使用java 8划分对象列表中的bigdecimal值？

Question

我有一个Invoice对象列表,其中包含属性频率和数量(BigDecimal).

1)需要在不考虑精度的情况下,根据频率乘以和改变量.在下面显示的代码中输出但是输出失去了一些精度.金额值应为100.

2)是否可以用Java 8流API替换发票列表中的金额值.尝试使用

invoiceList.forEach(s -> s.getAmount().multiply(new BigDecimal("10")));

但是这不会取代对象中的金额值.在这种情况下无法弄清楚使用replaceAll,尝试下面的代码,但给出编译错误,

 newList.replaceAll((v) -> v.getAmount().multiply(frequencyFactor.get(v.getFrequency())));

整个主要课程如下.无法更改任何数据类型或设计.我必须从Web服务处理获取的列表.

public class InvoiceMain {
    public static void main(String[] args) {
        List<Invoice> invoiceList = Arrays.asList(new Invoice("Quarterly", new BigDecimal(300)), new Invoice("Annually", new BigDecimal(1200)));
        Map<String, BigDecimal> frequencyFactor = new HashMap<>();
        frequencyFactor.put("Annually", new BigDecimal(1.0 / 12));
        frequencyFactor.put("Quarterly", new BigDecimal(1.0 / 3));
        for (Invoice invoice : invoiceList) {
            invoice.setAmount(invoice.getAmount().multiply(frequencyFactor.get(invoice.getFrequency())));
        }
        System.out.println(invoiceList);
    }
}

class Invoice {
    private String frequency;
    private BigDecimal amount;

    public Invoice(String frequency, BigDecimal amount) {
        super();
        this.frequency = frequency;
        this.amount = amount;
    }

    public String getFrequency() {
        return frequency;
    }

    public void setFrequency(String frequency) {
        this.frequency = frequency;
    }

    public BigDecimal getAmount() {
        return amount;
    }

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

    @Override
    public String toString() {
        return "Invoice [frequency=" + frequency + ", amount=" + amount + "]";
    }
}

输出是:

[Invoice [frequency=Quarterly, amount=99.999..], Invoice [frequency=Annually, amount=99.999...]]

所需的输出是

[Invoice [frequency=Quarterly, amount=100], Invoice [frequency=Annually, amount=100]]

Answer 1

该缺陷使用BigDecimal构造函数和double.然后BigDecimal无法确定精度并使用一个适合double的精度,也会引入一个inprecise值(来自double).

new BigDecimal(1.0/12)

将最佳方式更改为之后除以12:使用divideBy.

Map<String, BigDecimal> divisors = ...
divisors.put("Anually", BigDecimal.valueOf(12));
...
final Map<String, BigDecimal> finalDivisors = divisors;

BigDecimal sum = invoiceList.stream()
    .map(inv -> inv.amount.divideBy(finalDivisors.get(inv.frequency))
    .sum();

在实际代码中:

static class Invoice {
    final BigDecimal amount;
    final int months;

    Invoice(BigDecimal amount, int months) {
        this.amount = amount;
        this.months = months;
    }
}

public static void main(String[] args) {
    List<Invoice> invoices = Arrays.asList(
            new Invoice(new BigDecimal("12.50"), 3),
            new Invoice(new BigDecimal("120.50"), 12),
            new Invoice(new BigDecimal("7.25"), 3));
    BigDecimal sum = invoices.stream()
            .map(inv -> inv.amount.divide(BigDecimal.valueOf(inv.months),
                                          RoundingMode.HALF_UP))
            .reduce(BigDecimal.ZERO, BigDecimal::add);

    System.out.println("Sum: " + sum); // 16.63
}