Exercise: Adjust Kotlin for Java usage
Consider the following Kotlin elements:
package advanced.java
data class Money(
val amount: BigDecimal = BigDecimal.ZERO,
val currency: Currency = Currency.EUR,
) {
companion object {
fun eur(amount: String) =
Money(BigDecimal(amount), Currency.EUR)
fun usd(amount: String) =
Money(BigDecimal(amount), Currency.USD)
val ZERO_EUR = eur("0.00")
}
}
fun List<Money>.sum(): Money? {
if (isEmpty()) return null
val currency = this.map { it.currency }.toSet().single()
return Money(
amount = sumOf { it.amount },
currency = currency
)
}
operator fun Money.plus(other: Money): Money {
require(currency == other.currency)
return Money(amount + other.amount, currency)
}
enum class Currency {
EUR, USD
}
This is how they can be used in Kotlin:
fun main() {
val money1 = Money.eur("10.00")
val money2 = Money.eur("29.99")
println(listOf(money1, money2, money1).sum())
// Money(amount=49.99, currency=EUR)
println(money1 + money2)
// Money(amount=39.99, currency=EUR)
val money3 = Money.usd("10.00")
val money4 = Money()
val money5 = Money(BigDecimal.ONE)
val money6 = Money.ZERO_EUR
}
However, Java usage is not so convenient. Your task is to add appropriate annotations so it is more Java-friendly and can be used like this:
package advanced.java;
import java.math.BigDecimal;
import java.util.List;
public class JavaClass {
public static void main(String[] args) {
Money money1 = Money.eur("10.00");
Money money2 = Money.eur("29.99");
List<Money> moneyList =
List.of(money1, money2, money1);
System.out.println(MoneyUtils.plus(money1, money2));
// Money(amount=39.99, currency=EUR)
Money money3 = Money.usd("10.00");
Money money4 = new Money();
Money money5 = new Money(BigDecimal.ONE);
Money money6 = Money.ZERO_EUR;
}
}Once you are done with the exercise, you can check your solution here.
