Parses a well-formatted binary string and return it's decimal value, showcasing functional style in kotlin along the way

BinaryString.kt

import org.junit.Test
import kotlin.math.pow

class BinStringTest {
    @Test
    fun test() {
        println(BinaryString("111").toInt())      // 7
        println(BinaryString("10000").toInt())    // 16
        println(BinaryString("1000110").toInt())  // 70
        println(BinaryString("11111111").toInt())  // 255

        try {
            println(BinaryString("11BBCCC").toInt())  // error
        } catch (e: Exception) {
            println(e)
        }
    }
}

class BinaryString(private val str: String) {

    private val exponent = str.length - 1

    init {
        if (str.isEmpty() || !"[0-1]*".toRegex().matches(str)) {
            throw RuntimeException("Invalid binary string.")
        }
    }

    fun toInt(): Int {
        return str.toCharArray()
                .map { it.toString().toInt() }
                .foldIndexed(0f, {index, acc, digit ->
                    acc + digit * 2f.pow(exponent - index)
                })
                .toInt()
    }
}