Generic Classes in Kotlin

Introduction

Generic Classes is a fundamental concept every Kotlin developer should understand. Generics let you write type-safe containers and algorithms that work with many types while keeping compile-time checks.

Generic classes store values of parameterized types. 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

• Generic classes store values of parameterized types.
• Type parameter is declared after class name.
• Same class can be used with different types.

Syntax

class Box<T>(val value: T)

Generic Classes in Kotlin Example Program in Kotlin

class Box<T>(val value: T)

fun main(args: Array<String>) {
    val stringBox = Box("Kotlin")
    val intBox = Box(100)
    println("${stringBox.value}, ${intBox.value}")
}

Sample Output

Kotlin, 100

When to use

Use generics when building reusable containers, parsers, or algorithms that must work with more than one type.

How it works

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

2. val stringBox = Box("Kotlin") assigns or updates a value used later in the program.

3. val intBox = Box(100) assigns or updates a value used later in the program.

4. The println("${stringBox.value}, ${intBox.value}") statement writes a line to the console — this produces part of the sample output below.

5. Generic classes store values of parameterized types.

6. 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

• Understand the core idea: generic classes store values of parameterized types.
• Prefer readable names and small functions so examples map directly to real projects.
• Run and modify the example — change values and observe how the output changes.

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

• Generic classes store values of parameterized types.
• Type parameter is declared after class name.
• Same class can be used with different types.
• 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.