flutter-study-notebook/study_guide/Generics in Dart.md

Generics in Dart: A Comprehensive Guide

Generics allow you to write flexible, reusable code that can work with different types while maintaining type safety.

1. Generic Classes

You can create classes that work with different types by using type parameters.

Example:

class Box<T> {
  T contents;

  Box(this.contents);

  T getContents() {
    return contents;
  }
}

void main() {
  var intBox = Box<int>(42);
  var stringBox = Box<String>("Hello");

  print(intBox.getContents()); // Prints 42
  print(stringBox.getContents()); // Prints Hello
}

2. Generic Methods

Methods can also use generics to work with different types.

Example:

T first<T>(List<T> list) {
  if (list.isEmpty) {
    throw StateError("List is empty");
  }
  return list[0];
}

void main() {
  var numbers = [1, 2, 3];
  var strings = ["a", "b", "c"];

  print(first(numbers)); // Prints 1
  print(first(strings)); // Prints a
}

3. Type Bounds

You can restrict the types that can be used with a generic by using type bounds.

Example:

class NumberBox<T extends num> {
  T value;

  NumberBox(this.value);

  void square() {
    print(value * value);
  }
}

void main() {
  var intBox = NumberBox<int>(5);
  var doubleBox = NumberBox<double>(2.5);

  intBox.square(); // Prints 25
  doubleBox.square(); // Prints 6.25

  // var stringBox = NumberBox<String>("Hello"); // Compile-time error
}

4. Generic Collections

Dart's collection types are all generic.

Example:

void main() {
  List<int> numbers = [1, 2, 3];
  Map<String, int> ages = {"Alice": 30, "Bob": 25};
  Set<double> scores = {9.5, 8.7, 9.2};

  numbers.add(4); // OK
  // numbers.add("5"); // Compile-time error

  print(ages["Alice"]); // Prints 30
  print(scores.contains(9.2)); // Prints true
}

Conclusion

Generics in Dart provide a powerful way to write reusable, type-safe code. They are extensively used in Dart's core libraries and are a fundamental concept for writing efficient and flexible Dart and Flutter applications.