Aggregate List<X> to List<X> with Java 8 Stream API

Question

I've got the following class:

class Money {
  CurrencyUnit currencyUnit;
  BigDecimal amount;
}

In my application, I get some random list of Money objects:

currencyUnit | amount
---------------------
EUR          | 5.1
EUR          | 0
USD          | 1.09
EUR          | 42
USD          | 3

Now I'd like to use the Java 8 Stream API to create the following result (simply calling BigDecimal::add for each currencyUnit's amount):

currencyUnit | amount
---------------------
EUR          | 47.1
USD          | 4.09

What I already know/did:

Stream<Money> moneyStream = moneyList.stream();

And here it ends already. I know I can use a Collector to produce a Map<CurrencyUnit, List<Money>>:

moneyStream.collect(Collectors.groupingBy(m -> m.getCurrencyUnit());

But then I still have to go through all key-value-pairs and sum up the amount data.

What's the right (and possibly easiest way) to do that? It can't be that complicated, right? :)

---

EDIT: If it's not that clear what I need, here's my old-way Java code:

Map<CurrencyUnit, Money> map = new HashMap<>();
moneyList.stream().forEach(e -> {
    Money m = map.get(e.getCurrencyUnit());
    if(m == null) {
        m = new Money();
        m.setAmount(BigDecimal.ZERO);
        m.setCurrencyUnit(e.getCurrencyUnit());
        map.put(e.getCurrencyUnit(), m);
    }
    m.setAmount(m.getAmount().add(e.getAmount()));
});
return map.values();

---

EDIT 2: Another solution, which isn't really elegant:

List<Money> list = inputList.stream()
    .collect(Collectors.groupingBy(Money::getCurrencyUnit))
    .values().stream().map(ml -> {
        Money money = new Money();
        ml.forEach(m -> {
            if(money.getCurrencyUnit() == null) {
                money.setCurrencyUnit(m.getCurrencyUnit());
                money.setAmount(m.getAmount());
            } else {
                money.setAmount(money.getAmount().add(m.getAmount()));
            }
        });
        return money;
    }).collect(Collectors.toList());

Answer 1

You can use a groupingBy collector to group the objects by CurrencyUnit. Without second argument, the groupingBy method collects the elements into a list. However, you can also specify a downstream collector if you need something else.

You can use Collectors::summingInt and Collectors::summingLong for int and long. For BigDecimal, you can fall back to Collectors::reducing:

import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.reducing;

Map<CurrencyUnit, BigDecimal> result = moneyList.stream()
    .collect(
        groupingBy(
            Money::getCurrencyUnit,
            reducing(BigDecimal.ZERO, Money::getAmount, BigDecimal::add)));

---

Edit: You can also create List<Money>:

List<Money> result = moneyList.stream()
    .collect(
        groupingBy(
            Money::getCurrencyUnit,
            reducing(BigDecimal.ZERO, Money::getAmount, BigDecimal::add)))
    .entrySet().stream()
    .map(e -> new Money(e.getKey(), e.getValue())
    .collect(toList());