1.8新特性Comparator

一、普通排序：

  @Test
    public void testComparator() {
        List<Integer> list = Arrays.asList(1, 9, 3, 5, 0, 8);
        list.sort(Comparator.naturalOrder());
        System.out.println("升序：" + list);

        list.sort(Comparator.reverseOrder());
        System.out.println("降序：" + list);
    }

结果为：

升序：[0, 1, 3, 5, 8, 9]
降序：[9, 8, 5, 3, 1, 0]

二、根据对象属性排序：

1.根据属性排序

    public void testComparator() {
        List<Person> personList = new ArrayList<>();
        personList.add(new Person("Tom", 2));
        personList.add(new Person("Jerry", 4));
        personList.add(new Person("Lucy", 7));
        personList.add(new Person("Lily", 7));

        // 年龄升序
        personList.sort(Comparator.comparing(Person::getAge));
        System.out.println("年龄升序:" + personList);

        // 年龄降序
        personList.sort(Comparator.comparing(Person::getAge).reversed());
        System.out.println("年龄降序:" + personList);

        // 先年龄降序再名字升序
        personList.sort(Comparator.comparing(Person::getAge).reversed().thenComparing(Person::getName));
        System.out.println("先年龄降序再名字升序:" + personList);
    }

person对象：

public class Person {
    private String name;
    private Integer age;

    public Person(String name, Integer age) {
        this.name = name;
        this.age = age;
    }

    public Integer getAge() {
        return age;
    }

    @Override
    public String toString() {
        return "Person [name=" + name + ", age=" + age + "]";
    }
}

结果为：

年龄升序:[Person [name=Tom, age=2], Person [name=Jerry, age=4], Person [name=Lucy, age=7], Person [name=Lily, age=7]]
年龄降序:[Person [name=Lucy, age=7], Person [name=Lily, age=7], Person [name=Jerry, age=4], Person [name=Tom, age=2]]
先年龄降序再名字升序:[Person [name=Lily, age=7], Person [name=Lucy, age=7], Person [name=Jerry, age=4], Person [name=Tom, age=2]]

 2.根据属性类型排序

根据int double 。。。排序 ：即根据对象的  int double 。。。排序;

例如：

Comparator.comparingInt(Person::getAge).reversed().thenComparing(Person::getName))

3.排序对象存在null的情况：

   @Test
    public void testComparator() {
        List<Person> personList = new ArrayList<>();
        personList.add(new Person("Tom", 2));
        personList.add(new Person("Jerry", 4));
        personList.add(new Person("Lucy", 7));
        personList.add(new Person("Lily", 7));

        personList.add(null);
        // null的排序
        personList.sort(
                Comparator.nullsFirst(Comparator.comparing(Person::getAge).reversed().thenComparing(Person::getName)));
        System.out.println("null排第一:" + personList);
    }

三、比较器的常规用法：

八(三)、常用类之比较器Comparable Comparator