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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, "bob@example.com")); roster.add( new Person( "Fred", IsoChronology.INSTANCE.date(1980, 6, 20), Person.Sex.MALE, "fred@example.com")); roster.add( new Person( "Jane", IsoChronology.INSTANCE.date(1990, 7, 15), Person.Sex.FEMALE, "jane@example.com")); roster.add( new Person( "George", IsoChronology.INSTANCE.date(1990, 7, 15), Person.Sex.MALE, "george@example.com")); 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()); }