Challenge 2: Operator Overloading for a Money Type — Solution

data class Money(val cents: Int) : Comparable<Money> {
    operator fun plus(other: Money) = Money(cents + other.cents)
    override fun compareTo(other: Money): Int = cents.compareTo(other.cents)
}

fun main() {
    val price = Money(1999)
    val tax = Money(160)
    val total = price + tax
    println("Total: ${total.cents} cents")

    val budget = Money(2000)
    println("Over budget: ${total > budget}")
}

/*
Output:
Total: 2159 cents
Over budget: true

Notes:
- operator fun plus(other: Money) makes "price + tax" call
  price.plus(tax) automatically, returning a new Money with the summed
  cents value.
- compareTo is implemented by having Money implement Comparable<Money>
  (Course 1's generic-constraint pattern from Chapter 3) and overriding
  compareTo — Kotlin then maps >, <, >=, <= directly onto that single
  method, so "total > budget" works without any separate operator fun
  needed for ">" itself.
- Both operators map onto operations that already have an obvious,
  intuitive meaning for a Money type — exactly the kind of case operator
  overloading is meant for.
*/