Functions and Parameters

Functions, Collections, and Classes

Functions and Parameters

A function is a named piece of code that does a job and can give back a result. Dart functions say what type of result they return. You can pass values in, give them names with defaults, and even pass other functions.

int add(int a, int b) => a + b;

void greet(String name, {String prefix = 'Hello'}) {
  print('$prefix, $name!');
}

void runTwice(void Function() action) {
  action();
  action();
}

void main() {
  greet('Ada');               // Hello, Ada!
  runTwice(() => print('tick')); // tick tick
}

Lists, Sets, and Maps

Dart has three everyday containers:

• List — an ordered row of items, like a shopping list.
• Set — items where each one appears only once.
• Map — pairs of keys and values, like a phone book.

var names = ['Ada', 'Alan'];
var unique = {'a', 'b', 'a'};      // only one 'a'
var ages = {'Ada': 39, 'Alan': 41};
print(ages['Ada']);                // 39

Mapping, Filtering, and Reducing

These are ways to transform a collection: map changes each item, filter (called where in Dart) keeps some items, and reduce squashes everything into one result.

final upper = names.map((n) => n.toUpperCase()).toList();
print(upper); // [ADA, ALAN]

Classes and Objects

A class is a blueprint. An object is one thing built from that blueprint. A class holds data and the actions that work with it.

class Counter {
  int _count = 0; // _count is private to this file
  int get count => _count;
  void increment() => _count++;
  @override
  String toString() => 'Counter($_count)';
}

Constructors, Fields, and Methods

A constructor is the special recipe for building an object. Fields are the variables inside, and methods are the actions. Writing Circle(this.radius) saves a value straight into a field.

class Circle {
  final double radius;
  Circle(this.radius);
}

Inheritance and Overriding

Inheritance lets one class build on another. The child gets the parent's abilities and can override — replace — any it wants to do differently.

class Animal {
  void speak() => print('...');
}
class Dog extends Animal {
  @override
  void speak() => print('Woof!');
}