Java Stream
在Java8中，可以使用lambda表达式实例化比较器(Comparator)。
还可以颠倒自然顺序和比较器(Comparator)提供的顺序。
自然排序使用Comparable提供的排序，该排序必须由实例为流元素的类实现。
在本页中，将使用java 8 Stream sorted()方法对列表List, Map和Set进行排序。

1、使用Stream sorted()完成自然排序、比较器和反向排序

下面是sorted()方法的语法

  • sorted()：它使用自然顺序对流中的元素进行排序。元素类必须实现Comparable接口。
  • sorted(Comparator<? super T> comparator)：这里使用lambda表达式创建一个Comparator实例。可以按升序和降序对流元素进行排序。

下面的代码行将按自然顺序对列表进行排序。

  1. list.stream().sorted()

要反转自然顺序，Comparator提供reverseOrder()方法。

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

下面的代码行使用Comparator对列表进行排序。

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

为了颠倒顺序，Comparator提供reversed()方法。

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

2、在List中使用Stream sorted()方法

下面对Student列表进行排序操作。首先，将按自然顺序排序，然后使用比较器(Comparator)。
下面是颠倒自然排序和比较器提供的排序的例子。
SortList.java

  1. package com.concretepage;
  3. import java.util.ArrayList;
  4. import java.util.Comparator;
  5. import java.util.List;
  6. import java.util.stream.Collectors;
  7. public class SortList {
  8.     public static void main(String[] args) {
  9.         List<Student> list = new ArrayList<Student>();
  10.         list.add(new Student(1, "Mahesh", 12));
  11.         list.add(new Student(2, "Suresh", 15));
  12.         list.add(new Student(3, "Nilesh", 10));
  14.         System.out.println("---Natural Sorting by Name---");
  15.         List<Student> slist = list.stream().sorted().collect(Collectors.toList());
  16.         slist.forEach(e -> System.out.println("Id:"+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));
  18.         System.out.println("---Natural Sorting by Name in reverse order---");
  19.         slist = list.stream().sorted(Comparator.reverseOrder()).collect(Collectors.toList());
  20.         slist.forEach(e -> System.out.println("Id:"+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));  
  22.         System.out.println("---Sorting using Comparator by Age---");
  23.         slist = list.stream().sorted(Comparator.comparing(Student::getAge)).collect(Collectors.toList());
  24.         slist.forEach(e -> System.out.println("Id:"+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));
  26.         System.out.println("---Sorting using Comparator by Age with reverse order---");
  27.         slist = list.stream().sorted(Comparator.comparing(Student::getAge).reversed()).collect(Collectors.toList());
  28.         slist.forEach(e -> System.out.println("Id:"+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));
  29.     }
  30. }

Student.java

  1. package com.concretepage;
  2. public class Student implements Comparable<Student> {
  3.     private int id;
  4.     private String name;
  5.     private int age;
  6.     public Student(int id, String name, int age) {
  7.         this.id = id;
  8.         this.name = name;
  9.         this.age = age;
  10.     }
  11.     public int getId() {
  12.         return id;
  13.     }
  14.     public String getName() {
  15.         return name;
  16.     }
  17.     public int getAge() {
  18.         return age;
  19.     }
  20.     @Override
  21.     public int compareTo(Student ob) {
  22.         return name.compareTo(ob.getName());
  23.     }
  24.     @Override
  25.     public boolean equals(final Object obj) {
  26.         if (obj == null) {
  27.             return false;
  28.         }
  29.         final Student std = (Student) obj;
  30.         if (this == std) {
  31.             return true;
  32.         } else {
  33.             return (this.name.equals(std.name) && (this.age == std.age));
  34.         }
  35.     }
  36.     @Override
  37.     public int hashCode() {
  38.         int hashno = 7;
  39.         hashno = 13 * hashno + (name == null ? 0 : name.hashCode());
  40.         return hashno;
  41.     } 
  42. }

输出

  1. ---Natural Sorting by Name---
  2. Id:1, Name: Mahesh, Age:12
  3. Id:3, Name: Nilesh, Age:10
  4. Id:2, Name: Suresh, Age:15
  5. ---Natural Sorting by Name in reverse order---
  6. Id:2, Name: Suresh, Age:15
  7. Id:3, Name: Nilesh, Age:10
  8. Id:1, Name: Mahesh, Age:12
  9. ---Sorting using Comparator by Age---
  10. Id:3, Name: Nilesh, Age:10
  11. Id:1, Name: Mahesh, Age:12
  12. Id:2, Name: Suresh, Age:15
  13. ---Sorting using Comparator by Age with reverse order---
  14. Id:2, Name: Suresh, Age:15
  15. Id:1, Name: Mahesh, Age:12
  16. Id:3, Name: Nilesh, Age:10

3、在Set中使用Stream sorted()方法

下面对Student类的集合(Set)进行排序操作，此类必须重写equals()和hashCode()方法来标识唯一的元素。对于自然排序，学生类需要实现Comparable接口。
在下面的例子中，将使用自然排序和比较器提供的排序对集合进行排序。
SortSet.java

  1. package com.concretepage;
  3. import java.util.Comparator;
  4. import java.util.HashSet;
  5. import java.util.Set;
  6. public class SortSet {
  7.     public static void main(String[] args) {
  8.         Set<Student> set = new HashSet<Student>();
  9.         set.add(new Student(1, "Mahesh", 12));
  10.         set.add(new Student(2, "Suresh", 15));
  11.         set.add(new Student(3, "Nilesh", 10));
  13.         System.out.println("---Natural Sorting by Name---");
  14.         set.stream().sorted().forEach(e -> System.out.println("Id:"
  15.                                                               + e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));
  17.         System.out.println("---Natural Sorting by Name in reverse order---");
  18.         set.stream().sorted(Comparator.reverseOrder()).forEach(e -> System.out.println("Id:"
  19.                                                                                        + e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));  
  21.         System.out.println("---Sorting using Comparator by Age---");
  22.         set.stream().sorted(Comparator.comparing(Student::getAge))
  23.             .forEach(e -> System.out.println("Id:"+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));
  25.         System.out.println("---Sorting using Comparator by Age in reverse order---");
  26.         set.stream().sorted(Comparator.comparing(Student::getAge).reversed())
  27.             .forEach(e -> System.out.println("Id:"+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));
  28.     }
  29. }

输出

  1. ---Natural Sorting by Name---
  2. Id:1, Name: Mahesh, Age:12
  3. Id:3, Name: Nilesh, Age:10
  4. Id:2, Name: Suresh, Age:15
  5. ---Natural Sorting by Name in reverse order---
  6. Id:2, Name: Suresh, Age:15
  7. Id:3, Name: Nilesh, Age:10
  8. Id:1, Name: Mahesh, Age:12
  9. ---Sorting using Comparator by Age---
  10. Id:3, Name: Nilesh, Age:10
  11. Id:1, Name: Mahesh, Age:12
  12. Id:2, Name: Suresh, Age:15
  13. ---Sorting using Comparator by Age in reverse order---
  14. Id:2, Name: Suresh, Age:15
  15. Id:1, Name: Mahesh, Age:12
  16. Id:3, Name: Nilesh, Age:10

4、在Map中使用Stream sorted()方法

这里将按键和值对Map进行排序。
SortMap.java

  1. package com.concretepage;
  3. import java.util.Comparator;
  4. import java.util.HashMap;
  5. import java.util.Map;
  6. public class SortMap {
  7.     public static void main(String[] args) {
  8.         Map<Integer, String> map = new HashMap<>();
  9.         map.put(15, "Mahesh");
  10.         map.put(10, "Suresh");
  11.         map.put(30, "Nilesh");
  13.         System.out.println("---Sort by Map Value---");
  14.         map.entrySet().stream().sorted(Comparator.comparing(Map.Entry::getValue))
  15.             .forEach(e -> System.out.println("Key: "+ e.getKey() +", Value: "+ e.getValue()));
  17.         System.out.println("---Sort by Map Key---");
  18.         map.entrySet().stream().sorted(Comparator.comparing(Map.Entry::getKey))
  19.             .forEach(e -> System.out.println("Key: "+ e.getKey() +", Value: "+ e.getValue()));
  20.     }
  21. }

输出

  1. ---Sort by Map Value---
  2. Key: 15, Value: Mahesh
  3. Key: 30, Value: Nilesh
  4. Key: 10, Value: Suresh
  5. ---Sort by Map Key---
  6. Key: 10, Value: Suresh
  7. Key: 15, Value: Mahesh
  8. Key: 30, Value: Nilesh

下面要对值为自定义对象的Map进行排序。
SortMapOfCustomObject.java

  1. package com.concretepage;
  3. import java.util.Comparator;
  4. import java.util.HashMap;
  5. import java.util.Map;
  6. public class SortMapOfCustomObject {
  7.     public static void main(String[] args) {
  8.         Map<Integer, Student> map = new HashMap<>();
  9.         map.put(1, new Student(1, "Mahesh", 12));
  10.         map.put(2, new Student(2, "Suresh", 15));
  11.         map.put(3, new Student(3, "Nilesh", 10));
  12.         //Map Sorting by Value i.e student's natural ordering i.e by name
  13.         map.entrySet().stream().sorted(Comparator.comparing(Map.Entry::getValue))
  14.             .forEach(e -> {
  15.                 Integer key = (Integer)e.getKey();
  16.                 Student std = (Student)e.getValue();
  17.                 System.out.println("Key: " + key +", value: ("+ std.getId() +", "+ std.getName()+", "+ std.getAge()+")"); 
  18.             });
  19.     }
  20. }

输出

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