Preconditions.kt: Validate Your Kotlin

Input validation is important, so you're most likely familiar with the following pattern in Java:

public static <T> T getFifthElement(@Nonnull T[] array) {  
  if (array == null || array.length < 5) {
    throw new IllegalArgumentException(
        "array should be non-null and have at least five elements!");
  return array[4];

What would happen if we omitted all that verbose checking and went with a one-liner? The code would still break as expected, throwing either a NullPointerException or an ArrayIndexOutOfBoundsException. However, IllegalArgumentException clearly indicates that the calling code passes invalid arguments, making it a lot easier for the developer to identify the problem. Also note, that although we're annotating the parameter with @Nonnull, we still have to null-check: besides an IDE warning, nothing is preventing the calling code from actually passing in a null - you love Kotlin, don't you?

To make input validation a little less verbose, Kotlin provides a set of utility functions, contained in a file called Preconditions.kt. Most of the functions in this file have names starting with either require or check, let's dive in and see how they work.


What require() does is pretty straightforward:

public inline fun require(value: Boolean, lazyMessage: () -> Any): Unit {  
  if (!value) {
    val message = lazyMessage()
    throw IllegalArgumentException(message.toString())

It takes a value, that should resolve to Boolean, and a lazyMessage function that should return the error message. Why a function and not a plain String? It's not uncommon to do string concatenation to construct the error message, for example, including the argument into the message to make it more informative. When wrapped into a function, the concatenation (which can be expensive) will only happen when the precondition fails, and not at the time of the function call - that's a very nice optimization!

Let's rewrite our Java function in Kotlin using require():

fun <T> getFifthElement(array: Array<T>): T {  
  require(array.size >= 5) {
    "array should have at least five elements, but has ${array.size}!"
  return array[4]

Let's see what we have here:

  • First, note that the null-check is gone - compiler handles it for us!
  • The precondition has been flipped from array.length < 5 to array.size >= 5 and now reads more naturally: "function requires that the size of the array is greater or equal to 5".
  • The string creation, which now includes the size of the offending array, will only happen if the precondition fails, so we don't have to worry about performance in the happy case. Looks good!

There's a shorter version of require():

public inline fun require(value: Boolean): Unit =  
    require(value) { "Failed requirement." }

In my opinion, "Failed requirement" is not very suggestive as the error message, so I would recommend using the two-parameter version.


check() is almost a carbon copy of require():

public inline fun check(value: Boolean, lazyMessage: () -> Any): Unit {  
  if (!value) {
    val message = lazyMessage()
    throw IllegalStateException(message.toString())

The only difference is that check() throws IllegalStateException when the precondition fails. The difference is in the semantics:

  • IllegalArgumentException should be thrown when a method is called with an incorrect set of arguments.
  • IllegalStateException should be thrown when a method is called at an inappropriate time, i.e. the current state of the object does not allow that particular method to be called.

If this sounds a bit vague, the following snippet should help drive the point home. The code comes from KotlinPoet, a handy library that helps you generate Kotlin code.

class TypeSpec {  
  class Builder {
    fun companionObject(companionObject: TypeSpec) = apply {
      check(kind == Kind.CLASS || kind == Kind.INTERFACE) { 
        "$kind can't have a companion object" 
      require(companionObject.kind == Kind.COMPANION) { 
        "expected a companion object class but was $kind " 
      this.companionObject = companionObject

companionObject() is a builder method that adds a companion object definition to a TypeSpec. As you can see, both require() and check() are in play here:

  • check() verifies that the TypeSpec that's being created is either a class or an interface: adding a companion object to an enum or an annotation wouldn't make sense. This is a validation of the state of the current Builder object, and not of the argument of the method, hence the use of check().
  • require() verifies that companionObject actually represents a companion object. This is a validation of the argument, hence the code uses require().

Same as require(), check() comes in a shorter form which defaults the error message to "Check failed.". Again, I would not recommend using it a lot, as I strongly believe that an exception message should be as descriptive as possible to be helpful.


Kotlin's standard library contains a bunch of hidden gems, that can help you simplify your code and make it more idiomatic. Preconditions.kt contains a couple of handy functions, require() and check(), that assist in validating input and state of your objects in a nice and clean way. Sanitize your input, and keep writing Kotlin!