Kotlin Exception Handling

Failure is always a possibility

If debugging is the process of removing software bugs, then programming must be the process of putting them in.

Edsger W. Dijkstra

Kotlin finds basic errors when it analyzes the program. Errors that could not be detected at compile time must be dealt with at runtime. We can throw exception and here we will look at catch exceptions. Improved error handling is a better way to increase code reliability.

Beware of Exception Subtypes

The testMyCode() function throws IllegalArgumentException when incorrect code is provided.

fun testMyCode(code: Int) {
  if (code < 0) {
    throw IllegalArgumentException("Code must be positive")
  }
}

fun main() {
  try {
    testMyCode("-100".toInt())
  } catch (e: IllegalArgumentException) {
    print(e.message) // Code must be positive
  }

  try {
    testMyCode("0".toInt(1))
  } catch (e: IllegalArgumentException) {
    print(e.message) // the result will be not our code: It will be something like
    // radix 1 was not in valid range 2..36
  }
}

What happens to our second block? why is the exception throwing an error which we do not recognize? It comes from toInt() function where the library function could also throw the IllegalArgumentException. How could we avoid it? Well, we can define our own Exception such as IncorrectInputException and throws using own exception instead.

class IncorrectInputException(message: String): Exception(message)

fun testMyCode(code: Int) {
  if (code < 0) {
    throw IncorrectInputException("Code must be positive")
  }
}

fun main() {
  try {
    testMyCode("-100".toInt())
  } catch (e: IncorrectInputException) {
    print(e.message) // Code must be positive
  }

  try {
    testMyCode("0".toInt(1))
  } catch (e: IncorrectInputException) {
    print(e.message) // the result will be not our code: It will be something like
    // radix 1 was not in valid range 2..36
  }
}