Java 8 stream - merge collections of objects sharing the same Id

Question

I have a collection of invoices :

class Invoice {
  int month;
  BigDecimal amount
}

I\'d like to merge these invoices, so I get one invoice pe

Answer 1

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 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.