copy

Learn to copy data classes with copy.

The copy method

Data classes generate a copy method automatically. It produces a clone of the instance while changing some properties; the original is untouched.

Example: val q = p.copy(price = 20.0). Here q has the same properties as p but price becomes 20.0. p stays the same. Properties not mentioned as named arguments keep their original value. This is a common and safe way to modify data classes.

copy and named arguments

The copy method takes all parameters of the primary constructor as named arguments. You pass only the ones you want to change; the rest keep the original value.

Example: data class Product(val name: String, val price: Double). p.copy(name = "New") changes only name; price stays the same. p.copy() produces an exact clone. The order of named arguments does not matter; you can change any parameter.

Exercise Define the Product data class and use copy

Define a data class named Product with a primary constructor taking val name: String, val price: Double. In main create a Product("Pen", 15.0) instance. Produce a q instance with p.copy(price = 20.0). Print p and q separately with println.

Show example solution

data class Product(val name: String, val price: Double)

fun main() {
    val p = Product("Pen", 15.0)
    val q = p.copy(price = 20.0)
    println(p)
    println(q)
}

Test yourself

Match the copy pattern

p.copy(price = 20.0)

Clone with changed price
p.copy()

Exact clone
val q = ...

Assigning to a new instance
p unchanged

Original preserved
What does <code>val q = p.copy(price = 20.0)</code> do?
- Sets p's price to 20.0
- Produces an exact clone of p
- Produces a new clone with price 20.0, p unchanged
- Deletes p
Show answer

Produces a new clone with price 20.0, p unchanged — copy keeps the original and returns a modified clone.

Complete the copy call

val q = p.____(price = 20.0)

Show answer

val q = p.copy(price = 20.0)

Next lesson: equals and hashCode

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