Introduction to dart

abstract.dart

import 'dart:math';

void main() {

  final square = Square(side: 2.0);
  print(square.area);
  final circle = Circle(radius:14.0);
  printArea(circle);
}

void printArea(Shape shape) => print(shape.area);

abstract class Shape{
  double get area;
}

class Square implements Shape{
  Square({this.side});
  
  final double side;
  
  double get area => side * side;
}

class Circle implements Shape{
  Circle({this.radius});
  
  final double radius;
  
  double get area => 2 * pi * radius;
}

learn_classes.dart

void main() {
 
  final person = Person(name: "Ihwan", height: 1.7);
  print(person.sayHello());
  person.intro();
}

class Person{
  Person({this.name, this.age = 17, this.height});
  final String name;
  final int age;
  final double height;
  
  String sayHello() => "Hello $name";
  
  void intro() => print("hello, i'm $name, $age y.o & $height meter tall");
}

main.dart

void main() {
  //variable & declaration
  String name = 'Ihwan';
  int age = 22;
  double height = 1.7;
  var friend = 'Andi';
  final phi = 3.14;
  dynamic salary = '1000000';
  
  sayName("udin");
  describe(name, age);
  final String andi = describeFriend(friend: friend);
  print(andi);
}

//use Optional parameters, nullability and default values
void describe(String name, int age, [double height = 1.5]){
  //String interpolation
  print("Hello, I'm $name");
  print ('my name has ${name.length} letters');
  print('my age $age y.o & my height $height meter tall');
}

//named parameter
String describeFriend({String friend, dynamic salary}){
  salary = 'nothing';
  return "my friend name is $friend \n our salary is $salary";
}

//arrow operator
void sayName(String name) => print("hello $name");