How many times did you have to write code like this:

fun setCapacity(cap: Int) {
    if (cap < 0) {
        throw IllegalArgumentException()
    }
    ...
}

Instead, you can check arguments with require():

fun setCapacity(cap: Int) {
    require(cap > 0)
}

This makes the code a lot more fluent.

You can use require() to check for nested nulls:

fun printNameLength(p: Profile) {
    require(p.firstName != null)
}

But there's also requireNotNull() for that:

fun printNameLength(p: Profile) {
    requireNotNull(p.firstName)
}

Use check() to validate the state of your object. This is useful when you provide some object that the user may not have set up correctly:

private class HttpClient {
    var body: String? = null
    var url: String = ""

    fun postRequest() {
        check(body != null) {
            "Body must be set in POST requests"
        }
    }
    
    fun getRequest() {
        // This one is fine without body
    }
}

And again, there's a shortcut for null: checkNotNull().