}

/*
Alice Ann
Alice Jessy
Tom Smith

参考：

https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#thenComparing-java.util.Comparator

Java Comparator接口学习笔记

原文：https://www.cnblogs.com/Yaigu/p/9533335.html