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Java 8 list 对象字段升序降序

java 来源:囧囧小先生 1次浏览

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 of 
sorted()
 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() 

To reverse the natural ordering 
Comparator
 provides 
reverseOrder()
 method. We use it as follows.



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

The following line of code is using 
Comparator
 to sort the list.



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

To reverse the ordering, 
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 a 
List
 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()));
	}
} 

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;
        }	
} 

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 

Stream sorted() with Set

Here we are sorting the 
Set
 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 
Comparable
interface. 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()));
	}
} 

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 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 a 
Map
 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()));
	}
} 

Output




---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 

Here we are sorting a map whose values are custom objects. 


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()+")"); 
	    	});
	}
} 

Output




Key: 1, value: (1, Mahesh, 12)
Key: 3, value: (3, Nilesh, 10)
Key: 2, value: (2, Suresh, 15) 


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