Kotlin Programming: The Big Nerd Ranch Guide, 2nd Edition

Preconditions

Unexpected values can cause your program to behave in unintended ways. As a developer, you will spend plenty of time validating input to ensure you are working with the values you intend. Some sources of exceptions are common, like unexpected null values. To make it easier to validate input and debug to avoid certain common issues, Kotlin provides some convenience functions as part of its standard library. They allow you to use a built-in function to throw an exception with a custom message.

These functions are called precondition functions, because they allow you to define preconditions – conditions that must be true before some piece of code is executed.

You have seen a number of ways in this chapter to guard against ...