toString
- Kotlin course
- Intermediate
- Lesson 16/116
- 5 min
Learn automatic toString generation.
Automatic toString generation
Data classes generate the toString method automatically. It produces a readable form with the class name and property values.
Example: data class Book(val title: String, val author: String). Calling println(Book("Kotlin", "JetBrains")) outputs Book(title=Kotlin, author=JetBrains). In normal classes without an override, toString prints a memory address; data classes avoid this. It is ideal for debugging and logging.
The toString format
The toString format of data classes is ClassName(prop1=value1, prop2=value2). Property names and values are connected with =.
Example: data class Money(val amount: Double, val currency: String). Money(100.0, "USD").toString() returns Money(amount=100.0, currency=USD). Because println calls toString automatically, println(m) and println(m.toString()) produce the same output.
Practice toString with the Book data class
Define a data class named Book with a primary constructor taking val title: String, val author: String. In main create a Book("Kotlin", "JetBrains") instance. First print the result of b.toString(), then print b directly with println.
Starter code
// Define the Book data class
// with title and author properties
// instantiate in main
// print toString and the instance directlySample solution
data class Book(val title: String, val author: String)
fun main() {
val b = Book("Kotlin", "JetBrains")
println(b.toString())
println(b)
}Expected output
Book(title=Kotlin, author=JetBrains) Book(title=Kotlin, author=JetBrains)
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the toString call
data class Book(val title: String, val author: String)
val b = Book("Kotlin", "JetBrains")
println(b.____())data class Book(val title: String, val author: String)
val b = Book("Kotlin", "JetBrains")
println(b.toString())Explanation: toString returns the readable string of the instance.
2 Match the toString parts
| Term | Meaning |
|---|---|
Book |
Class name |
title=Kotlin |
Property and value |
, |
Separator |
println(b) |
Automatic toString |
3 <code>data class Book(val title: String, val author: String)</code> is defined. What does <code>println(Book("Kotlin", "JetBrains"))</code> print?
- Book(title=Kotlin, author=JetBrains) Correct answer
- Book@1a2b3c
- Kotlin - JetBrains
- Book
Explanation: A data class toString shows the class name and property-value pairs.