Lamaba表达式五——Sorted

原文链接：https://blog.youkuaiyun.com/lsgqjh/article/details/63686383
国外对Java8一系列总结的不错， 翻译过来给大家共享
这篇文章将会讲解Java 8 Stream sorted()示例， 我们能够以自然序或着用Comparator 接口定义的排序规则来排序一个流。Comparator 能用用lambada表达式来初始化， 我们还能够逆序一个已经排序的流。
接下来我们将会使用java 8 的流式sorted排序List 、Map 、 Set
1、sorted() 默认使用自然序排序， 其中的元素必须实现Comparable 接口
2、sorted(Comparator<? super T> comparator) ：我们可以使用lambada 来创建一个Comparator 实例。可以按照升序或着降序来排序元素。
下面代码以自然序排序一个list

list.stream().sorted() 
   
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自然序逆序元素，使用Comparator 提供的reverseOrder() 方法

list.stream().sorted(Comparator.reverseOrder()) 
   
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使用Comparator 来排序一个list

list.stream().sorted(Comparator.comparing(Student::getAge)) 
   
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把上面的元素逆序

list.stream().sorted(Comparator.comparing(Student::getAge).reversed()) 
   
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Stream sorted() with List

我们排序一组装着Student 类对象的List 集合。 首先我们使用自然序， 接着我们使用Comparator 分别进行升序和降序：
SortList.java

package com.concretepage;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;
public class SortList {
    public static void main(String[] args) {
        List<Student> list = new ArrayList<Student>();
        list.add(new Student(1, "Mahesh", 12));
        list.add(new Student(2, "Suresh", 15));
        list.add(new Student(3, "Nilesh", 10));

        System.out.println("---Natural Sorting by Name---");
        List<Student> slist = list.stream().sorted().collect(Collectors.toList());
        slist.forEach(e -> System.out.println("Id:"+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));

        System.out.println("---Natural Sorting by Name in reverse order---");
        slist = list.stream().sorted(Comparator.reverseOrder()).collect(Collectors.toList());
        slist.forEach(e -> System.out.println("Id:"+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));        

        System.out.println("---Sorting using Comparator by Age---");
        slist = list.stream().sorted(Comparator.comparing(Student::getAge)).collect(Collectors.toList());
        slist.forEach(e -> System.out.println("Id:"+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));

        System.out.println("---Sorting using Comparator by Age with reverse order---");
        slist = list.stream().sorted(Comparator.comparing(Student::getAge).reversed()).collect(Collectors.toList());
        slist.forEach(e -> System.out.println("Id:"+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));
    }
} 
   
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* Student.java *

package com.concretepage;
public class Student implements Comparable<Student> {
    private int id;
    private String name;
    private int age;
    public Student(int id, String name, int age) {
        this.id = id;
        this.name = name;
        this.age = age;
    }
    public int getId() {
        return id;
    }
    public String getName() {
        return name;
    }
    public int getAge() {
        return age;
    }
    @Override
    public int compareTo(Student ob) {
        return name.compareTo(ob.getName());
    }
        @Override
        public boolean equals(final Object obj) {
          if (obj == null) {
             return false;
          }
          final Student std = (Student) obj;
          if (this == std) {
             return true;
          } else {
             return (this.name.equals(std.name) && (this.age == std.age));
          }
        }
        @Override
        public int hashCode() {
          int hashno = 7;
          hashno = 13 * hashno + (name == null ? 0 : name.hashCode());
          return hashno;
        }   
} 
   
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* Output *

---Natural Sorting by Name---
Id:1, Name: Mahesh, Age:12
Id:3, Name: Nilesh, Age:10
Id:2, Name: Suresh, Age:15
---Natural Sorting by Name in reverse order---
Id:2, Name: Suresh, Age:15
Id:3, Name: Nilesh, Age:10
Id:1, Name: Mahesh, Age:12
---Sorting using Comparator by Age---
Id:3, Name: Nilesh, Age:10
Id:1, Name: Mahesh, Age:12
Id:2, Name: Suresh, Age:15
---Sorting using Comparator by Age with reverse order---
Id:2, Name: Suresh, Age:15
Id:1, Name: Mahesh, Age:12
Id:3, Name: Nilesh, Age:10 
   
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