本文介绍 Java 8 中 Stream API 的 sorted 方法使用方法，包括自然排序、使用 Comparator 进行升序和降序排序，并通过示例展示如何对 List、Set 和 Map 进行排序。
原文：https://www.concretepage.com/java/jdk-8/java-8-stream-sorted-example
Stream sorted() using Natural Ordering, Comparator and Reverse Ordering
Find the syntax ofsorted() method. 1.
sorted(): It sorts the elements of stream using natural ordering. The element class must implement Comparable interface. 2.
sorted(Comparator<? super T> comparator): Here we create an instance of Comparator using lambda expression. We can sort the stream elements in ascending and descending order. The following line of code will sort the list in natural ordering.
list.stream().sorted()
Comparator provides reverseOrder() method. We use it as follows.list.stream().sorted(Comparator.reverseOrder())
Comparator to sort the list.list.stream().sorted(Comparator.comparing(Student::getAge))
Comparator provides reversed() method. We use this method as follows.list.stream().sorted(Comparator.comparing(Student::getAge).reversed())
Stream sorted() with List
Here we are sorting aList of objects of Student class. First we will sort by natural ordering and then using Comparator. We will reverse both ordering natural ordering as well as ordering provided by Comparator in our example. 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()));
}
} 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;
}
} ---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
Stream sorted() with Set
Here we are sorting theSet of objects of Student class. This class must override equals() and hashCode()methods to identify unique elements. For natural ordering Student class needs to implement Comparableinterface. In our example we will sort our Set using natural ordering as well as ordering provided by Comparator. SortSet.java
package com.concretepage;
import java.util.Comparator;
import java.util.HashSet;
import java.util.Set;
public class SortSet {
public static void main(String[] args) {
Set<Student> set = new HashSet<Student>();
set.add(new Student(1, "Mahesh", 12));
set.add(new Student(2, "Suresh", 15));
set.add(new Student(3, "Nilesh", 10));
System.out.println("---Natural Sorting by Name---");
set.stream().sorted().forEach(e -> System.out.println("Id:"
+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));
System.out.println("---Natural Sorting by Name in reverse order---");
set.stream().sorted(Comparator.reverseOrder()).forEach(e -> System.out.println("Id:"
+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));
System.out.println("---Sorting using Comparator by Age---");
set.stream().sorted(Comparator.comparing(Student::getAge))
.forEach(e -> System.out.println("Id:"+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));
System.out.println("---Sorting using Comparator by Age in reverse order---");
set.stream().sorted(Comparator.comparing(Student::getAge).reversed())
.forEach(e -> System.out.println("Id:"+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));
}
} ---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 in reverse order--- Id:2, Name: Suresh, Age:15 Id:1, Name: Mahesh, Age:12 Id:3, Name: Nilesh, Age:10
Stream sorted() with Map
Here we are sorting aMap by key as well as value. SortMap.java
package com.concretepage;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;
public class SortMap {
public static void main(String[] args) {
Map<Integer, String> map = new HashMap<>();
map.put(15, "Mahesh");
map.put(10, "Suresh");
map.put(30, "Nilesh");
System.out.println("---Sort by Map Value---");
map.entrySet().stream().sorted(Comparator.comparing(Map.Entry::getValue))
.forEach(e -> System.out.println("Key: "+ e.getKey() +", Value: "+ e.getValue()));
System.out.println("---Sort by Map Key---");
map.entrySet().stream().sorted(Comparator.comparing(Map.Entry::getKey))
.forEach(e -> System.out.println("Key: "+ e.getKey() +", Value: "+ e.getValue()));
}
} ---Sort by Map Value--- Key: 15, Value: Mahesh Key: 30, Value: Nilesh Key: 10, Value: Suresh ---Sort by Map Key--- Key: 10, Value: Suresh Key: 15, Value: Mahesh Key: 30, Value: Nilesh
SortMapOfCustomObject.java
package com.concretepage;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;
public class SortMapOfCustomObject {
public static void main(String[] args) {
Map<Integer, Student> map = new HashMap<>();
map.put(1, new Student(1, "Mahesh", 12));
map.put(2, new Student(2, "Suresh", 15));
map.put(3, new Student(3, "Nilesh", 10));
//Map Sorting by Value i.e student's natural ordering i.e by name
map.entrySet().stream().sorted(Comparator.comparing(Map.Entry::getValue))
.forEach(e -> {
Integer key = (Integer)e.getKey();
Student std = (Student)e.getValue();
System.out.println("Key: " + key +", value: ("+ std.getId() +", "+ std.getName()+", "+ std.getAge()+")");
});
}
} Key: 1, value: (1, Mahesh, 12) Key: 3, value: (3, Nilesh, 10) Key: 2, value: (2, Suresh, 15)
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