Java8排序stream.sorted()

使用Java8排序stream.sorted()

http://www.concretepage.com/java/jdk-8/java-8-stream-

sorted-example
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 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() 

1
自然序逆序元素，使用Comparator 提供的reverseOrder() 方法

list.stream().sorted(Comparator.reverseOrder()) 

1
使用Comparator 来排序一个list

list.stream().sorted(Comparator.comparing(Student::getAge)) 

1
把上面的元素逆序

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

1
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 list = new ArrayList();
        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 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()));
    }
} 

• Student.java *

  package com.concretepage;
    public class Student implements Comparable {
        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;
            }   
    } 
  

• 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