Secondary constructor
- Kotlin course
- Intermediate
- Lesson 2/116
- 5 min
Learn multiple constructors with secondary constructors.
Secondary constructor
When you need multiple constructors in a class, you use a secondary constructor. It starts with the constructor keyword and its body must delegate to another constructor with this(...).
Example: class Book(val title: String) { constructor(t: String, a: String) : this(t) { println(a) } }. The second constructor first calls the primary constructor via this(t), then runs its own body. Every secondary constructor must start with a this delegation.
The this(...) call
A secondary constructor delegates to another constructor with this(...) at the start of its body. The call must be the first statement. The primary constructor and init blocks run first, then the secondary body runs.
Example: constructor(name: String, age: Int) : this(name) { println(age) }. Here this(name) runs the primary constructor first, then the secondary body runs. This order guarantees that properties are always initialized.
Practice Add a secondary constructor to Book
Define a class named Book. Its primary constructor takes val title: String. Add a secondary constructor that takes title and author, delegates with this(title) and writes println("Author: $author") in its body. In main call the two-parameter constructor.
Starter code
// Define the Book class
// primary: val title: String
// secondary: title and author, delegating with this(title)
// call the two-parameter constructor in mainSample solution
class Book(val title: String) {
constructor(t: String, a: String) : this(t) {
println("Author: $a")
}
}
fun main() {
val b = Book("Kotlin", "JetBrains")
}Expected output
Author: JetBrains
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the parts of a secondary constructor
| Term | Meaning |
|---|---|
constructor(t, a) |
Secondary signature |
: this(t) |
Primary delegation |
{ println(a) } |
Extra body |
class Book(val title) |
Primary constructor |
2 When is a secondary constructor used?
- When the class has only one parameter
- When primary constructors are forbidden
- When different parameter combinations are needed for the same class Correct answer
- When the class will not be inherited
Explanation: A secondary constructor is added when different parameter sets are needed.
3 Complete the secondary constructor body
class Book(val title: String) {
constructor(t: String, a: String) ____ this(t) {
println(a)
}
}class Book(val title: String) {
constructor(t: String, a: String) : this(t) {
println(a)
}
}Explanation: The colon binds the this call.