Why Preconditions (require, check, error) in Kotlin? - Purpose & Use Cases

The Big Idea

What if a tiny check could save your whole program from crashing unexpectedly?

The Scenario

Imagine you write a program that takes user input for age and you want to make sure the age is not negative. Without preconditions, you have to write many if statements everywhere to check if the input is valid before using it.

The Problem

Manually checking conditions everywhere makes your code long, hard to read, and easy to forget. If you miss a check, your program might crash or behave strangely later, making bugs hard to find.

The Solution

Kotlin's preconditions like require, check, and error let you quickly and clearly state what must be true before your code runs. They stop the program early with helpful messages if something is wrong, keeping your code clean and safe.

Before vs After

✗ Before

if (age < 0) {
    throw IllegalArgumentException("Age cannot be negative")
}
// continue with age

✓ After

require(age >= 0) { "Age cannot be negative" }
// continue with age

What It Enables

This lets you catch problems early and write safer, easier-to-understand code that clearly states its rules.

Real Life Example

When building a banking app, you can use require to ensure withdrawal amounts are positive before processing, preventing mistakes and protecting users.

Key Takeaways

Manual checks clutter code and risk missing errors.

Preconditions provide a simple way to enforce rules upfront.

They improve code safety and clarity by stopping errors early.