String Comparison in Kotlin

Introduction

String Comparison 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.

Use == and != for structural (content) comparison of strings. 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

• Use == and != for structural (content) comparison of strings.
• Use === and !== for referential equality when needed.
• The compareTo function compares strings lexicographically.

Syntax

if (str1 == str2) { ... }

String Comparison in Kotlin Example Program in Kotlin

fun main(args: Array<String>) {
    val a = "Kotlin"
    val b = "Kotlin"
    val c = String(charArrayOf('K','o','t','l','i','n'))
    println("a == b : ${a == b}")
    println("a == c : ${a == c}")
    println("a === c : ${a === c}")
}

Sample Output

a == b : true
a == c : true
a === c : false

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

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

4. val c = String(charArrayOf('K','o','t','l','i','n')) assigns or updates a value used later in the program.

5. The println("a == b : ${a == b}") statement writes a line to the console — this produces part of the sample output below.

6. The println("a == c : ${a == c}") statement writes a line to the console — this produces part of the sample output below.

7. The println("a === c : ${a === c}") statement writes a line to the console — this produces part of the sample output below.

8. Use == and != for structural (content) comparison of strings.

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

• Use == and != for structural (content) comparison of strings.
• Use === and !== for referential equality when needed.
• The compareTo function compares strings lexicographically.
• 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.