JDK8新特性：使用stream、Comparator和Method Reference实现集合的优雅排序

大家对java接口Comparator和Comparable都不陌生，JDK8里面Comparable还和以前一样，没有什么改动；但是Comparator在之前基础上增加了很多static和default方法。本文主要结合JDK的stream编程，学习下Comparator。阅读本文需要一些前置知识，可以参考如下文章。

JDK8新特性：接口的静态方法和默认方法
http://blog.csdn.net/aitangyong/article/details/54134385

JDK8新特性：函数式接口@FunctionalInterface的使用说明

http://blog.csdn.net/aitangyong/article/details/54137067

JDK8新特性：lambda入门
http://blog.csdn.net/aitangyong/article/details/54317539

JDK8新特性：使用Method References实现方法复用，简化lambda表达式
http://blog.csdn.net/aitangyong/article/details/54586197

可以使用Stream.sort对集合进行排序，sort有2个重载方法，区别如下。

1. // Student实现Comparable接口,默认按照id升序排列
2. public class Student implements Comparable<Student>{
3.  
4. private int id;
5.  
6. private int age;
7.  
8. private String name;
9.  
10. private Address address;
11.  
12. public Student(int id, int age, String name, Address address) {
13. this.id = id;
14. this.age = age;
15. this.name = name;
16. this.address = address;
17. }
18.  
19. public int getId() {
20. return id;
21. }
22.  
23. public void setId(int id) {
24. this.id = id;
25. }
26.  
27. public int getAge() {
28. return age;
29. }
30.  
31. public void setAge(int age) {
32. this.age = age;
33. }
34.  
35. public String getName() {
36. return name;
37. }
38.  
39. public void setName(String name) {
40. this.name = name;
41. }
42.  
43. public Address getAddress() {
44. return address;
45. }
46.  
47. public void setAddress(Address address) {
48. this.address = address;
49. }
50.  
51. @Override
52. public String toString() {
53. return "Student [id=" + id + ", age=" + age + ", name=" + name + ", address=" + address + "]";
54. }
55.  
56. @Override
57. public int compareTo(Student o) {
58. return this.id - o.id;
59. }
60.  
61. }

stream().sorted()/Comparator.naturalOrder()/Comparator.reverseOrder()，要求元素必须实现Comparable接口。

1. import java.util.ArrayList;
2. import java.util.Comparator;
3. import java.util.List;
4. import java.util.stream.Collectors;
5.  
6. public class TestComparator {
7.  
8. public static void main(String[] args) {
9. List<Student> students = buildStudents();
10.  
11. // 按照默认顺序排序
12. List<Student> ascList1 = students.stream().sorted().collect(Collectors.toList());
13. System.out.println(ascList1);
14.  
15. // 按照自然序排序(其实就是默认顺序)
16. List<Student> ascList2 = students.stream().sorted(Comparator.naturalOrder()).collect(Collectors.toList());
17. System.out.println(ascList2);
18.  
19. // 按照默认顺序的相反顺序排序
20. List<Student> descList = students.stream().sorted(Comparator.reverseOrder()).collect(Collectors.toList());
21. System.out.println(descList);
22.  
23. }
24.  
25. private static List<Student> buildStudents() {
26. List<Student> students = new ArrayList<>();
27. students.add(new Student(10, 20, "aty", new Address("d")));
28. students.add(new Student(1, 22, "qun", new Address("c")));
29. students.add(new Student(1, 26, "Zen", new Address("b")));
30. students.add(new Student(5, 23, "aty", new Address("a")));
31. return students;
32. }
33.  
34. }
35.  
36.  

如果Student没有实现Comparable接口，效果如下：

接下来测试，都不要求Student实现Comparable接口，这里直接给出Student和Address实体类。

1. public class Student {
2.  
3. private int id;
4.  
5. private int age;
6.  
7. private String name;
8.  
9. private Address address;
10.  
11. public Student(int id, int age, String name, Address address) {
12. this.id = id;
13. this.age = age;
14. this.name = name;
15. this.address = address;
16. }
17.  
18. public int getId() {
19. return id;
20. }
21.  
22. public void setId(int id) {
23. this.id = id;
24. }
25.  
26. public int getAge() {
27. return age;
28. }
29.  
30. public void setAge(int age) {
31. this.age = age;
32. }
33.  
34. public String getName() {
35. return name;
36. }
37.  
38. public void setName(String name) {
39. this.name = name;
40. }
41.  
42. public Address getAddress() {
43. return address;
44. }
45.  
46. public void setAddress(Address address) {
47. this.address = address;
48. }
49.  
50. @Override
51. public String toString() {
52. return "Student [id=" + id + ", age=" + age + ", name=" + name + ", address=" + address + "]";
53. }
54.  
55. }

1. public class Address {
2. private String address;
3.  
4. public Address(String address) {
5. super();
6. this.address = address;
7. }
8.  
9. public String getAddress() {
10. return address;
11. }
12.  
13. public void setAddress(String address) {
14. this.address = address;
15. }
16.  
17. @Override
18. public String toString() {
19. return "Address [address=" + address + "]";
20. }
21.  
22.  
23. }

Comparator.comparing(Function
keyExtractor)生成1个Comparator对象，要求keyExtractor.apply()返回值一定要实现Comparable接口。比如下面代码extractIdWay1和extractIdWay2都是等价的，从Student对象中提取id属性，而id是int类型(Integer实现了Comparable)。

1. import java.util.ArrayList;
2. import java.util.Comparator;
3. import java.util.List;
4. import java.util.function.Function;
5. import java.util.stream.Collectors;
6.  
7. public class TestComparator {
8.  
9. public static void main(String[] args) {
10. List<Student> students = buildStudents();
11.  
12. // 使用lambda表达式创建Function对象
13. Function<Student, Integer> extractIdWay1 = (student) -> student.getId();
14.  
15. // 使用方法引用简化lambda
16. Function<Student, Integer> extractIdWay2 = Student::getId;
17.  
18. // Comparator.comparing(Function keyExtractor)
19. Comparator<Student> byId = Comparator.comparing(extractIdWay2);
20.  
21. // 升序
22. List<Student> ascList = students.stream().sorted(byId).collect(Collectors.toList());
23. System.out.println(ascList);
24.  
25. // 降序
26. List<Student> descList = students.stream().sorted(byId.reversed()).collect(Collectors.toList());
27. System.out.println(descList);
28.  
29. }
30.  
31. private static List<Student> buildStudents() {
32. List<Student> students = new ArrayList<>();
33. students.add(new Student(10, 20, "aty", new Address("d")));
34. students.add(new Student(1, 22, "qun", new Address("c")));
35. students.add(new Student(1, 26, "Zen", new Address("b")));
36. students.add(new Student(5, 23, "aty", new Address("a")));
37. return students;
38. }
39.  
40. }

由于Student.getAddress()返回的对象没有实现Comparable接口，所以不能通过Comparator.comparing()创建一个Comparator对象。

如果我们想安装Address(没有实现Comparable接口)排序怎么办呢？使用另一种形式的comparing方法：

1. import java.util.ArrayList;
2. import java.util.Comparator;
3. import java.util.List;
4. import java.util.stream.Collectors;
5.  
6. public class TestComparator {
7.  
8. public static void main(String[] args) {
9. List<Student> students = buildStudents();
10.  
11. Comparator<Address> cmpAddr = Comparator.comparing(Address::getAddress);
12. Comparator<Student> byAddress = Comparator.comparing(Student::getAddress, cmpAddr);
13. List<Student> sortedAddressList = students.stream().sorted(byAddress).collect(Collectors.toList());
14. System.out.println(sortedAddressList);
15. }
16.  
17. private static List<Student> buildStudents() {
18. List<Student> students = new ArrayList<>();
19. students.add(new Student(10, 20, "aty", new Address("d")));
20. students.add(new Student(1, 22, "qun", new Address("c")));
21. students.add(new Student(1, 26, "Zen", new Address("b")));
22. students.add(new Student(5, 23, "aty", new Address("a")));
23. return students;
24. }
25.  
26. }

这种形式的comparing()接收2个参数，第一个参数提取要排序的key，第二个参数指定排序的Comparator。自己指定比较器，可以灵活定制比较逻辑。比如，我们想实现字符串不区分大小写比较。

1. //getName()返回String本身已经实现了Comparable,但是我们可以自己传递一个不区分大小写的比较器
2. Comparator<Student> byName = Comparator.comparing(Student::getName, String.CASE_INSENSITIVE_ORDER);
3. List<Student> sortedNameList = students.stream().sorted(byName).collect(Collectors.toList());
4. System.out.println(sortedNameList);

comparingDouble()、comparingLong()、comparingInt()不过是comparing()更具体的版本，使用方式相同。

1. public static void main(String[] args) {
2. List<Student> students = buildStudents();
3.  
4. Comparator<Student> byAge1 = Comparator.comparingInt(Student::getAge);
5. Comparator<Student> byAge2 = Comparator.comparing(Student::getAge);
6. List<Student> sortedAgeList1 = students.stream().sorted(byAge1).collect(Collectors.toList());
7. List<Student> sortedAgeList2 = students.stream().sorted(byAge2).collect(Collectors.toList());
8. System.out.println(sortedAgeList1);
9. System.out.println(sortedAgeList2);
10. }
11.  
12. private static List<Student> buildStudents() {
13. List<Student> students = new ArrayList<>();
14. students.add(new Student(10, 20, "aty", new Address("d")));
15. students.add(new Student(1, 22, "qun", new Address("c")));
16. students.add(new Student(1, 26, "Zen", new Address("b")));
17. students.add(new Student(5, 23, "aty", new Address("a")));
18. return students;
19. }

Comparator.nullsFirst()和Comparator.nullsLast()，前面我们创建的Student列表中没有null，如果有null的话，上面的代码都会抛异常。而这2个方法就是用来处理null的，一个认为null比所有非null都小，一个认为比所有都大。

1. public class TestComparator {
2.  
3. public static void main(String[] args) {
4. List<Student> students = buildStudents();
5. Comparator<Student> nullNotAllowed = Comparator.comparing(Student::getId);
6. Comparator<Student> allowNullComparator = Comparator.nullsFirst(nullNotAllowed);
7.  
8. // 正常排序
9. List<Student> result1 = students.stream().sorted(allowNullComparator).collect(Collectors.toList());
10. System.out.println(result1);
11.  
12. // 抛异常
13. List<Student> result2 = students.stream().sorted(nullNotAllowed).collect(Collectors.toList());
14. System.out.println(result2);
15.  
16. }
17.  
18. private static List<Student> buildStudents() {
19. List<Student> students = new ArrayList<>();
20. students.add(new Student(10, 20, "aty", new Address("d")));
21. students.add(new Student(1, 22, "qun", new Address("c")));
22. students.add(new Student(1, 26, "Zen", new Address("b")));
23. students.add(new Student(5, 23, "aty", new Address("a")));
24. students.add(null);
25. return students;
26. }
27.  
28. }

至此Comparator的static方法已经介绍完毕，接下来我们看下它的default方法。

reversed()前面已经介绍了，返回一个新的比较器(排序顺序相反)

thenComparing()系列方法与comparing()使用方法类似

如果我们先按照id排序，id相等的话再按照name排序，那么可以这样写。

1. public static void main(String[] args) {
2. List<Student> students = buildStudents();
3.  
4. // id升序
5. Comparator<Student> byIdASC = Comparator.comparing(Student::getId);
6.  
7. // named不分区大小写降序
8. Comparator<Student> byNameDESC = Comparator.comparing(Student::getName, String.CASE_INSENSITIVE_ORDER)
9. .reversed();
10.  
11. // 联合排序
12. Comparator<Student> finalComparator = byIdASC.thenComparing(byNameDESC);
13.  
14. List<Student> result = students.stream().sorted(finalComparator).collect(Collectors.toList());
15. System.out.println(result);
16. }
17.  
18. private static List<Student> buildStudents() {
19. List<Student> students = new ArrayList<>();
20. students.add(new Student(10, 20, "aty", new Address("d")));
21. students.add(new Student(1, 22, "qun", new Address("c")));
22. students.add(new Student(1, 26, "Zen", new Address("b")));
23. students.add(new Student(5, 23, "aty", new Address("a")));
24. return students;
25. }