Check Instructions

Check Instructions are like assert - they act like a constraints that must be met before it could proceed. Usually, they are used to validate function arguments and results. Check instructions typically throw exceptions when they fail. They are easier to write and produce more comprehensible code.

require()

This require() acts like preconditions and normally used to validate function arguments, so it typically appears at the beginning of function bodies. These tests cannot be checked at compile time. Preconditions are relatively easy to include in our code, but sometimes they can be turned into unit tests.

data class DayOfMonth(val dayNo: Int) {
  init {
    require(dayNo in 1..31) {
      "Day of month is out of range. Input: $dayNo"
    }
  }
}

DayOfMonth(10) //DayOfMonth(dayNo=15)
DayOfMonth(35) //IllegalArgumentException: Day of month is out of range. Input: 35

requireNotNull()

The requireNotNull() tests its first argument and returns that argument if it is not null. Otherwise, it produces an IllegalArgumentException.

fun doubleIt(n: Int?): Int {
  requireNotNull(n) {
    "the input cannot be null"
  }
  return n * n
}

doubleIt(null) // IllegalArgumentException the input cannot be null"

check()

The check() is identical to require() except that it throws IllegalStateException. It is typically used at the end of a function, to verify that the results are valid.

val myFile = DataFile("test.txt")

fun modifyContent(canModify: Boolean) {
  if (canModify)
    myFile.writeText("OK")

  check(myFile.exists()) {
    "${myFile.name} does not exist!"
  }
}

modifyContent(false) //IllegalStateException: test.txt does not exist!

assert()

To avoid commenting and uncommenting check() statements, assert() allows us to enable and disable assert() checks. assert() comes from Java. Assertions are disabled by default, and are only engaged if we explicitly turn them on using a command line flag. In Kotlin, this flag is -ea.

Usually, the recommendation is to use require() and check() which are always available without special configuration.