Java8 - sum求和，将 List 集合转为 Map，key去重(groupingBy)，sorted排序

package com.example.core.mydemo.java8;

public class GoodsPriceDTO {
    private Integer id;
    private String goodName;
    private Integer amount;

    //重写toString方法,System可以打印输出对象
    @Override
    public String toString() {
        return "GoodsPriceDTO{" +
                "id=" + id +
                ", goodName='" + goodName + '\'' +
                ", amount=" + amount +
                '}';
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getGoodName() {
        return goodName;
    }

    public void setGoodName(String goodName) {
        this.goodName = goodName;
    }

    public Integer getAmount() {
        return amount;
    }

    public void setAmount(Integer amount) {
        this.amount = amount;
    }
}

package com.example.core.mydemo.java8;

import java.util.*;
import java.util.stream.Collectors;

public class GoodsTotalTest {
    public static void main(String[] args) {
        List<GoodsPriceDTO> goodsPriceDTOS = new ArrayList<GoodsPriceDTO>();
        GoodsPriceDTO dto = new GoodsPriceDTO();
        dto.setId(1);
        dto.setGoodName("Apple1");
        dto.setAmount(27);
        goodsPriceDTOS.add(dto);

        GoodsPriceDTO dto2 = new GoodsPriceDTO();
        dto2.setId(2);
        dto2.setGoodName("Orange");
        dto2.setAmount(28);
        goodsPriceDTOS.add(dto2);

        GoodsPriceDTO dto3 = new GoodsPriceDTO();
        dto3.setId(3);
        dto3.setGoodName("Banana");
        dto3.setAmount(29);
        goodsPriceDTOS.add(dto3);

        GoodsPriceDTO dto4 = new GoodsPriceDTO();
        dto4.setId(4);
        dto4.setGoodName("Apple4");
        dto4.setAmount(23);
        goodsPriceDTOS.add(dto4);

        //重复键key
        GoodsPriceDTO dto5 = new GoodsPriceDTO();
        dto5.setId(4);
        dto5.setGoodName("Apple5");
        dto5.setAmount(100);
        goodsPriceDTOS.add(dto5);


        int amt = Optional.ofNullable(goodsPriceDTOS)
                .orElseGet(ArrayList::new)
                .stream()
                .filter(x -> x != null && ("Apple".equals(x.getGoodName()))).mapToInt(GoodsPriceDTO::getAmount).sum();
        System.out.println("amt=" + amt);

        //Java8 - 将 List 集合转为 Map
        //List转map
        Map<Integer,GoodsPriceDTO> map = Optional.ofNullable(goodsPriceDTOS)
                .orElseGet(ArrayList::new)
                .stream().collect(Collectors.toMap(GoodsPriceDTO::getId,GoodsPriceDTO->GoodsPriceDTO,(oldValue,newValue)->oldValue));
        System.out.println("map=" + map);

        //List转map -2
        Map<Integer,String> map2 = Optional.ofNullable(goodsPriceDTOS)
                .orElseGet(ArrayList::new)
                .stream().collect(Collectors.toMap(GoodsPriceDTO::getId,GoodsPriceDTO::getGoodName,(oldValue,newValue)->oldValue));
        System.out.println("map2=" + map2);

        //如果出现相同的key，那么会抛出重复key的异常
        //Duplicate key com.example.core.mydemo.java8.GoodsPriceDTO@20ad9418
        //输出： map2={1=Apple1, 2=Orange, 3=Banana, 4=Apple4}

        //刚才上面出现重复的ID，是根据值进行覆盖，在某些情况下需要映射成列表。即：List -> Map<Integer, List>的情况。
        Map<Integer,List<GoodsPriceDTO>> map3 = Optional.ofNullable(goodsPriceDTOS)
                .orElseGet(ArrayList::new)
                .stream().collect(Collectors.groupingBy(GoodsPriceDTO::getId));
        System.out.println("map3=" + map3);
        //map3={1=[GoodsPriceDTO{id=1, goodName='Apple1', amount=27}], 2=[GoodsPriceDTO{id=2, goodName='Orange', amount=28}], 3=[GoodsPriceDTO{id=3, goodName='Banana', amount=29}], 4=[GoodsPriceDTO{id=4, goodName='Apple4', amount=23}, GoodsPriceDTO{id=4, goodName='Apple5', amount=100}]}

        //Comparator 升序
        List<GoodsPriceDTO> newSortedList = Optional.ofNullable(goodsPriceDTOS)
                .orElseGet(ArrayList::new)
                .stream().sorted(Comparator.comparing(GoodsPriceDTO::getAmount)).collect(Collectors.toList());

        //按价格排序：
        newSortedList.stream().forEach(x -> {
            System.out.println("x=" + x);
        });

        //Comparator 降序
        List<GoodsPriceDTO> newSortedList1 = Optional.ofNullable(goodsPriceDTOS)
                .orElseGet(ArrayList::new)
                .stream().sorted(new Comparator<GoodsPriceDTO>() {
                    @Override
                    public int compare(GoodsPriceDTO o1, GoodsPriceDTO o2) {
                        return o2.getAmount().compareTo(o1.getAmount());
                    }
                }).collect(Collectors.toList());

        //按价格排序：
        newSortedList1.stream().forEach(x -> {
            System.out.println("x1=" + x);
        });



        //升序
        List<GoodsPriceDTO> newSortedList2 = Optional.ofNullable(goodsPriceDTOS)
                .orElseGet(ArrayList::new)
                .stream().sorted((a,b) ->(a.getAmount() - b.getAmount())).collect(Collectors.toList());

        //按价格排序：
        newSortedList2.stream().forEach(x -> {
            System.out.println("x2=" + x);
        });

        //降序
        List<GoodsPriceDTO> newSortedList3 = Optional.ofNullable(goodsPriceDTOS)
                .orElseGet(ArrayList::new)
                .stream().sorted((a,b) ->(b.getAmount() - a.getAmount())).collect(Collectors.toList());

        //按价格排序：
        newSortedList3.stream().forEach(x -> {
            System.out.println("x3=" + x);
        });

    }
}