Solution: Adjust Kotlin for Java usage

@file:JvmName("MoneyUtils")
package advanced.java

import java.math.BigDecimal

data class Money @JvmOverloads constructor(
    val amount: BigDecimal = BigDecimal.ZERO,
    val currency: Currency = Currency.EUR,
) {
    companion object {
        @JvmStatic
        fun eur(amount: String) =
            Money(BigDecimal(amount), Currency.EUR)

        @JvmStatic
        fun usd(amount: String) =
            Money(BigDecimal(amount), Currency.USD)

        @JvmField
        val ZERO_EUR = eur("0.00")
    }
}

@JvmName("sumMoney")
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
}