数据结构:map、Collectors、groupingBy

Question

    //TODO: aggregate bills that have same billNumber, date and customerId
    Map<Integer, BigDecimal> map = bills.stream().collect(Collectors.groupingBy(Bill::getBillNumber,
        Collectors.reducing(BigDecimal.ZERO, Bill::getNetAmount, BigDecimal::add)));

我想合计帐单的净额，但第一张帐单应该按billNumber、date和CustomerID合计。

所以这个程序的结果应该是：30.50

34.50

15.00

10.00

目前它只按BillNumber聚合。

Answer 1

首先创建一个复合键：

//@AllArgsConstructor // better to use Lombok if it's possible
//@Getter
//@EqualsAndHashCode
public class CompositeBillKey {
    private final Integer billNumber;
    private final Date billDate;
    private final String customerId;

    public Integer getBillNumber() {
        return billNumber;
    }

    public Date getBillDate() {
        return billDate;
    }

    public String getCustomerId() {
        return customerId;
    }

    public CompositeBillKey(Integer billNumber, Date billDate, String customerId) {
        this.billNumber = billNumber;
        this.billDate = billDate;
        this.customerId = customerId;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        CompositeBillKey that = (CompositeBillKey) o;
        return Objects.equals(billNumber, that.billNumber) && Objects.equals(billDate, that.billDate) && Objects.equals(customerId, that.customerId);
    }

    @Override
    public int hashCode() {
        return Objects.hash(billNumber, billDate, customerId);
    }
}

那么您可以执行以下操作：

Map<CompositeBillKey, BigDecimal> billNumberToNetAmount = bills.stream()
        .collect(Collectors.groupingBy(b -> 
                      new CompositeBillKey(b.getBillNumber(), b.getBillDate(), b.getCustomerId()),
                Collectors.reducing(BigDecimal.ZERO, Bill::getNetAmount, BigDecimal::add)));

输出：

30.50
34.50
15.00
10.00