PreConditionsExample.kt

fun main(args: Array<String>) {
    RequestBody(null, 25)
}

data class RequestBody(val name: String?, val age: Int?) {
    
    init {

        checkNotNull(name) {
            "This can't be null" // Custom message for IllegalStateException
        }

        checkNotNull(age) // There is a default message too
        
        require(age ?: 0 < 18) { // defenitely not these kind of conditions :P
            "Custom message for IllegalArgumentException"
        }
    }
}