String Concatenation in Kotlin

Introduction

String Concatenation is a fundamental concept every Kotlin developer should understand. Strings appear in almost every program — user input, file paths, API responses, and UI labels. Kotlin’s standard library provides concise helpers for common text tasks.

Strings can be joined using the + operator. 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

• Strings can be joined using the + operator.
• The plusAssign operator += can append text to an existing string variable.
• Concatenation creates a new string because String is immutable.

Syntax

var result = str1 + str2

String Concatenation in Kotlin Example Program in Kotlin

fun main(args: Array<String>) {
    val first = "Hello, "
    val second = "Kotlin"
    val message = first + second
    println(message)
}

Sample Output

Hello, Kotlin

When to use

Use string functions when formatting output, validating user input, splitting CSV data, or building URLs and file paths.

How it works

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

2. val first = "Hello, " assigns or updates a value used later in the program.

3. val second = "Kotlin" assigns or updates a value used later in the program.

4. val message = first + second assigns or updates a value used later in the program.

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

6. Strings can be joined using the + operator.

7. 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

• Prefer string templates ("Hello, $name") over concatenation for readability.
• Use trim(), isBlank(), and isEmpty() to validate user input consistently.
• Choose StringBuilder when building large strings inside loops.

Common Mistakes

• Calling substring with wrong end indices — prefer substringAfter / substringBefore helpers.
• Using == when case-insensitive comparison is needed — use equals(other, ignoreCase = true).
• Concatenating inside tight loops instead of using StringBuilder.

Key Points

• Strings can be joined using the + operator.
• The plusAssign operator += can append text to an existing string variable.
• Concatenation creates a new string because String is immutable.
• 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.