Project structure and build basics in Kotlin

Introduction

Organizing your Kotlin project and building it correctly helps your code work well and be easy to manage.

When starting a new Kotlin app or library

When adding new files or features to your project

When you want to create a runnable program from your code

When sharing your code with others or publishing it

When fixing errors related to missing files or dependencies

Syntax

Kotlin

ProjectRoot/
 ├── build.gradle.kts
 ├── settings.gradle.kts
 ├── src/
 │    ├── main/
 │    │    ├── kotlin/
 │    │    │    └── YourCode.kt
 │    │    └── resources/
 │    └── test/
 │         ├── kotlin/
 │         └── resources/

build.gradle.kts is the Kotlin script file that tells how to build your project.

src/main/kotlin holds your main Kotlin code files.

Examples

This is a simple build script using Gradle Kotlin DSL to set up a Kotlin JVM project.

Kotlin

plugins {
    kotlin("jvm") version "1.8.0"
}

repositories {
    mavenCentral()
}

dependencies {
    implementation(kotlin("stdlib"))
}

This file names your project, which helps Gradle identify it.

Kotlin

settings.gradle.kts

rootProject.name = "MyKotlinApp"

Sample Program

This example shows a minimal Kotlin project setup with a build script and a main program that prints a message.

Kotlin

plugins {
    kotlin("jvm") version "1.8.0"
}

repositories {
    mavenCentral()
}

dependencies {
    implementation(kotlin("stdlib"))
}

// src/main/kotlin/Main.kt
fun main() {
    println("Hello, Kotlin project!")
}

Important Notes

Always keep your source code inside src/main/kotlin for main code and src/test/kotlin for tests.

Use Gradle commands like ./gradlew build to compile and build your project.

Keep your build scripts simple at first, then add dependencies as needed.

Summary

A clear project structure keeps your Kotlin code organized and easy to find.

Build scripts tell tools how to compile and package your code.

Using standard folders and Gradle helps your project work smoothly and be easy to share.

Practice

1. Which folder in a typical Kotlin project contains the main source code files?

easy

A. resources/main

B. src/test/kotlin

C. build/libs

D. src/main/kotlin

Solution

1. Step 1: Understand standard Kotlin project folders

   The src/main/kotlin folder is where the main Kotlin source files are placed.

2. Step 2: Differentiate from other folders

   src/test/kotlin is for test code, build/libs is for compiled output, and resources/main is for resource files.

3. Final Answer:

   src/main/kotlin -> Option D

4. Quick Check:

   Main source code folder = src/main/kotlin [OK]

Hint: Main code is always under src/main/kotlin folder [OK]

Common Mistakes:

• Confusing test folder with main source folder
• Thinking build output folders contain source code
• Mixing resource folders with source code folders

2. Which file is commonly used to define build instructions in a Kotlin project using Gradle?

easy

A. settings.gradle

B. build.gradle.kts

C. gradle.properties

D. manifest.xml

Solution

1. Step 1: Identify Gradle build script files

   Gradle uses build.gradle.kts (Kotlin DSL) or build.gradle (Groovy DSL) to define build instructions.

2. Step 2: Differentiate from other Gradle-related files

   settings.gradle configures multi-project builds, gradle.properties holds properties, and manifest.xml is unrelated.

3. Final Answer:

   build.gradle.kts -> Option B

4. Quick Check:

   Build instructions file = build.gradle.kts [OK]

Hint: Build scripts are named build.gradle.kts for Kotlin DSL [OK]

Common Mistakes:

• Confusing settings.gradle with build.gradle.kts
• Thinking gradle.properties contains build logic
• Mixing XML files with Gradle scripts

3. Given this Gradle build snippet, what will be the output directory for compiled Kotlin classes?

kotlin {
    jvmToolchain(17)
}

tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).configureEach {
    kotlinOptions.jvmTarget = "17"
}

medium

A. build/classes/kotlin/main

B. src/main/kotlin

C. build/libs

D. out/production

Solution

1. Step 1: Understand Gradle default output folders

   By default, Kotlin compiled classes go to build/classes/kotlin/main for the main source set.

2. Step 2: Differentiate source and output folders

   src/main/kotlin is source code, build/libs holds packaged jars, and out/production is not standard for Gradle Kotlin projects.

3. Final Answer:

   build/classes/kotlin/main -> Option A

4. Quick Check:

   Compiled classes output = build/classes/kotlin/main [OK]

Hint: Compiled classes go under build/classes/kotlin/main by default [OK]

Common Mistakes:

• Confusing source folder with output folder
• Thinking build/libs holds class files instead of jars
• Assuming non-standard output folders

4. You see this error when running Gradle: Could not find method kotlin() for arguments. What is the likely cause?

medium

A. The Kotlin plugin is not applied in build.gradle.kts

B. The source folder is missing

C. The JVM target version is invalid

D. The Gradle wrapper is outdated

Solution

1. Step 1: Understand the error message context

   The error means Gradle does not recognize the kotlin {} block, usually because the Kotlin plugin is missing.

2. Step 2: Check common causes

   Missing plugins { kotlin("jvm") version "x.y.z" } causes this error. Source folder or JVM target issues cause different errors. Outdated Gradle wrapper usually causes version incompatibility errors.

3. Final Answer:

   The Kotlin plugin is not applied in build.gradle.kts -> Option A

4. Quick Check:

   Missing Kotlin plugin causes kotlin() method error [OK]

Hint: Check if Kotlin plugin is applied when kotlin() method errors occur [OK]

Common Mistakes:

• Assuming source folder missing causes this error
• Blaming JVM target version without checking plugin
• Ignoring plugin application in build script

5. You want to add a new resource folder to your Kotlin project for configuration files. Where should you place it to follow standard Gradle project structure?

hard

A. src/resources/main

B. resources/config

C. src/main/resources

D. config/resources

Solution

1. Step 1: Recall standard resource folder location

   Gradle expects resource files in src/main/resources to include them in the build output.

2. Step 2: Differentiate from incorrect folder structures

   Folders like resources/config, src/resources/main, or config/resources are not standard and won't be automatically included.

3. Final Answer:

   src/main/resources -> Option C

4. Quick Check:

   Resource files go under src/main/resources [OK]

Hint: Put resource files in src/main/resources for Gradle to find them [OK]

Common Mistakes:

• Placing resources outside src/main folder
• Using non-standard folder names
• Not including resources in build output