Java 8 新特性：Comparator.naturalOrder | 自然排序

Java
naturalOrder是比较器功能接口的静态方法。
Java 8中引入的Comparator.naturalOrder方法返回一个比较器，该比较器以自然顺序比较可比较对象。
对于自然排序，一个类需要实现Comparable并定义compareTo方法。
一个对象的集合根据compareTo方法以自然排序进行排序。
像Integer、String和Date这样的Java类实现了Comparable接口并覆盖了其compareTo方法，它们以词表顺序(lexicographic-order)进行排序。
从Java源代码中找到naturalOrder方法声明。

static <T extends Comparable<? super T>> Comparator<T> naturalOrder()

为了扭转自然排序，可以使用Comparator.reverseOrder方法。
提供一些例子，说明如何使用Comparator.naturalOrder与Stream.sorted、Collections.sort、List.sort和Arrays.sort一起对对象的集合进行自然排序。

与 Stream.sorted 一起使用

Stream.sorted返回一个由这个流的元素组成的流，根据提供的比较器进行排序。
StreamSortedDemo.java

import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
public class StreamSortedDemo {
    public static void main(String[] args) {
        List<Integer> numList = Arrays.asList(12, 10, 15, 8, 11);
        numList.stream().sorted(Comparator.naturalOrder()).forEach(n -> System.out.print(n + " "));
        System.out.println("\n-----------");
        List<String> strList = Arrays.asList("Varanasi", "Allahabad", "Kanpur", "Noida");
        strList.stream().sorted(Comparator.naturalOrder()).forEach(s -> System.out.print(s + " "));
        System.out.println("\n-----------"); 
        List<Student> stdList = Student.getStudentList();
        stdList.stream().sorted(Comparator.naturalOrder()).forEach(s -> System.out.print(s.getName() + " "));
        System.out.println("\n-----------"); 
        //reverse order of natural order using Comparator.reverseOrder()
        stdList = Student.getStudentList();
        stdList.stream().sorted(Comparator.reverseOrder()).forEach(s -> System.out.print(s.getName() + " ")); 
    }
}

Student.java

import java.util.Arrays;
import java.util.List;
public class Student implements Comparable<Student> {
    private String name;
    private int age;
    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public int getAge() {
        return age;
    }
    @Override
    public int compareTo(Student s) {
        return name.compareTo(s.getName());
    }
    public static List<Student> getStudentList() {
        Student s1 = new Student("Ram", 18);
        Student s2 = new Student("Shyam", 22);
        Student s3 = new Student("Mohan", 19);
        Student s4 = new Student("Mahesh", 20);
        Student s5 = new Student("Krishna", 21);
        List<Student> list = Arrays.asList(s1, s2, s3, s4, s5);
        return list;
    }
}

输出

8 10 11 12 15 
-----------
Allahabad Kanpur Noida Varanasi 
-----------
Krishna Mahesh Mohan Ram Shyam 
-----------
Shyam Ram Mohan Mahesh Krish

与 Collections.sort 一起使用

Collections.sort根据给定的比较器实例对指定的列表进行排序。
CollectionsSortDemo.java

import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
public class CollectionsSortDemo {
    public static void main(String[] args) {
        List<Integer> numList = Arrays.asList(12, 10, 15, 8, 11);
        Collections.sort(numList, Comparator.naturalOrder());
        numList.forEach(n -> System.out.print(n + " "));
        System.out.println("\n-----------");
        List<String> strList = Arrays.asList("Varanasi", "Allahabad", "Kanpur", "Noida");
        Collections.sort(strList, Comparator.naturalOrder());
        strList.forEach(s -> System.out.print(s + " "));
        System.out.println("\n-----------"); 
        List<Student> stdList = Student.getStudentList();
        Collections.sort(stdList, Comparator.naturalOrder());
        stdList.forEach(s -> System.out.print(s.getName() + " ")); 
    }
}

与 List.sort 一起使用

List.sort根据给定的比较器实例对这个列表进行排序。
ListSortDemo.java

import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
public class ListSortDemo {
    public static void main(String[] args) {
        List<Integer> numList = Arrays.asList(12, 10, 15, 8, 11);
        numList.sort(Comparator.naturalOrder());
        numList.forEach(n -> System.out.print(n + " "));
        System.out.println("\n-----------");
        List<String> strList = Arrays.asList("Varanasi", "Allahabad", "Kanpur", "Noida");
        strList.sort(Comparator.naturalOrder());
        strList.forEach(s -> System.out.print(s + " "));
        System.out.println("\n-----------"); 
        List<Student> stdList = Student.getStudentList();
        stdList.sort(Comparator.naturalOrder());
        stdList.forEach(s -> System.out.print(s.getName() + " "));
    }
}

与 Arrays.sort 一起使用

Arrays.sort根据指定的比较器引起的顺序对指定的对象阵列进行排序。
ArraysSortDemo.java

import java.util.Arrays;
import java.util.Comparator;
public class ArraysSortDemo {
    public static void main(String[] args) {
        Student s1 = new Student("Ram", 18);
        Student s2 = new Student("Shyam", 22);
        Student s3 = new Student("Mohan", 19);
        Student[] stdArray = { s1, s2, s3 };
        Arrays.sort(stdArray, Comparator.naturalOrder());
        for (Student s : stdArray) {
            System.out.print(s.getName() + " ");
        }
    }
}