java遗珠之变强了的Comparator

Posted 2022-09-08 吴冬冬

一个更为强大的Comparator，可以说再也不用写侵入式的Comparable接口，而且连Comparator的lambda都可以不写了。

我先来看需要排序的类：

public class Person 
    public enum Sex 
        MALE, FEMALE
    

    private String name;
    private LocalDate birthday;
    private Sex gender;
    private String emailAddress;

    Person(String nameArg, LocalDate birthdayArg,
           Sex genderArg, String emailArg) 
        name = nameArg;
        birthday = birthdayArg;
        gender = genderArg;
        emailAddress = emailArg;
    

    public int getAge() 
        return birthday
                .until(IsoChronology.INSTANCE.dateNow())
                .getYears();
    

    public void printPerson() 
        System.out.println(name + ", " + this.getAge());
    

    public Sex getGender() 
        return gender;
    

    public String getName() 
        return name;
    

    public String getEmailAddress() 
        return emailAddress;
    

    public LocalDate getBirthday() 
        return birthday;
    
    
    public static List<Person> createRoster() 

        List<Person> roster = new ArrayList<>();
        roster.add(
                new Person(
                        "Bob",
                        IsoChronology.INSTANCE.date(2000, 9, 12),
                        Person.Sex.MALE, "[e-mail]"));
        roster.add(
                new Person(
                        "Fred",
                        IsoChronology.INSTANCE.date(1980, 6, 20),
                        Person.Sex.MALE,
                        "[e-mail]"));
        roster.add(
                new Person(
                        "Jane",
                        IsoChronology.INSTANCE.date(1990, 7, 15),
                        Person.Sex.FEMALE, "[e-mail]"));
        roster.add(
                new Person(
                        "George",
                        IsoChronology.INSTANCE.date(1990, 7, 15),
                        Person.Sex.MALE, "[e-mail]"));

        return roster;
    

编写一个测试类，并初始化数据

public class ComparotorTest 
    private static List<Person> roster;

    @BeforeClass
    public static void setup() 
        roster = Person.createRoster();
        for (Person p : roster) 
            p.printPerson();
        

    

对Person的单个字段进行排序

    @Test
    public void testWithComparing() 
        System.out.println("Comparator.comparing");
        roster.sort(Comparator.comparing(Person::getBirthday));
        roster.stream().forEach(person -> person.printPerson());
    

对Person的多个字段进行排序

    @Test
    public void testWithComparing() 
        System.out.println("Comparator.comparing");
        roster.sort(Comparator.comparing(Person::getBirthday).thenComparing(Person::getName));
        roster.stream().forEach(person -> person.printPerson());
    

对其中的某个字段倒序

    @Test
    public void testWithComparing() 
        System.out.println("Comparator.comparing");
        roster.sort(Comparator.comparing(Person::getBirthday).reversed().thenComparing(Person::getName));
        roster.stream().forEach(person -> person.printPerson());
    

让其中的一个字段为空的时候放在前面或者后面

    @Test
    public void testWithComparing() 
        System.out.println("Comparator.comparing");
        roster.sort(Comparator.comparing(Person::getBirthday,Comparator.nullsLast(Comparator.reverseOrder())).thenComparing(Person::getName));
        roster.stream().forEach(person -> person.printPerson());