Challenge 1: A Class with an Init Block — Solution

class BankAccount(val owner: String, var balance: Double) {
    init {
        require(balance >= 0) { "Starting balance cannot be negative" }
    }

    fun deposit(amount: Double) {
        balance += amount
    }
}

fun main() {
    val account = BankAccount("Philip", 100.0)
    account.deposit(50.0)
    account.deposit(25.0)
    println("${account.owner}'s balance: ${account.balance}")
}

/*
Output:
Philip's balance: 175.0

Notes:
- The init block runs once, at construction time, and uses require() to
  throw an IllegalArgumentException immediately if balance starts negative
  (e.g. BankAccount("Philip", -10.0) would crash right there).
- deposit() mutates the var property balance directly via +=, since
  balance was declared as var in the primary constructor.
- Two deposits (50.0, then 25.0) bring the balance from 100.0 to 175.0.
*/