Java 8 list 对象字段升序降序

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 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)