Shallow Copy vs. Deep Copy

In this lesson you will learn the difference between a shallow and a deep copy in Kotlin, so you can avoid bugs caused by unintentionally shared data. This is for anyone searching for "Kotlin shallow copy deep copy".

Assigning a Kotlin class instance to another variable actually copies a reference to the same object. That is the shallow-copy situation for classes, and changing one of them affects the other.

In the sample code, val shallow = original makes shallow point at the same Address instance as original. So setting shallow.city = "Osaka" also changes what original.city prints.

A common stumbling block for beginners is the bug where "I copied it, but the original changed too". Whenever you work with reference-type variables you need to keep this shallow-copy behaviour in mind.

In real-world development, when you want genuinely independent data, a data class's .copy() method duplicates an instance easily.

📖 Reference code

class Address(var city: String)

fun main() {
    val original = Address("Tokyo")
    val shallow = original
    shallow.city = "Osaka"
    println(original.city)
}

✍️ Your code

Type your code, then press "Run"

🧪 This site can't compile or run Kotlin directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).