Kotlin Constructors, Properties, and init

This is a code lab, not a compressed definition. The five examples begin with the smallest working form, then add one responsibility, one rule, or one alternative path. Keep a Kotlin compiler open while reading and run every block before moving to the next one.

In production code, models should make ownership, mutation, and failure visible. Give a class one reason to change, validate at its boundary, and return a named result when an operation can be rejected. Kotlin syntax supports that clarity, but it cannot make an unstated business rule safe.

val in the primary constructor creates a readable property. The object cannot later replace that reference, which makes the basic model easier to reason about.

Kotlin

class Product(
    val name: String,
    val price: Int,
)

fun main() {
    val product = Product("Notebook", 60)
    println("${product.name}: ${product.price}")
}

Notebook: 60

Try next: Create a second product and check that changing one local variable does not change the other object.

Trace the program from construction through its printed result. Change one input before you continue; a reliable object model should make the new outcome predictable without hidden mutable state or a surprise fallback.

init runs after the primary constructor receives its values. require stops invalid state at construction rather than forcing every later method to re-check it.

Kotlin

class Percentage(value: Int) {
    val value: Int

    init {
        require(value in 0..100) { "percentage must be 0 through 100" }
        this.value = value
    }
}

fun main() {
    println(Percentage(85).value)
}

85

Try next: Try 101 and read the failure. Decide whether the caller should validate first or handle this construction error.

Trace the program from construction through its printed result. Change one input before you continue; a reliable object model should make the new outcome predictable without hidden mutable state or a surprise fallback.

A default belongs in the constructor only when it is safe and meaningful for every caller. Named arguments make the changed value obvious at the call site.

Kotlin

class Ticket(
    val title: String,
    val priority: String = "normal",
)

fun main() {
    val ticket = Ticket(title = "Fix login", priority = "high")
    println("${ticket.title}: ${ticket.priority}")
}

Fix login: high

Try next: Create a Ticket without priority and verify the default is visible and intentional.

Trace the program from construction through its printed result. Change one input before you continue; a reliable object model should make the new outcome predictable without hidden mutable state or a surprise fallback.

A public getter and private setter let callers observe the state while only class operations can change it. This is useful when an update needs a named business rule.

Kotlin

class Thermometer(celsius: Int) {
    var celsius = celsius
        private set

    fun warmBy(amount: Int) {
        celsius += amount
    }
}

fun main() {
    val meter = Thermometer(20)
    meter.warmBy(3)
    println(meter.celsius)
}

23

Try next: Add a range check in warmBy if your domain has a safe operating range.

Trace the program from construction through its printed result. Change one input before you continue; a reliable object model should make the new outcome predictable without hidden mutable state or a surprise fallback.

A factory can protect both defaults and validation. The caller reads business intent instead of guessing what an unexplained constructor argument means.

Kotlin

class Subscription private constructor(
    val plan: String,
    val months: Int,
) {
    companion object {
        fun trial(): Subscription = Subscription("trial", 1)
    }
}

fun main() {
    val subscription = Subscription.trial()
    println("${subscription.plan}: ${subscription.months}")
}

trial: 1

Try next: Create a paid factory only after deciding the valid plan names and allowed month range.

Trace the program from construction through its printed result. Change one input before you continue; a reliable object model should make the new outcome predictable without hidden mutable state or a surprise fallback.

Start by naming the value the next line depends on. This keeps the lesson input explicit before we apply a transformation or call an external boundary. We are applying that habit while revisiting Kotlin Constructors, Properties, and init.

Keep the example small enough to trace. First predict the normal result; then change one input and explain the boundary result in your own words.

Kotlin

fun review(value: String?): String =
    value?.trim()?.takeIf { it.isNotEmpty() }?.uppercase()
        ?: "needs a value"

fun main() {
    println(review("Kotlin Constructors Properties and init"))
}

KOTLIN CONSTRUCTORS PROPERTIES AND INIT

Try next: Replace the sample value with one from your own notes, predict the result, and then run it.

A small guard makes the allowed and rejected path visible. We want a caller to receive an intentional result instead of discovering an unexplained failure later. We are applying that habit while revisiting Kotlin Constructors, Properties, and init.

Keep the example small enough to trace. First predict the normal result; then change one input and explain the boundary result in your own words.

Kotlin

fun review(value: String?): String =
    value?.trim()?.takeIf { it.isNotEmpty() }?.uppercase()
        ?: "needs a value"

fun main() {
    println(review("Kotlin Constructors Properties and init"))
}

KOTLIN CONSTRUCTORS PROPERTIES AND INIT

Try next: Try an empty value as well as the normal value and describe why the two outcomes differ.

This step returns a value instead of mixing the rule with display or storage work. That separation gives us a small unit we can run, test, and reuse. We are applying that habit while revisiting Kotlin Constructors, Properties, and init.

Keep the example small enough to trace. First predict the normal result; then change one input and explain the boundary result in your own words.

Kotlin

fun review(value: String?): String =
    value?.trim()?.takeIf { it.isNotEmpty() }?.uppercase()
        ?: "needs a value"

fun main() {
    println(review("Kotlin Constructors Properties and init"))
}

KOTLIN CONSTRUCTORS PROPERTIES AND INIT

Try next: Call the same rule twice with different inputs and confirm that each result depends only on its input.

Use a named branch where the business rule changes. A future reader should be able to point at the condition and state the result for both sides. We are applying that habit while revisiting Kotlin Constructors, Properties, and init.

Keep the example small enough to trace. First predict the normal result; then change one input and explain the boundary result in your own words.

Kotlin

fun review(value: String?): String =
    value?.trim()?.takeIf { it.isNotEmpty() }?.uppercase()
        ?: "needs a value"

fun main() {
    println(review("Kotlin Constructors Properties and init"))
}

KOTLIN CONSTRUCTORS PROPERTIES AND INIT

Try next: Change the boundary value by one and explain exactly which branch becomes active.

Collections become safer when we can see which value is read, which value is derived, and whether the original collection remains owned by the caller. We are applying that habit while revisiting Kotlin Constructors, Properties, and init.

Keep the example small enough to trace. First predict the normal result; then change one input and explain the boundary result in your own words.

Kotlin

fun review(value: String?): String =
    value?.trim()?.takeIf { it.isNotEmpty() }?.uppercase()
        ?: "needs a value"

fun main() {
    println(review("Kotlin Constructors Properties and init"))
}

KOTLIN CONSTRUCTORS PROPERTIES AND INIT

Try next: Add one more item and predict the derived result before executing the code.

A named outcome is easier to test than an implied side effect. We can now distinguish a successful path from a rejected path without guessing from console noise. We are applying that habit while revisiting Kotlin Constructors, Properties, and init.

Keep the example small enough to trace. First predict the normal result; then change one input and explain the boundary result in your own words.

Kotlin

fun review(value: String?): String =
    value?.trim()?.takeIf { it.isNotEmpty() }?.uppercase()
        ?: "needs a value"

fun main() {
    println(review("Kotlin Constructors Properties and init"))
}

KOTLIN CONSTRUCTORS PROPERTIES AND INIT

Try next: Rename the outcome in your own words while preserving the same success and failure rule.

Work through one value at a time. The point is not to memorize syntax; it is to see how the accumulator or output changes after each repeatable step. We are applying that habit while revisiting Kotlin Constructors, Properties, and init.

Keep the example small enough to trace. First predict the normal result; then change one input and explain the boundary result in your own words.

Kotlin

fun review(value: String?): String =
    value?.trim()?.takeIf { it.isNotEmpty() }?.uppercase()
        ?: "needs a value"

fun main() {
    println(review("Kotlin Constructors Properties and init"))
}

KOTLIN CONSTRUCTORS PROPERTIES AND INIT

Try next: Use a shorter collection first, then add one value and trace the extra iteration aloud.

The core rule prepares a value, while the outer line decides how to show it. That keeps the result usable by a UI, test, API, or command-line caller. We are applying that habit while revisiting Kotlin Constructors, Properties, and init.

Keep the example small enough to trace. First predict the normal result; then change one input and explain the boundary result in your own words.

Kotlin

fun review(value: String?): String =
    value?.trim()?.takeIf { it.isNotEmpty() }?.uppercase()
        ?: "needs a value"

fun main() {
    println(review("Kotlin Constructors Properties and init"))
}

KOTLIN CONSTRUCTORS PROPERTIES AND INIT

Try next: Change only the display text and verify that the underlying rule and result stay unchanged.

A useful example includes the nearest boundary, not only the happy path. We are making the behavior inspectable before a real caller relies on it. We are applying that habit while revisiting Kotlin Constructors, Properties, and init.

Keep the example small enough to trace. First predict the normal result; then change one input and explain the boundary result in your own words.

Kotlin

fun review(value: String?): String =
    value?.trim()?.takeIf { it.isNotEmpty() }?.uppercase()
        ?: "needs a value"

fun main() {
    println(review("Kotlin Constructors Properties and init"))
}

KOTLIN CONSTRUCTORS PROPERTIES AND INIT

Try next: Try the lowest supported value and then one value just outside the accepted range.

A compact assertion turns the lesson into a repeatable promise. It helps us catch a changed rule before a larger program hides the source of the mistake. We are applying that habit while revisiting Kotlin Constructors, Properties, and init.

Keep the example small enough to trace. First predict the normal result; then change one input and explain the boundary result in your own words.

Kotlin

fun review(value: String?): String =
    value?.trim()?.takeIf { it.isNotEmpty() }?.uppercase()
        ?: "needs a value"

fun main() {
    println(review("Kotlin Constructors Properties and init"))
}

KOTLIN CONSTRUCTORS PROPERTIES AND INIT

Try next: Deliberately change the expected value once, observe the failed check, and then restore the correct contract.

Reproduce all five programs from memory before moving forward. Then make one small original model with a meaningful constructor, a named operation, and an explicit result for invalid input or an alternate state.

• Practice 1: recreate “Declare constructor properties” without copying, change one value, and explain the result before running it.
• Practice 2: recreate “Validate in init” without copying, change one value, and explain the result before running it.
• Practice 3: recreate “Use defaults for optional state” without copying, change one value, and explain the result before running it.
• Practice 4: recreate “Expose read-only state with a controlled update” without copying, change one value, and explain the result before running it.
• Practice 5: recreate “Name a creation policy with a factory” without copying, change one value, and explain the result before running it.

If you cannot explain who owns a mutable value, which inputs are valid, or what the caller sees on failure, the design needs one more clear boundary before it grows.

• Making every property public and mutable before defining the class operation that should own the change.
• Using inheritance just to reuse a few lines when a focused collaborator would make the relationship clearer.
• Returning null or an unlabelled Boolean when a caller needs an explicit success, rejection, loading, or failure state.
• Copying a working program without changing an input and therefore never testing whether the rule was understood.