List 排序

Java API针对集合类型的排序提供了2个方法

void java.util.Collections.sort(List<T> list)
void java.util.Collections.sort(List<T> list, Comparator<? super T> c)

如果集合里面元素都实现了Comparable接口，则可以直接调用第一个方法排序，又称为内部排序。如果需要按照自己的想法排序，可以调用第二个方法，并传一个Comparator进去定义排序规则。
Java针对数组也实现了排序方法，可以调用Arrays.sort方法来排序。

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;

public class ListArraySort {
    static class Book {
        String name;
        double price;
        
        public Book(String name, double price) {
            this.name = name;
            this.price = price;
        }
        
        public String getName() {
            return name;
        }
        
        public String toString() {
            return name + " " + price;
        }
    }
    
    static class Person implements Comparable<Person> {
        String name;
        int age;
        
        public Person(String name, int age) {
            this.name = name;
            this.age = age;
        }
        
        public int getAge() {
            return age;
        }
        
        @Override
        public int compareTo(Person o) {
            // 按照年龄升序排列
            return this.getAge() - o.getAge();
        }
        
        public String toString() {
            return name + " " + age;
        }
    }
    
    public static void main(String[] args) {
        testInnerSort();
        testOuterSort();
        testArraySort();
    }
    
    public static void testInnerSort() {
        List<Person> listA = new ArrayList<Person>();
        listA.add(new Person("Messi", 32));
        listA.add(new Person("Neymar", 27));
        
        java.util.Collections.sort(listA);
        for (Person person : listA) {
            System.out.println(person);
        }
    }
    
    public static void testOuterSort() {
        List<Book> listB = new ArrayList<>();
        listB.add(new Book("Data Structures and Algorithm Analysis in C", 23.7));
        listB.add(new Book("Introduction to Algorithm", 35));
        
        java.util.Collections.sort(listB, new Comparator<Book>() {
            @Override
            public int compare(Book o1, Book o2) {
                // 按照书名降序排列
                return o2.getName().compareTo(o1.getName());
            }
        });
        for (Book book : listB) {
            System.out.println(book);
        }
    }
    
    public static void testArraySort() {
        Integer[] array = new Integer[] {5, 4, 6, 3, 2, 8, 9, 12, 3};
        Arrays.sort(array, new Comparator<Integer>() {
            @Override
            public int compare(Integer o1, Integer o2) {
                // 降序排列
                return o2 - o1;
            }
        });
        System.out.println(Arrays.asList(array));
    }
}

输出

Neymar 27
Messi 32
Introduction to Algorithm 35.0
Data Structures and Algorithm Analysis in C 23.7
[12, 9, 8, 6, 5, 4, 3, 3, 2]