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)