Kotlin Properties in Java

Introduction

Kotlin Properties in Java is a fundamental concept every Kotlin developer should understand. Kotlin and Java compile to the same JVM bytecode, so you can mix both languages in one project with minimal friction.

Kotlin properties generate getter and setter methods for Java. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• Kotlin properties generate getter and setter methods for Java.
• Boolean isPropertyName() style getter is generated for Boolean properties.
• Property name casing affects generated method names.

Syntax

var title: String

Kotlin Properties in Java Example Program in Kotlin

class Article {
    var title: String = "Kotlin Interop"
}

fun main(args: Array<String>) {
    val article = Article()
    println(article.title)
    article.title = "Updated Title"
    println(article.title)
}

Sample Output

Kotlin Interop
Updated Title

When to use

Use interop when migrating a Java codebase incrementally or calling mature Java libraries from Kotlin.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. var title: String = "Kotlin Interop" assigns or updates a value used later in the program.

3. val article = Article() assigns or updates a value used later in the program.

4. The println(article.title) statement writes a line to the console — this produces part of the sample output below.

5. article.title = "Updated Title" assigns or updates a value used later in the program.

6. The println(article.title) statement writes a line to the console — this produces part of the sample output below.

7. Kotlin properties generate getter and setter methods for Java.

8. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

Common Mistakes

Key Points

• Kotlin properties generate getter and setter methods for Java.
• Boolean isPropertyName() style getter is generated for Boolean properties.
• Property name casing affects generated method names.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Nullability annotations help Kotlin and Java agree on which values can be null at boundaries.
• Semicolons at the end of statements are optional in Kotlin.