String Length and Substring in Kotlin

Introduction

String Length and Substring 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.

The length property returns the number of characters in a string. 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

• The length property returns the number of characters in a string.
• substring(start) and substring(start, end) extract part of a string.
• String indices start from zero like arrays.

Syntax

val len = text.length
val part = text.substring(0, 4)

String Length and Substring in Kotlin Example Program in Kotlin

fun main(args: Array<String>) {
    val text = "Little Drops"
    println("Length: ${text.length}")
    println("Substring: ${text.substring(0, 6)}")
}

Sample Output

Length: 12
Substring: Little

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 text = "Little Drops" assigns or updates a value used later in the program.

3. The println("Length: ${text.length}") statement writes a line to the console — this produces part of the sample output below.

4. The println("Substring: ${text.substring(0, 6)}") statement writes a line to the console — this produces part of the sample output below.

5. The length property returns the number of characters in a string.

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

• 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

• The length property returns the number of characters in a string.
• substring(start) and substring(start, end) extract part of a string.
• String indices start from zero like arrays.
• 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.