Kotlin Get Started: First File and main

This page is a compact code lab, not a one-paragraph definition. The five examples move from the smallest valid form to a boundary case or reusable function. Keep a local Kotlin compiler open while reading: type each example, make the suggested change, and observe the output before continuing.

In real software, the same discipline protects larger systems. Name inputs clearly, validate data at the boundary, avoid logging secrets, and make an error or alternate result visible to the caller. Kotlin's syntax helps, but the programmer still owns the business rule.

A no-argument main is enough for many learning programs and command tools. Keep it small: invoke one piece of logic, print the result, and leave external work outside until the local rule is correct.

Kotlin

fun main() {
    println("Program started")
}

Program started

Try next: Save this as Main.kt and run it from your Kotlin environment. Confirm the file and program name shown by your tool.

Read the example in order: identify the input, trace the Kotlin expression or branch, then compare the printed result with the stated rule. A small change should produce a predictable new result; if it does not, make the contract explicit before adding more syntax.

A main function may accept an Array of String values supplied by the command line. Treat arguments as untrusted input: check their shape before assuming a missing or malformed value has a safe meaning.

Kotlin

fun main(args: Array<String>) {
    val first = args.firstOrNull() ?: "guest"
    println("Hello, " + first)
}

Hello, guest (when no argument is supplied)

Try next: Run with one argument such as Kotlin. Then decide whether a real command should accept, reject, or default a missing argument.

Read the example in order: identify the input, trace the Kotlin expression or branch, then compare the printed result with the stated rule. A small change should produce a predictable new result; if it does not, make the contract explicit before adding more syntax.

main should explain the sequence of the program, while a helper owns one transformation. This separation lets a test exercise the helper without setting up command-line arguments or capturing console output.

Kotlin

fun formatCourse(name: String): String {
    return name.trim().uppercase()
}

fun main() {
    println(formatCourse("  kotlin  "))
}

KOTLIN

Try next: Try a blank string. If blank input is invalid, add a focused guard in formatCourse rather than silently returning an empty label.

Read the example in order: identify the input, trace the Kotlin expression or branch, then compare the printed result with the stated rule. A small change should produce a predictable new result; if it does not, make the contract explicit before adding more syntax.

A package gives related files a namespace and avoids collisions as a project grows. It should reflect an ownership boundary; do not create deep package trees before the code actually has distinct responsibilities.

Kotlin

package learning.kotlin

fun main() {
    println("package declaration is the first line")
}

package declaration is the first line

Try next: Create a second file in the same package and call a top-level function from it. Keep package names lowercase and stable.

Read the example in order: identify the input, trace the Kotlin expression or branch, then compare the printed result with the stated rule. A small change should produce a predictable new result; if it does not, make the contract explicit before adding more syntax.

The compiler is part of the learning loop. A type or syntax error identifies an assumption that the program cannot prove; make the smallest correction, rerun, and understand why the original code was rejected.

Kotlin

fun main() {
    val answer: Int = 42
    println(answer)
}

42

Try next: Temporarily replace 42 with a quoted value and read the compiler message. Restore the number after understanding the type mismatch.

Read the example in order: identify the input, trace the Kotlin expression or branch, then compare the printed result with the stated rule. A small change should produce a predictable new result; if it does not, make the contract explicit before adding more syntax.

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 Get Started: First File and main.

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 Get Started First File and main"))
}

KOTLIN GET STARTED FIRST FILE AND MAIN

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 Get Started: First File and main.

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 Get Started First File and main"))
}

KOTLIN GET STARTED FIRST FILE AND MAIN

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 Get Started: First File and main.

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 Get Started First File and main"))
}

KOTLIN GET STARTED FIRST FILE AND MAIN

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 Get Started: First File and main.

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 Get Started First File and main"))
}

KOTLIN GET STARTED FIRST FILE AND MAIN

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 Get Started: First File and main.

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 Get Started First File and main"))
}

KOTLIN GET STARTED FIRST FILE AND MAIN

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 Get Started: First File and main.

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 Get Started First File and main"))
}

KOTLIN GET STARTED FIRST FILE AND MAIN

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 Get Started: First File and main.

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 Get Started First File and main"))
}

KOTLIN GET STARTED FIRST FILE AND MAIN

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 Get Started: First File and main.

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 Get Started First File and main"))
}

KOTLIN GET STARTED FIRST FILE AND MAIN

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 Get Started: First File and main.

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 Get Started First File and main"))
}

KOTLIN GET STARTED FIRST FILE AND MAIN

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 Get Started: First File and main.

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 Get Started First File and main"))
}

KOTLIN GET STARTED FIRST FILE AND MAIN

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

A tutorial becomes useful only when you can produce the behavior without copying the last code block. Complete the five short repetitions below, then write one new version that uses your own names and values.

• Practice 1: reproduce “Create a runnable main” from memory, change one input, and explain the result before running it.
• Practice 2: reproduce “Pass command-line arguments” from memory, change one input, and explain the result before running it.
• Practice 3: reproduce “Call a named helper” from memory, change one input, and explain the result before running it.
• Practice 4: reproduce “Use package names deliberately” from memory, change one input, and explain the result before running it.
• Practice 5: reproduce “Read a compiler error as feedback” from memory, change one input, and explain the result before running it.

For a stronger check, explain one invalid input and the response the caller should receive. A generic crash, an unlabelled default, or a printed error is not a complete failure contract.

• Copying a code block without changing an input, then assuming the rule is understood.
• Using a convenient fallback where the caller actually needs a validation error, retry state, or explicit missing result.
• Hiding mutable state, external work, or an exception behind a short expression that a future reader cannot safely review.
• Treating type inference as a reason to hide an important numeric range, nullable value, or public API contract.