Type Constraints in Kotlin

Introduction

Type Constraints 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.

Type constraints limit generic type parameters. 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

• Type constraints limit generic type parameters.
• Use where clause or upper bound syntax : BoundType.
• Ensures type supports required operations.

Syntax

fun <T : Number> show(value: T)

Type Constraints in Kotlin Example Program in Kotlin

fun <T : Comparable<T>> max(a: T, b: T): T {
    return if (a >= b) a else b
}

fun main(args: Array<String>) {
    println(max("apple", "banana"))
    println(max(10, 25))
}

Sample Output

banana
25

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. return if (a >= b) a else b assigns or updates a value used later in the program.

3. The println(max("apple", "banana")) statement writes a line to the console — this produces part of the sample output below.

4. The println(max(10, 25)) statement writes a line to the console — this produces part of the sample output below.

5. Type constraints limit generic type parameters.

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

Common Mistakes

Key Points

• Type constraints limit generic type parameters.
• Use where clause or upper bound syntax : BoundType.
• Ensures type supports required operations.
• 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.