Generic class

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.

Exercise 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).

Show example solution

class Box<T>(val value: T)

fun main() {
    val intBox = Box<Int>(42)
    println(intBox.value)
}

Test yourself

How is a <strong>generic class</strong> declared?
- class Box(T)
- class Box<T extends T>
- class Box<T>
- class Box: T
Show answer

class Box<T> — A generic class is written as class Box<T>.

Complete the generic Box definition

class Box____(val value: T)

Show answer

class Box<T>(val value: T)

Match the generic Box parts

class Box<T>

Class declaration
val value: T

Type-parameterized property
Box<Int>(5)

Concrete type call
intBox.value

Access

Next lesson: Generic functions

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