Generic class
- Kotlin course
- Intermediate
- Lesson 47/116
- 5 min
Learn generic classes with class Box<T>.
Generic class
A generic class takes a type parameter. It is declared as class Box<T>; T stands for any type.
Example: class Box<T>(val value: T). This class is used as Box<Int>(5) or Box<String>("hello"). T is a type parameter; it is replaced by a concrete type at the call site. Generic classes boost reuse; you write the same logic for many types. Type safety is checked at compile time.
Using Box
When a generic class is called, the concrete type is given in angle brackets. T is replaced by that type.
Example: val intBox = Box<Int>(5); val strBox = Box<String>("hello"). intBox.value is an Int, strBox.value is a String. The type parameter is fixed at the call site; wrong types cause a compile error. This reduces runtime errors. Generic classes are common in collections, wrappers and stores.
Practice The Box<T> generic class
Define the generic class class Box<T>(val value: T). In main declare val intBox = Box<Int>(42) and print its value with println(intBox.value).
Starter code
// Define the Box<T> generic class
// with a value: T property
// instantiate with Int in main and printSample solution
class Box<T>(val value: T)
fun main() {
val intBox = Box<Int>(42)
println(intBox.value)
}Expected output
42
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 How is a <strong>generic class</strong> declared?
- class Box(T)
- class Box<T extends T>
- class Box<T> Correct answer
- class Box: T
Explanation: A generic class is written as class Box<T>.
2 Complete the generic Box definition
class Box____(val value: T)class Box<T>(val value: T)Explanation: <T> is the type parameter in angle brackets.
3 Match the generic Box parts
| Term | Meaning |
|---|---|
class Box<T> |
Class declaration |
val value: T |
Type-parameterized property |
Box<Int>(5) |
Concrete type call |
intBox.value |
Access |