Custom Getters and Setters in Kotlin

Introduction

Custom Getters and Setters is a fundamental concept every Kotlin developer should understand. Classes bundle data and behaviour. Kotlin reduces boilerplate with concise constructors, properties, and sensible defaults compared to Java.

Properties can define custom get and set logic while keeping field-like syntax. 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

• Properties can define custom get and set logic while keeping field-like syntax.
• Use a backing field when the stored value differs from the computed accessor logic.
• Custom accessors encapsulate validation and derived values cleanly.

Syntax

var size: Int = 0
    get() = field
    set(value) { field = if (value < 0) 0 else value }

Custom Getters and Setters in Kotlin Example Program in Kotlin

class Rectangle(var width: Int, var height: Int) {
    val area: Int
        get() = width * height

    var side: Int = 0
        set(value) {
            field = if (value < 0) 0 else value
            width = field
            height = field
        }
}

fun main() {
    val rect = Rectangle(4, 5)
    println("Area: ${rect.area}")
    rect.side = 6
    println("After side set: ${rect.width}x${rect.height}, area=${rect.area}")
}

Sample Output

Area: 20
After side set: 6x6, area=36

When to use

Use classes to model entities with state and behaviour — users, orders, view models, or service objects.

How it works

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

2. Properties can define custom get and set logic while keeping field-like syntax.

3. 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

• Keep constructors short; move complex setup to init blocks or factory functions.
• Expose behaviour through methods rather than public mutable fields.
• Mark classes final (default) unless inheritance is intentional.

Common Mistakes

• Skipping the example and only reading the definition — hands-on practice cements the concept.
• Copying syntax without understanding nullable vs non-nullable types or scope rules.
• Ignoring compiler warnings that often point to safer alternatives.

Key Points

• Properties can define custom get and set logic while keeping field-like syntax.
• Use a backing field when the stored value differs from the computed accessor logic.
• Custom accessors encapsulate validation and derived values cleanly.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Semicolons at the end of statements are optional in Kotlin.