使用 Streams 将 BigDecimals 相加 [英] Adding up BigDecimals using Streams

问题描述

我有一组 BigDecimal(在本例中为 LinkedList)，我想将它们加在一起.是否可以为此使用流?

I have a collection of BigDecimals (in this example, a LinkedList) that I would like to add together. Is it possible to use streams for this?

我注意到 Stream 类有几个方法

I noticed the Stream class has several methods

Stream::mapToInt
Stream::mapToDouble
Stream::mapToLong

每个都有一个方便的 sum() 方法.但是，正如我们所知，float 和 double 算术几乎总是一个坏主意.

Each of which has a convenient sum() method. But, as we know, float and double arithmetic is almost always a bad idea.

那么，有没有一种方便的方法来总结 BigDecimals?

So, is there a convenient way to sum up BigDecimals?

这是我目前的代码.

public static void main(String[] args) {
    LinkedList<BigDecimal> values = new LinkedList<>();
    values.add(BigDecimal.valueOf(.1));
    values.add(BigDecimal.valueOf(1.1));
    values.add(BigDecimal.valueOf(2.1));
    values.add(BigDecimal.valueOf(.1));

    // Classical Java approach
    BigDecimal sum = BigDecimal.ZERO;
    for(BigDecimal value : values) {
        System.out.println(value);
        sum = sum.add(value);
    }
    System.out.println("Sum = " + sum);

    // Java 8 approach
    values.forEach((value) -> System.out.println(value));
    System.out.println("Sum = " + values.stream().mapToDouble(BigDecimal::doubleValue).sum());
    System.out.println(values.stream().mapToDouble(BigDecimal::doubleValue).summaryStatistics().toString());
}

如您所见，我正在使用 BigDecimal::doubleValue() 总结 BigDecimal，但这(如预期的那样)并不精确.

As you can see, I am summing up the BigDecimals using BigDecimal::doubleValue(), but this is (as expected) not precise.

后人回答

这两个答案都非常有帮助.我想补充一点:我的真实场景不涉及原始 BigDecimal 的集合，它们包含在发票中.但是，我可以通过使用 map() 函数来修改 Aman Agnihotri 的答案来解决这个问题:

Both answers were extremely helpful. I wanted to add a little: my real-life scenario does not involve a collection of raw BigDecimals, they are wrapped in an invoice. But, I was able to modify Aman Agnihotri's answer to account for this by using the map() function for stream:

public static void main(String[] args) {

    LinkedList<Invoice> invoices = new LinkedList<>();
    invoices.add(new Invoice("C1", "I-001", BigDecimal.valueOf(.1), BigDecimal.valueOf(10)));
    invoices.add(new Invoice("C2", "I-002", BigDecimal.valueOf(.7), BigDecimal.valueOf(13)));
    invoices.add(new Invoice("C3", "I-003", BigDecimal.valueOf(2.3), BigDecimal.valueOf(8)));
    invoices.add(new Invoice("C4", "I-004", BigDecimal.valueOf(1.2), BigDecimal.valueOf(7)));

    // Classical Java approach
    BigDecimal sum = BigDecimal.ZERO;
    for(Invoice invoice : invoices) {
        BigDecimal total = invoice.unit_price.multiply(invoice.quantity);
        System.out.println(total);
        sum = sum.add(total);
    }
    System.out.println("Sum = " + sum);

    // Java 8 approach
    invoices.forEach((invoice) -> System.out.println(invoice.total()));
    System.out.println("Sum = " + invoices.stream().map((x) -> x.total()).reduce((x, y) -> x.add(y)).get());
}

static class Invoice {
    String company;
    String invoice_number;
    BigDecimal unit_price;
    BigDecimal quantity;

    public Invoice() {
        unit_price = BigDecimal.ZERO;
        quantity = BigDecimal.ZERO;
    }

    public Invoice(String company, String invoice_number, BigDecimal unit_price, BigDecimal quantity) {
        this.company = company;
        this.invoice_number = invoice_number;
        this.unit_price = unit_price;
        this.quantity = quantity;
    }

    public BigDecimal total() {
        return unit_price.multiply(quantity);
    }

    public void setUnit_price(BigDecimal unit_price) {
        this.unit_price = unit_price;
    }

    public void setQuantity(BigDecimal quantity) {
        this.quantity = quantity;
    }

    public void setInvoice_number(String invoice_number) {
        this.invoice_number = invoice_number;
    }

    public void setCompany(String company) {
        this.company = company;
    }

    public BigDecimal getUnit_price() {
        return unit_price;
    }

    public BigDecimal getQuantity() {
        return quantity;
    }

    public String getInvoice_number() {
        return invoice_number;
    }

    public String getCompany() {
        return company;
    }
}

推荐答案

原答案

是的，这是可能的:

Original answer

Yes, this is possible:

List<BigDecimal> bdList = new ArrayList<>();
//populate list
BigDecimal result = bdList.stream()
        .reduce(BigDecimal.ZERO, BigDecimal::add);

它的作用是:

1. 获取一个List<BigDecimal>.
2. 把它变成一个Stream<BigDecimal>

3. 调用reduce方法.

1. Obtain a List<BigDecimal>.
2. Turn it into a Stream<BigDecimal>

3. Call the reduce method.

3.1.我们提供一个用于加法的标识值，即 BigDecimal.ZERO.

3.1. We supply an identity value for addition, namely BigDecimal.ZERO.

3.2.我们指定 BinaryOperator，它通过方法引用 BigDecimal::add 添加两个 BigDecimal.

3.2. We specify the BinaryOperator<BigDecimal>, which adds two BigDecimal's, via a method reference BigDecimal::add.

编辑后更新答案

我看到您添加了新数据，因此新答案将变为:

Updated answer, after edit

I see that you have added new data, therefore the new answer will become:

List<Invoice> invoiceList = new ArrayList<>();
//populate
Function<Invoice, BigDecimal> totalMapper = invoice -> invoice.getUnit_price().multiply(invoice.getQuantity());
BigDecimal result = invoiceList.stream()
        .map(totalMapper)
        .reduce(BigDecimal.ZERO, BigDecimal::add);

除了我添加了一个 totalMapper 变量之外，它几乎是相同的，它具有从 Invoice 到 BigDecimal 的函数并返回该发票的总价.

It is mostly the same, except that I have added a totalMapper variable, that has a function from Invoice to BigDecimal and returns the total price of that invoice.

然后我获得一个Stream，将其映射到一个Stream，然后将其归约为一个BigDecimal.

Then I obtain a Stream<Invoice>, map it to a Stream<BigDecimal> and then reduce it to a BigDecimal.

现在，从 OOP 设计的角度来看，我建议您也实际使用您已经定义的 total() 方法，这样会变得更容易:

Now, from an OOP design point I would advice you to also actually use the total() method, which you have already defined, then it even becomes easier:

List<Invoice> invoiceList = new ArrayList<>();
//populate
BigDecimal result = invoiceList.stream()
        .map(Invoice::total)
        .reduce(BigDecimal.ZERO, BigDecimal::add);

这里我们直接使用map方法中的方法引用.

Here we directly use the method reference in the map method.

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