Adding up BigDecimals using Streams

Question

Original answer

Yes, this is possible:

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

What it does is:

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

3. Call the reduce method.

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

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

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);

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.

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

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);

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

This post already has a checked answer, but the answer doesn't filter for null values. The correct answer should prevent null values by using the Object::nonNull function as a predicate.

BigDecimal result = invoiceList.stream()
    .map(Invoice::total)
    .filter(Objects::nonNull)
    .filter(i -> (i.getUnit_price() != null) && (i.getQuantity != null))
    .reduce(BigDecimal.ZERO, BigDecimal::add);

This prevents null values from attempting to be summed as we reduce.

You can sum up the values of a BigDecimal stream using a reusable Collector named summingUp:

BigDecimal sum = bigDecimalStream.collect(summingUp());

The Collector can be implemented like this:

public static Collector<BigDecimal, ?, BigDecimal> summingUp() {
    return Collectors.reducing(BigDecimal.ZERO, BigDecimal::add);
}

Use this approach to sum the list of BigDecimal:

List<BigDecimal> values = ... // List of BigDecimal objects
BigDecimal sum = values.stream().reduce((x, y) -> x.add(y)).get();

This approach maps each BigDecimal as a BigDecimal only and reduces them by summing them, which is then returned using the get() method.

Here's another simple way to do the same summing:

List<BigDecimal> values = ... // List of BigDecimal objects
BigDecimal sum = values.stream().reduce(BigDecimal::add).get();

Update

If I were to write the class and lambda expression in the edited question, I would have written it as follows:

import java.math.BigDecimal;
import java.util.LinkedList;

public class Demo
{
  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)));

    // Java 8 approach, using Method Reference for mapping purposes.
    invoices.stream().map(Invoice::total).forEach(System.out::println);
    System.out.println("Sum = " + invoices.stream().map(Invoice::total).reduce((x, y) -> x.add(y)).get());
  }

  // This is just my style of writing classes. Yours can differ.
  static class Invoice
  {
    private String company;
    private String number;
    private BigDecimal unitPrice;
    private BigDecimal quantity;

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

    public Invoice(String company, String number, BigDecimal unitPrice, BigDecimal quantity)
    {
      setCompany(company);
      setNumber(number);
      setUnitPrice(unitPrice);
      setQuantity(quantity);
    }

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

    public String getCompany()
    {
      return company;
    }

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

    public String getNumber()
    {
      return number;
    }

    public void setNumber(String number)
    {
      this.number = number;
    }

    public BigDecimal getUnitPrice()
    {
      return unitPrice;
    }

    public void setUnitPrice(BigDecimal unitPrice)
    {
      this.unitPrice = unitPrice;
    }

    public BigDecimal getQuantity()
    {
      return quantity;
    }

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

If you don't mind a third party dependency, there is a class named Collectors2 in Eclipse Collections which contains methods returning Collectors for summing and summarizing BigDecimal and BigInteger. These methods take a Function as a parameter so you can extract a BigDecimal or BigInteger value from an object.

List<BigDecimal> list = mList(
        BigDecimal.valueOf(0.1),
        BigDecimal.valueOf(1.1),
        BigDecimal.valueOf(2.1),
        BigDecimal.valueOf(0.1));

BigDecimal sum =
        list.stream().collect(Collectors2.summingBigDecimal(e -> e));
Assert.assertEquals(BigDecimal.valueOf(3.4), sum);

BigDecimalSummaryStatistics statistics =
        list.stream().collect(Collectors2.summarizingBigDecimal(e -> e));
Assert.assertEquals(BigDecimal.valueOf(3.4), statistics.getSum());
Assert.assertEquals(BigDecimal.valueOf(0.1), statistics.getMin());
Assert.assertEquals(BigDecimal.valueOf(2.1), statistics.getMax());
Assert.assertEquals(BigDecimal.valueOf(0.85), statistics.getAverage());

Note: I am a committer for Eclipse Collections.