如何使用Java Stream API统计小吃店每日客流总量

开发者阿泽

2026-10-4

实现方案

完全可以通过Stream API简化实现，核心思路是把每条预订记录扁平映射为「日期-对应客流」的条目，再直接分组累加即可，无需手动判断哈希表是否存在对应键，简化后代码如下：

import java.time.LocalDate;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class SnackBarOccupanceTest {
  public static void main(String args[]){
    SnackBarStat snackBarStat1 = new SnackBarStat();
    snackBarStat1.setStartDate(LocalDate.of(2021, 9, 13));
    snackBarStat1.setCustomers(2);
    snackBarStat1.setNumberOfDays(3);

    SnackBarStat snackBarStat2 = new SnackBarStat();
    snackBarStat2.setStartDate(LocalDate.of(2021, 9, 12));
    snackBarStat2.setCustomers(3);
    snackBarStat2.setNumberOfDays(2);

    SnackBarStat snackBarStat3 = new SnackBarStat();
    snackBarStat3.setStartDate(LocalDate.of(2021, 9, 13));
    snackBarStat3.setCustomers(1);
    snackBarStat3.setNumberOfDays(1);

    List<SnackBarStat> snackBarStats = List.of(snackBarStat1, snackBarStat2, snackBarStat3);
    
    // Stream API实现核心逻辑
    Map<LocalDate, Integer> occupancePerDate = snackBarStats.stream()
      .flatMap(stat -> IntStream.range(0, stat.getNumberOfDays())
        .mapToObj(i -> Map.entry(stat.getStartDate().plusDays(i), stat.getCustomers()))
      )
      .collect(Collectors.groupingBy(
        Map.Entry::getKey,
        Collectors.summingInt(Map.Entry::getValue)
      ));

    System.out.println(occupancePerDate);
  }
}