We will be using IntelliJ IDEA because it provides great integration of Gradle with Kotlin, and it is a really great IDE to work on. You can also use Android Studio for it.

In the following steps, we will be creating a Kotlin project with the Gradle build system. First, we will select the Create New Project option from the menu. Then, follow these steps:

version '1.0-SNAPSHOT'

buildscript {
  ext.kotlin_version = '1.1.4-3'

  repositories {
      mavenCentral()
  }
  dependencies {
      classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
  }
}

apply plugin: 'java'
apply plugin: 'kotlin'

sourceCompatibility = 1.8

repositories {
  mavenCentral()
}

dependencies {
  compile "org.jetbrains.kotlin:kotlin-stdlib-jre8:$kotlin_version"
  testCompile group: 'junit', name: 'junit', version: '4.12'
}

compileKotlin {
  kotlinOptions.jvmTarget = "1.8"
}
compileTestKotlin {
  kotlinOptions.jvmTarget = "1.8"
}

apply plugin: 'application'
mainClassName = "HelloWorldKt"

The default structure of the project, when you create a new project in IntelliJ, is as illustrated:

project
   - src
       - main (root)
           - kotlin
           - java

If you want to have a different structure of the project, you should declare it in build.gradle. You can do it by adding the following lines in build.gradle.

The corresponding sourceSets property should be updated if not using the default convention:

sourceSets {
   main.kotlin.srcDirs += 'src/main/myKotlin'
   main.java.srcDirs += 'src/main/myJava'
}

Though you can keep Kotlin and Java files under the same package, it’s a good practice to keep them separated.

Check out the How to build a self-executable jar with Gradle and Kotlin recipe in this chapter.