I have a collection of invoices :
class Invoice {
int month;
BigDecimal amount
}
I'd like to merge these invoices, so I get one invoice per month, and the amount is the sum of the invoices amount for this month.
For example:
invoice 1 : {month:1,amount:1000}
invoice 2 : {month:1,amount:300}
invoice 3 : {month:2,amount:2000}
Output:
invoice 1 : {month:1,amount:1300}
invoice 2 : {month:2,amount:2000}
How can I do this with java 8 streams?
EDIT : as my Invoice class was mutable and it was not a problem to modify them, I choosed Eugene's solution
Collection<Invoice> invoices = list.collect(Collectors.toMap(Invoice::getMonth, Function.identity(), (left, right) -> {
left.setAmount(left.getAmount().add(right.getAmount()));
return left;
})).values();
If you could add the following copy constructor and merge method to your Invoice
class:
public Invoice(Invoice another) {
this.month = another.month;
this.amount = another.amount;
}
public Invoice merge(Invoice another) {
amount = amount.add(another.amount); // BigDecimal is immutable
return this;
}
You could reduce as you want, as follows:
Collection<Invoice> result = list.stream()
.collect(Collectors.toMap(
Invoice::getMonth, // use month as key
Invoice::new, // use copy constructor => don't mutate original invoices
Invoice::merge)) // merge invoices with same month
.values();
I'm using Collectors.toMap
to do the job, which has three arguments: a function that maps elements of the stream to keys, a function that maps elements of the stream to values and a merge function that is used to combine values when there are collisions on the keys.
If you are OK returning a Collection
it would look like this:
Collection<Invoice> invoices = list.collect(Collectors.toMap(Invoice::getMonth, Function.identity(), (left, right) -> {
left.setAmount(left.getAmount().add(right.getAmount()));
return left;
})).values();
If you really need a List
:
list.stream().collect(Collectors.collectingAndThen(Collectors.toMap(Invoice::getMonth, Function.identity(), (left, right) -> {
left.setAmount(left.getAmount().add(right.getAmount()));
return left;
}), m -> new ArrayList<>(m.values())));
Both obviously assume that Invoice
is mutable...
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