Startups on Kotlin: How JetBrains' Language is Changing Business

Why Startups Choose Kotlin: An Unexpected Turn

When we hear "startup," Python, JavaScript, or Go usually come to mind. But over the past three years, quietly and without much fanfare, a revolution has taken place. Kotlin — a language created by JetBrains (the same folks who made PyCharm and IntelliJ IDEA) — has become a real secret weapon for many successful projects.

Why? Because startups are about speed and survival. You need to build an MVP (Minimum Viable Product) in a month, not six months. Kotlin lets you write code that works simultaneously on Android, the backend, and even in the browser. And it's beautiful. Let's look at real success stories and see the code that helped companies grow.

Story 1: Trello and "Mature" Kotlin

In 2017, the Trello team (which was already part of Atlassian) made a bold decision: rewrite the mobile app in Kotlin. At the time, it was a risk — the language wasn't yet officially supported by Google for Android. But the results exceeded expectations.

What did they gain?

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• A 30% reduction in bugs thanks to Kotlin's null safety.
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• 1.5x faster development due to concise syntax.
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• Easy migration — Kotlin is 100% compatible with Java, so old code didn't have to be thrown away.
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Here's what a typical Trello function looks like in Kotlin (simplified example):

data class Card(\n    val id: String,\n    val title: String,\n    val description: String?,\n    val dueDate: LocalDate?\n)\n\nfun displayCard(card: Card): String {\n    // Safe call using the Elvis operator\n    val desc = card.description ?: "No description"\n    val date = card.dueDate?.format(DateTimeFormatter.ofPattern("dd.MM.yyyy")) ?: "No deadline"\n    return "📌 ${card.title}\\n📝 $desc\\n📅 $date"\n}

Notice: ? after the type means the value can be null. This eliminates the infamous NullPointerException that kills startups at the worst possible moment.

Story 2: Cash App and Financial Stability

Square (now Block) uses Kotlin in its popular Cash App. This isn't just "another wallet" — it's a financial system processing billions of dollars. A mistake here costs real money.

Cash App switched to Kotlin for the backend. Yes, Kotlin works great on the server via the Ktor framework or Spring Boot. The main advantage? Coroutines — lightweight threads that allow processing thousands of requests without lag.

Code example: asynchronous payment with coroutines

suspend fun processPayment(\n    userId: String, \n    amount: BigDecimal, \n    currency: Currency\n): Result<Transaction> {\n    return withContext(Dispatchers.IO) {\n        try {\n            // Check balance\n            val balance = userRepository.getBalance(userId)\n            if (balance < amount) {\n                return@withContext Result.failure(InsufficientFundsException())\n            }\n            // Charge funds\n            val transaction = paymentGateway.charge(userId, amount, currency)\n            userRepository.updateBalance(userId, balance - amount)\n            Result.success(transaction)\n        } catch (e: Exception) {\n            // Log the error, but don't crash the app\n            logger.error("Payment failed for user $userId", e)\n            Result.failure(e)\n        }\n    }\n}

The keywords suspend and withContext are Kotlin magic. They allow you to write asynchronous code as if it were synchronous. No callbacks and no promise hell.

Story 3: Pinterest and Development Speed

Pinterest is one of the first major startups to fully switch to Kotlin for Android. In 2020, they