Java列表List的列表List排序

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　　假设一个Java列表中的元素也都是一些列表，那么对这些子列表以它们的长度进行排序。

import java.util.Collection;

import java.util.HashMap;

import java.util.List;

import java.util.Map;

import static java.util.stream.Collectors.groupingBy;

import static java.util.stream.Collectors.toList;

/**

 * (**) Sorting a list of lists according to length of sublists

 */

public class P28 {

    public static <T> List<List<T>> lsort(List<List<T>> list) {

        list.sort((xs1, xs2) -> xs1.size() - xs2.size());

        return list;

    }

    public static List<List<String>> lfsort(List<List<String>> list) {

        Map<Integer, Integer> freqs = new HashMap<>();

        list.stream().map(List::size).forEach(l -> freqs.put(l, freqs.compute(l, (k, v) -> v == null ? 1 : v + 1)));

        return list

                .stream()

                .sorted((xs1, xs2) -> freqs.get(xs1.size()) - freqs.get(xs2.size()))

                .collect(toList());

    }

    /*

    Same as scala code. This code does not works correctly because elements with same length are at different position

     */

    public static List<List<String>> lfsort1(List<List<String>> list) {

        return lsort(

                lsort(list)

                        .stream()

                        .collect(groupingBy(List::size))

                        .values().stream().collect(toList()))

                .stream()

                .flatMap(Collection::stream)

                .collect(toList());

    }

}

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