Generic class

Learn generic classes with <T>.

What is a generic class?

A generic class is a class that takes a type parameter. The type parameter is written inside angle brackets: <T>. T is a placeholder; it is replaced by a real type when the class is used.

Example: class Box<T> { T value; Box(this.value); }. Here Box works with any type: Box<int>, Box<String>. The type parameter makes a single class definition reusable for many types. Compile-time type safety is enforced; assigning the wrong type is a compile error.

The Box<T> example

To use Box<T> with a concrete type, write the type argument explicitly: var b = Box<int>(42);. Now b.value is an int.

Dart often infers the type: writing var b = Box(42); lets Dart figure out that T is int. But explicit typing improves readability. You do not need to redefine the class for each type; a single Box<T> definition works for numbers, text and custom types. This is the core power of generic classes.

Exercise Define and use a Box<T> class

Define a generic class named Box<T>. Give it a T value field and a Box(this.value) constructor. In main, create an instance with var b = Box<int>(42); and call print(b.value).

Show example solution

// Define the Box<T> class
class Box<T> {
  T value;
  Box(this.value);
}

void main() {
  var b = Box<int>(42);
  print(b.value);
}

Test yourself

Why use a generic class?
- To shrink the memory footprint of a class
- To combine type safety and reuse in one definition
- To hide constructors
- To speed up runtime
Show answer

To combine type safety and reuse in one definition — A generic class uses one definition for many types without losing type safety.

Add the type parameter to Box

class Box____ {
  T value;
  Box(this.value);
}

Show answer

class Box<T> {
  T value;
  Box(this.value);
}

Match the Box<T> parts

Box

Class name
T

Type parameter
value

Field name

Next lesson: Generic methods

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