在 Java 中使用 stream.sorted() 对列表进行排序

Sorting a list with stream.sorted() in Java

提问by Ivan C

I'm interested in sorting a list from a stream. This is the code I'm using:

我对从流中排序列表很感兴趣。这是我正在使用的代码：

list.stream()
    .sorted((o1, o2)->o1.getItem().getValue().compareTo(o2.getItem().getValue()))
    .collect(Collectors.toList());

Am I missing something? The list is not sorting.

我错过了什么吗？该列表未排序。

It should sort the lists according to the item with the lowest value.

它应该根据具有最低值的项目对列表进行排序。

for (int i = 0; i < list.size(); i++)
{
   System.out.println("list " + (i+1));
   print(list, i);
}

And the print method:

和打印方法：

public static void print(List<List> list, int i)
{
    System.out.println(list.get(i).getItem().getValue());
}

采纳答案by Matt

This is not like Collections.sort()where the parameter reference gets sorted. In this case you just get a sorted stream that you need to collect and assign to another variable eventually:

这不像Collections.sort()参数引用被排序的地方。在这种情况下，您只会得到一个排序的流，您需要收集它并最终分配给另一个变量：

List result = list.stream().sorted((o1, o2)->o1.getItem().getValue().
                                   compareTo(o2.getItem().getValue())).
                                   collect(Collectors.toList());

You've just missed to assign the result

你刚刚错过了分配结果

回答by Tanmay Baid

It seems to be working fine:

它似乎工作正常：

List<BigDecimal> list = Arrays.asList(new BigDecimal("24.455"), new BigDecimal("23.455"), new BigDecimal("28.455"), new BigDecimal("20.455"));
System.out.println("Unsorted list: " + list);
final List<BigDecimal> sortedList = list.stream().sorted((o1, o2) -> o1.compareTo(o2)).collect(Collectors.toList());
System.out.println("Sorted list: " + sortedList);

Example Input/Output

示例输入/输出

Unsorted list: [24.455, 23.455, 28.455, 20.455]
Sorted list: [20.455, 23.455, 24.455, 28.455]

Are you sure you are not verifying list instead of sortedList[in above example] i.e. you are storing the result of stream()in a new Listobject and verifying that object?

您确定您不是在验证列表而不是sortedList[在上面的示例中] 即您将结果存储stream()在一个新List对象中并验证该对象吗？

回答by River

Use list.sortinstead:

使用list.sort来代替：

list.sort((o1, o2) -> o1.getItem().getValue().compareTo(o2.getItem().getValue()));

and make it more succinct using Comparator.comparing:

并使用Comparator.comparing以下方法使其更简洁：

list.sort(Comparator.comparing(o -> o.getItem().getValue()));

After either of these, listitself will be sorted.

在其中任何一个之后，list本身将被排序。

Your issue is that list.stream.sortedreturnsthe sorted data, it doesn't sort in place as you're expecting.

您的问题是 返回排序后的数据，它没有按照您的预期进行排序。list.stream.sorted

回答by Venkat IndianEagle

Collection<Map<Item, Integer>> itemCollection = basket.values();
Iterator<Map<Item, Integer>> itemIterator =   itemCollection.stream().sorted(new TestComparator()).collect(Collectors.toList()).iterator();



package com.ie.util;

import com.ie.item.Item;

import java.util.Comparator;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

public class TestComparator implements Comparator<Map<Item, Integer>> {

// comparator is used to sort the Items based on the price


    @Override
    public int compare(Map<Item, Integer> o1, Map<Item, Integer> o2) {


      //  System.out.println("*** compare method will be called *****");


        Item item1 = null;
        Item item2 = null;


        Set<Item> itemSet1 = o1.keySet();
        Iterator<Item> itemIterator1 = itemSet1.iterator();
        if(itemIterator1.hasNext()){
           item1 =   itemIterator1.next();
        }

        Set<Item> itemSet2 = o2.keySet();
        Iterator<Item> itemIterator2 = itemSet2.iterator();
        if(itemIterator2.hasNext()){
            item2 =   itemIterator2.next();
        }


        return -item1.getPrice().compareTo(item2.getPrice());


    }
}

**** this is helpful to sort the nested map objects like Map> here i sorted based on the Item object price .

**** 这有助于对嵌套地图对象进行排序，例如 Map> 此处我根据 Item 对象 price 进行排序。

回答by Udaya Shankara Gandhi Thalabat

Java 8 provides different utility api methods to help us sort the streams better.

Java 8 提供了不同的实用 api 方法来帮助我们更好地对流进行排序。

If your list is a list of Integers(or Double, Long, String etc.,) then you can simply sort the list with default comparators provided by java.

如果您的列表是整数（或双精度、长整型、字符串等）列表，那么您可以简单地使用 java 提供的默认比较器对列表进行排序。

List<Integer> integerList = Arrays.asList(1, 4, 3, 4, 5);

Creating comparator on fly:

即时创建比较器：

integerList.stream().sorted((i1, i2) -> i1.compareTo(i2)).forEach(System.out::println);

With default comparator provided by java 8 when no argument passed to sorted():

当没有参数传递给 sorted() 时，使用 java 8 提供的默认比较器：

integerList.stream().sorted().forEach(System.out::println); //Natural order

If you want to sort the same list in reverse order:

如果要以相反的顺序对同一列表进行排序：

 integerList.stream().sorted(Comparator.reverseOrder()).forEach(System.out::println); // Reverse Order

If your list is a list of user defined objects, then:

如果您的列表是用户定义对象的列表，则：

List<Person> personList = Arrays.asList(new Person(1000, "First", 25, 30000),
        new Person(2000, "Second", 30, 45000),
        new Person(3000, "Third", 35, 25000));

Creating comparator on fly:

即时创建比较器：

personList.stream().sorted((p1, p2) -> ((Long)p1.getPersonId()).compareTo(p2.getPersonId()))
        .forEach(person -> System.out.println(person.getName()));

Using Comparator.comparingLong() method(We have comparingDouble(), comparingInt() methods too):

使用 Comparator.comparingLong() 方法（我们也有 compareDouble()、comparingInt() 方法）：

personList.stream().sorted(Comparator.comparingLong(Person::getPersonId)).forEach(person -> System.out.println(person.getName()));

Using Comparator.comparing() method(Generic method which compares based on the getter method provided):

使用 Comparator.comparing() 方法（基于提供的 getter 方法进行比较的通用方法）：

personList.stream().sorted(Comparator.comparing(Person::getPersonId)).forEach(person -> System.out.println(person.getName()));

We can do chaining too using thenComparing() method:

我们也可以使用 thenComparing() 方法进行链接：

personList.stream().sorted(Comparator.comparing(Person::getPersonId).thenComparing(Person::getAge)).forEach(person -> System.out.println(person.getName())); //Sorting by person id and then by age.

Person class

人物类

public class Person {
    private long personId;
    private String name;
    private int age;
    private double salary;

    public long getPersonId() {
        return personId;
    }

    public void setPersonId(long personId) {
        this.personId = personId;
    }

    public Person(long personId, String name, int age, double salary) {
        this.personId = personId;
        this.name = name;
        this.age = age;

        this.salary = salary;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public double getSalary() {
        return salary;
    }

    public void setSalary(double salary) {
        this.salary = salary;
    }
}