java 8 groupingBy

純手記

package com.company.demo0.demo0;

import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.stream.Collectors;

public class Ttest {

    public static void main(String[] args) {

        List<XXX> makeXXXList = makeXXXList();

        // 按type分組
        Map<Integer, List<XXX>> collect = makeXXXList.stream().collect(Collectors.groupingBy(XXX::getType));
        System.out.println(collect);
        //{0=[XXX{name='n0', type=0, price=0}, XXX{name='n5', type=0, price=5}], 1=[XXX{name='n1', type=1, price=1}, XXX{name='n4', type=1, price=4}], 2=[XXX{name='n2', type=2, price=2}, XXX{name='n3', type=2, price=3}]}
        System.out.println("===");
        //按type分組記算總的price
        Map<Integer, BigDecimal> collect1 = makeXXXList.stream().collect(Collectors.groupingBy(XXX::getType,
                Collectors.reducing(BigDecimal.ZERO, XXX::getPrice, BigDecimal::add)));
        System.out.println(collect1);
        // {0=5, 1=5, 2=5}
        System.out.println("===");
        // 按type分組求最大price的XXX
        Map<Integer, XXX> collect2 = makeXXXList.stream().collect(Collectors.groupingBy(XXX::getType,
                Collectors.collectingAndThen(Collectors.maxBy(Comparator.comparing(XXX::getPrice)), Optional::get)));
        System.out.println(collect2);

        System.out.println("===");
        // 按type分組求最大price
        Map<Integer, Optional<BigDecimal>> collect3 = makeXXXList.stream().collect
                (Collectors.groupingBy(XXX::getType, Collectors.mapping(x -> x.getPrice(), Collectors.maxBy(BigDecimal::compareTo))));
        System.out.println(collect3);
        // {0=Optional[5], 1=Optional[4], 2=Optional[3]}






List<Map<String, Integer>> mapArrayList = new ArrayList<Map<String, Integer>>();

        HashMap<String, Integer> stringIntegerHashMap = new HashMap<>();
        stringIntegerHashMap.put("A", 1);
        mapArrayList.add(stringIntegerHashMap);

        stringIntegerHashMap = new HashMap<>();
        stringIntegerHashMap.put("A", 2);
        mapArrayList.add(stringIntegerHashMap);

        stringIntegerHashMap = new HashMap<>();
        stringIntegerHashMap.put("B", 3);
        mapArrayList.add(stringIntegerHashMap);

        stringIntegerHashMap = new HashMap<>();
        stringIntegerHashMap.put("B", 4);
        mapArrayList.add(stringIntegerHashMap);

        Map<String, List<Integer>> collect = mapArrayList.stream()
                .flatMap(x -> x.entrySet().stream())
                .collect(Collectors.groupingBy(Map.Entry::getKey, Collectors.mapping(Map.Entry::getValue, Collectors.toList())));
        System.out.println(collect);

        Map<String, Integer> collect2 = mapArrayList.stream()
                .flatMap(x -> x.entrySet().stream()).
                        collect(Collectors.groupingBy(Map.Entry::getKey, Collectors.reducing(0, Map.Entry::getValue, Integer::sum)));
        System.out.println(collect2);







    }

    private static List<XXX> makeXXXList(){
        List<XXX> list = new ArrayList<>();
        list.add(new XXX("n0", 0, BigDecimal.ZERO));
        list.add(new XXX("n1", 1, new BigDecimal(1)));
        list.add(new XXX("n2", 2, new BigDecimal(2)));
        list.add(new XXX("n3", 2, new BigDecimal(3)));
        list.add(new XXX("n4", 1, new BigDecimal(4)));
        list.add(new XXX("n5", 0, new BigDecimal(5)));
        return list;
    }

    private static class XXX{

        private String name;
        private Integer type;
        private BigDecimal price;

        public XXX(String name, Integer type, BigDecimal price) {
            this.name = name;
            this.type = type;
            this.price = price;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public Integer getType() {
            return type;
        }

        public void setType(Integer type) {
            this.type = type;
        }

        public BigDecimal getPrice() {
            return price;
        }

        public void setPrice(BigDecimal price) {
            this.price = price;
        }

        @Override
        public String toString() {
            return "XXX{" +
                    "name='" + name + '\'' +
                    ", type=" + type +
                    ", price=" + price +
                    '}';
        }
    }
}