List的排序

Java API针对集合类型排序提供了两种支持：
java.util.Collections.sort(java.util.List)
java.util.Collections.sort(java.util.List, java.util.Comparator)

1种实现Comparator接口

 public static void main(String[] a) {

        ArrayList<P> arrayList = new ArrayList<>();
        arrayList.add(new P(34, "大爷34"));
        arrayList.add(new P(3, "大爷3"));
        arrayList.add(new P(5, "大爷5"));
        arrayList.add(new P(10, "大爷10"));
        arrayList.add(new P(22, "大爷22"));


        //使用 Comparator接口来实现排序
        //通过int值排序
        arrayList.sort(new Comparator<P>() {
            @Override
            public int compare(P o1, P o2) {
                //o1-o2(结果是负数)升序
                //o2-o1(结果是正整数)降序
                return o1.age - o2.age;
            }
        });

        //通过string来进行排序
        arrayList.sort(new Comparator<P>() {
            @Override
            public int compare(P o1, P o2) {
                //o1.name.compareTo(o2.name)升序
                //o2.name.compareTo(o1.name)降序
                return o1.name.compareTo(o2.name);
            }
        });


        //Comparable排序法，要求元素实现此接口。
        Collections.sort(arrayList);
        for (P p : arrayList) {
            System.out.println(p.name);
        }
    }

二种 P implements Comparable

 static class P implements Comparable<P> {
        P(int a, String n) {
            name = n;
            age = a;
        }

        public String name;
        public int age;

        @Override
        public int compareTo(@NotNull P o) {
            return age - o.age;
        }
    }

Kotlin排序

        //排序
            var list = mutableListOf<Person>()
            list.add(Person(50, "大爷5"))
            list.add(Person(60, "大爷6"))
            list.add(Person(10, "大爷1"))
            list.add(Person(20, "大爷2"))
            //升序
//            list.sortBy { it.age }
            //降序
            list.sortByDescending { it.age }
            //多条件排序先根据age 升序排列，若age相同，根据name升序排列
            list.sortWith(compareBy({ it.age }, { it.name }))
            list.forEach { println(it.age) }