Kotlin for Beginners: 5 Fatal Mistakes and How to Avoid Them

Introduction: Why Kotlin Seems Easy but Throws Surprises

Kotlin is a modern, concise, and incredibly convenient language. Many beginners come to it from Java or Python and think: "Well, everything here is obvious!". And indeed, Kotlin's syntax is clean, nullable types protect against NPE, and functional features like let, apply, and run look magically simple. But it is precisely this apparent simplicity that often becomes a trap.

In this article, we will break down the most common mistakes when learning Kotlin that beginners make. You will learn how to avoid them and get ready-made code examples. If you are just starting your journey in learning Kotlin for beginners, this article will save you many hours of debugging.

Mistake #1: Ignoring Null Safety or, Conversely, Paranoia

Kotlin has null safety built directly into the type system. This is a blessing, but beginners often fall into two extremes. The first is complete ignorance and using the !! (not-null assertion) operator at every step. The second is overusing the Elvis operator ?: and chains of ?.let, which turn the code into unreadable mush.

What the Mistake Looks Like

// Bad: total !! — kills all of Kotlin's safety\nfun processUser(user: User?) {\n    val name = user!!.name  // If user == null — crash!\n    println(name.uppercase())\n}\n\n// Bad: too many nested lets\nfun getUserCity(user: User?): String? {\n    return user?.let {\n        it.address?.let { addr ->\n            addr.city?.let { city ->\n                city.name\n            }\n        }\n    }\n}\n

How to Do It Right

// Good: use safe call + Elvis\nfun processUserSafe(user: User?) {\n    val name = user?.name ?: "Guest"\n    println(name.uppercase())\n}\n\n// Excellent: extensions and scope functions without a zoo of nesting\nfun getUserCityClean(user: User?): String? {\n    return user?.address?.city?.name\n}\n

Tip: Use the !! operator only if you are 100% sure the value is not null (for example, after a check in an if). In all other cases — use ?. and ?:.

This is one of the main Kotlin problems for beginners — they either fear null or do not understand how to work with it. Remember: Kotlin does not prohibit null; it forces you to think about it.

Mistake #2: Not Understanding the Difference Between var, val, and Constants

It seems that val is a final variable, like final in Java. But no. val is a read-only reference, not an immutable object. Beginners often confuse val with a compile-time constant (const val) and are surprised why their code doesn't work as expected.

Example of Confusion

// Beginner thinks: "val = immutable"\nval list = mutableListOf(1, 2, 3)\nlist.add(4)  // Works! But the beginner is shocked\n\n// This, however, is a compilation error\nval name = "Alice"\nname = "Bob"  // Error: val cannot be reassigned\n

How to Distinguish

// Compile-time constant — only for primitives and String\nconst val PI = 3.14159\n\n// Read-only reference — can be computed at runtime\nval currentTime = System.currentTimeMillis()\n\n// True immutability — use immutable collections\nval immutableList = listOf(1, 2, 3)  // listOf, not mutableListOf\n

Golden Rule: Use val everywhere possible. If a variable needs to change — only then use var. And const val — only for top-level constants or inside an object/companion object.

Mistake #3: Overusing companion object and object Singletons

Kotlin offers an elegant way to create singletons via ob