How to sum a list

Short answer

Turn a List<Integer> into a number stream with mapToInt() and call sum():

void main() {
    var numbers = List.of(4, 8, 15, 16, 23, 42);
    int sum = numbers.stream().mapToInt(Integer::intValue).sum();
 
    IO.println(sum);
}

Output:

Summing a field

For objects, extract the field you need:

record Item(String name, int quantity, double price) {}
 
void main() {
    var cart = List.of(new Item("Tea", 2, 3.5), new Item("Cake", 1, 12.0));
 
    IO.println(cart.stream().mapToInt(Item::quantity).sum());
    IO.println(cart.stream().mapToDouble(i -> i.quantity() * i.price()).sum());
}

Output:

3
19.0

Money

A sum of BigDecimal values is calculated with reduce(), zero and addition:

void main() {
    var payments = List.of(new BigDecimal("19.99"), new BigDecimal("5.01"), new BigDecimal("0.10"));
    BigDecimal total = payments.stream().reduce(BigDecimal.ZERO, BigDecimal::add);
 
    IO.println(total);
}

Output:

25.10

Sums by group

summingInt() inside groupingBy() calculates a sum for each group:

record Sale(String city, int amount) {}
 
void main() {
    var sales = List.of(new Sale("Berlin", 100), new Sale("Paris", 50), new Sale("Berlin", 70));
 
    var byCity = sales.stream()
            .collect(Collectors.groupingBy(Sale::city, TreeMap::new, Collectors.summingInt(Sale::amount)));
    IO.println(byCity);
}

Output:

{Berlin=170, Paris=50}

Common mistake

Adding money amounts as double. Rounding errors accumulate:

void main() {
    var prices = List.of(0.1, 0.2, 0.3);
    double total = 0;
    for (double price : prices) {
        total += price;
    }
    IO.println(total);
}

Output:

0.6000000000000001

Thanks for reading!

Short answer

Summing a field

Money

Sums by group

Common mistake

Functions used in this recipe

Thanks for reading!