Extension Object - Kotlin

The Extension Object pattern allows you to add new behavior to an existing class without modifying its source code, using a separate “extension” object. This is achieved by having the extension object hold an instance of the original class and delegate method calls to it, potentially adding pre- or post-processing logic. It’s a form of delegation and promotes the Open/Closed Principle.

In Kotlin, this is elegantly implemented using delegation via the by keyword. The extension object implements the same interface as the original class, then delegates all calls to the wrapped instance. This provides a clean and concise way to extend functionality without inheritance or modification of the original class. The example demonstrates extending the String class to provide a method for counting vowels, without altering the String class itself.

// Original class (no modification allowed)
class StringProcessor(private val str: String) {
    fun process(): String {
        return str.uppercase()
    }
}

// Extension Object
class VowelCounterExtension(private val stringProcessor: StringProcessor) : StringProcessor by stringProcessor {
    fun countVowels(): Int {
        val vowels = "AEIOUaeiou".toRegex()
        return stringProcessor.process().count { vowels.contains(it) }
    }
}

// Usage
fun main() {
    val original = StringProcessor("Hello World")
    val extended = VowelCounterExtension(original)

    println(original.process()) // Output: HELLO WORLD
    println(extended.countVowels()) // Output: 3
    println(extended.process()) //Delegated call to original class
}