Quick Answer

Adding functionality to classes you don't own

Understanding the Issue

Extension functions allow adding new functions to existing classes without modifying their source code. They're resolved statically (based on the declared type) and don't actually modify the original class.

The Problem

This code demonstrates the issue:

Kotlin Error 
// Problem: Utility class
object StringUtils {
    fun capitalize(s: String): String {
        return s.replaceFirstChar { it.uppercase() }
    }
}

The Solution

Here's the corrected code:

Kotlin Fixed 
// Solution: Extension function
fun String.capitalize(): String {
    return this.replaceFirstChar { it.uppercase() }
}

// Usage:
val name = "alice".capitalize()

// Extension properties:
val String.isEmail get() = this.matches(Regex("^\S+@\S+\.\S+$"))

Key Takeaways

Use extensions to add utility functions in a more discoverable and natural way.