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Hey Kotlin enthusiasts! Today, we’re diving into a hidden gem of Kotlin — the Contracts API. If you’ve ever felt like your code needs a bit more clarity or your functions could use a little more predictability, then buckle up! The Contracts API is here to save the day. And don’t worry, we’ll make this journey as fun and engaging as possible using some everyday analogies. Ready? Let’s go!

What are Contracts?

Imagine you’re about to sign a lease for a new apartment. The contract you sign specifies what you can and cannot do. You agree to pay rent on time, and the landlord agrees to maintain the property. This agreement gives both parties clarity and confidence.

In Kotlin, Contracts do something similar for your code. They provide a way to specify the relationship between input and output in your functions. This makes your code more predictable, and Kotlin’s compiler can use these contracts to optimize your code and catch potential issues.

Why Use Contracts?

Contracts help in improving code readability, maintainability, and performance. They allow you to declare the behavior of your functions, making it clear what the function guarantees about its inputs and outputs. This leads to safer code and happier developers!

The Basics of Contracts API

Let’s start with a simple example. Suppose you have a function that checks if a string is not null or blank:

fun isNotNullOrBlank(str: String?): Boolean {
    return str != null && str.isNotBlank()
}

This function works fine, but the compiler doesn’t know that after calling isNotNullOrBlank, the string is guaranteed to be non-null and non-blank if the function returns true. This is where Contracts come in.

Breaking It Down: Step by Step
  1. Import the Contracts Library: First, you need to import the contracts library:
import kotlin.contracts.*

2. Define the Contract: Inside your function, you can define the contract using contract { ... }. Here’s how you can define the contract for isNotNullOrBlank:

fun isNotNullOrBlank(str: String?): Boolean {
    contract {
        returns(true) implies (str != null && str.isNotBlank())
    }
    return str != null && str.isNotBlank()
}

3. Using the Function: Now, when you use this function, the compiler understands the contract and can make smarter decisions:

fun main() {
    val text: String? = "Hello, World!"

    if (isNotNullOrBlank(text)) {
        // The compiler knows text is non-null and non-blank here
        println("Text is valid: $text")
    }
}

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Everyday Analogy: The Shopping List

Think of a contract as a shopping list. Before you go shopping, you make a list of items you need. This list ensures that when you return, you have everything you need for the week. Similarly, a contract in Kotlin ensures that when a function is called, certain conditions are met, making the rest of your code more predictable.

More Fun with Contracts

Let’s look at another example. Imagine you have a function that checks if a list is not empty and then processes the first element:

fun <T> processFirstElement(list: List<T>?) {
    contract {
        returns() implies (list != null && list.isNotEmpty())
    }
    if (list != null && list.isNotEmpty()) {
        println("Processing ${list[0]}")
    }
}

Now, when you call processFirstElement, the compiler knows that if the function doesn’t return early, the list is guaranteed to be non-null and non-empty

fun main() {
    val items: List<String>? = listOf("Kotlin", "Java", "C++")
    if (items != null && items.isNotEmpty()) {
        processFirstElement(items)
        // The compiler knows items is non-null and non-empty here
        println("First item: ${items[0]}")
    }
}   
Wrapping Up

Contracts in Kotlin are like having a clear agreement or a shopping list for your code. They specify what your functions guarantee about their inputs and outputs, making your code more predictable, readable, and maintainable.

So, the next time you find yourself writing functions that return certain conditions or validate inputs, remember the Contracts API. It’s a powerful tool that can make your Kotlin code smarter and safer. Happy coding!

This article is previously published on proandroiddev.com

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