Java from zero: a record keeps the cents

A record is a class with the constructor, the fields, and the readers written for you. Money has one component, cents. add returns a new Money. It does not change the one you already hold.

A record is a value you do not mutate#

A record gives you a constructor, accessors, and equality from the components you name. Money has one component, cents. add does not change this. It returns a new Money whose cents are the sum. addExact throws if the sum does not fit in an int, so a silent wrap to a negative price cannot happen. Three lattes are 450 + 450 + 450. The original latte is still 450 after the expression, because each call built a different object.

1. latte450
2. add latte900
3. add latte1350

public class Main {
    record Money(int cents) {
        Money add(Money other) {
            return new Money(Math.addExact(this.cents, other.cents));
        }
    }

    public static void main(String[] args) {
        Money latte = new Money(450);
        Money total = latte.add(latte).add(latte);
        System.out.println(total.cents());
    }
}

1350

add returns a new object#

latte is 450 cents. latte.add(latte) is a new Money of 900. Calling add(latte) on that result makes 1350. latte.cents() is still 450. The original object did not change. That is easier to follow than a method that updates a field in place, because each name keeps the value you gave it.

Math.addExact throws ArithmeticException if the sum does not fit in an int, instead of wrapping around to a negative price. Three lattes are nowhere near that limit. The call is there so a huge sum fails loudly.

Format at the edge#

1350 cents is $13.50 when you show it to a person. Divide by 100 only in the line that formats the receipt. The object keeps the int. The next lesson lets a function accept any object that has the one method it calls.