nice avarage

package com.company.avarages;

import java.util.List;
import java.util.function.BinaryOperator;
import java.util.function.UnaryOperator;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class MainBetter {
    static private final List<Double> nums=IntStream.rangeClosed(100,3000)
            .asDoubleStream()
            .boxed()
            .collect(Collectors.toList());
    interface Post {
        Double sum(Double sum, Double size);
    }

    interface Mapper extends UnaryOperator<Double>{}

    interface Reducer extends BinaryOperator<Double> {}

    static class Summers{
        static Post division=(sum, size)->sum/size;
    }

    static class Mappers{
        static Mapper identity=x->x;
    }

    static class Reducers{
        static Reducer summer=(a,b)->a+b;
    }

    static class Average{
        private final Mapper mapper;
        private final Post post;
        private final Reducer reducer;

        public Average(Mapper mapper, Post post, Reducer reducer) {
            this.mapper = mapper;
            this.post = post;
            this.reducer = reducer;
        }

        public Double average(List<Double> nums) {
            return post.sum(
                    nums.stream()
                            .map(mapper)
                            .reduce(reducer).get()
                    ,
                    (double)nums.size()
            );
        }
    }
    public static void main(String[] args) {
        System.out.print(
                new Average(Mappers.identity, Summers.division, Reducers.summer)
                        .average(nums)
        );
    }
}