void main() {
  print('Hello, World!');
}

// Variable declaration
var name = 'Dart';              // Type inference
String language = 'Dart';       // Explicit type
int number = 42;                // Integer
double pi = 3.14159;           // Double
bool isActive = true;           // Boolean
List<String> fruits = ['apple', 'banana'];  // List
Map<String, int> scores = {     // Map
  'Alice': 100,
  'Bob': 90
};

// Constants
const maxValue = 100;
final currentTime = DateTime.now();

// Null safety
String? nullableName;           // Nullable
late String lateName;          // Late initialization

String text = 'Hello World';

// String methods
print(text.length);           // 11
print(text.toUpperCase());    // HELLO WORLD
print(text.toLowerCase());    // hello world
print(text.contains('World')); // true
print(text.replaceAll('World', 'Dart')); // Hello Dart

// String interpolation
String name = 'Dart';
print('Hello, $name!');       // Hello, Dart!
print('Sum: 3');       // Sum: 3

// Multi-line strings
String multiLine = '''
  Line 1
  Line 2
  Line 3
''';

// Lists
List<int> numbers = [1, 2, 3, 4, 5];
numbers.add(6);               // Add element
numbers.remove(3);            // Remove element
numbers.length;               // Get length
numbers.first;                // First element
numbers.last;                 // Last element

// Sets
Set<String> fruits = {'apple', 'banana', 'orange'};
fruits.add('mango');          // Add element
fruits.remove('banana');      // Remove element
fruits.contains('apple');     // Check existence

// Maps
Map<String, int> scores = {
  'Alice': 100,
  'Bob': 90
};
scores['Charlie'] = 95;       // Add/update
scores.remove('Bob');         // Remove
scores.containsKey('Alice');  // Check key

// If-else
if (age >= 18) {
  print('Adult');
} else if (age >= 13) {
  print('Teenager');
} else {
  print('Child');
}

// Switch
switch (status) {
  case 1:
    print('Active');
    break;
  case 2:
    print('Inactive');
    break;
  default:
    print('Unknown');
}

// Loops
for (var i = 0; i < 5; i++) {
  print(i);
}

for (var item in items) {
  print(item);
}

while (condition) {
  // Do something
}

do {
  // Do something
} while (condition);

// Basic function
int add(int a, int b) {
  return a + b;
}

// Arrow function
int multiply(int a, int b) => a * b;

// Optional parameters
void greet(String name, [String? title]) {
  print('Hello, ${title ?? ''} $name');
}

// Named parameters
void setUser({String? name, int? age}) {
  print('Name: $name, Age: $age');
}

// Default values
void connect({String host = 'localhost', int port = 8080}) {
  print('Connecting to $host:$port');
}

// Function as parameter
void process(Function callback) {
  callback();
}

// Future
Future<String> fetchData() async {
  await Future.delayed(Duration(seconds: 2));
  return 'Data loaded';
}

// Async/await
void main() async {
  try {
    final data = await fetchData();
    print(data);
  } catch (e) {
    print('Error: $e');
  }
}

// Stream
Stream<int> countStream() async* {
  for (var i = 1; i <= 5; i++) {
    await Future.delayed(Duration(seconds: 1));
    yield i;
  }
}

// Using streams
void main() async {
  await for (final count in countStream()) {
    print(count);
  }
}

class Person {
  // Instance variables
  String name;
  int age;
  
  // Constructor
  Person(this.name, this.age);
  
  // Named constructor
  Person.anonymous() : name = 'Anonymous', age = 0;
  
  // Getter
  String get description => '$name is $age years old';
  
  // Setter
  set age(int value) {
    if (value < 0) throw ArgumentError('Age cannot be negative');
    age = value;
  }
  
  // Method
  void greet() {
    print('Hello, I am $name');
  }
}

// Creating objects
final person = Person('Alice', 25);
person.greet();

// Abstract class
abstract class Animal {
  String name;
  Animal(this.name);
  
  void makeSound();
}

// Interface
class Flyable {
  void fly() {
    print('Flying...');
  }
}

// Implementation
class Bird extends Animal implements Flyable {
  Bird(String name) : super(name);
  
  @override
  void makeSound() {
    print('$name says: Tweet!');
  }
  
  @override
  void fly() {
    print('$name is flying');
  }
}

// Using inheritance
final bird = Bird('Sparrow');
bird.makeSound();
bird.fly();

// Mixin
mixin Logger {
  void log(String message) {
    print('Log: $message');
  }
}

mixin Validator {
  bool isValid(String input) {
    return input.isNotEmpty;
  }
}

// Using mixins
class User with Logger, Validator {
  String name;
  User(this.name);
  
  void updateName(String newName) {
    if (isValid(newName)) {
      name = newName;
      log('Name updated to $newName');
    }
  }
}

// Using the class
final user = User('Alice');
user.updateName('Bob');

// Stateless widget
class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      padding: EdgeInsets.all(16),
      child: Column(
        children: [
          Text('Hello, Flutter!'),
          ElevatedButton(
            onPressed: () {
              print('Button pressed');
            },
            child: Text('Click me'),
          ),
        ],
      ),
    );
  }
}

// Stateful widget
class CounterWidget extends StatefulWidget {
  @override
  _CounterWidgetState createState() => _CounterWidgetState();
}

class _CounterWidgetState extends State<CounterWidget> {
  int count = 0;
  
  void increment() {
    setState(() {
      count++;
    });
  }
  
  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        Text('Count: $count'),
        ElevatedButton(
          onPressed: increment,
          child: Text('Increment'),
        ),
      ],
    );
  }
}

// Push to new screen
Navigator.push(
  context,
  MaterialPageRoute(
    builder: (context) => SecondScreen(),
  ),
);

// Pop current screen
Navigator.pop(context);

// Push and replace
Navigator.pushReplacement(
  context,
  MaterialPageRoute(
    builder: (context) => NewScreen(),
  ),
);

// Push and remove until
Navigator.pushAndRemoveUntil(
  context,
  MaterialPageRoute(
    builder: (context) => HomeScreen(),
  ),
  (route) => false,
);

// Provider
class CounterProvider extends ChangeNotifier {
  int _count = 0;
  int get count => _count;
  
  void increment() {
    _count++;
    notifyListeners();
  }
}

// Using provider
class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Consumer<CounterProvider>(
      builder: (context, counter, child) {
        return Column(
          children: [
            Text('Count: ${counter.count}'),
            ElevatedButton(
              onPressed: counter.increment,
              child: Text('Increment'),
            ),
          ],
        );
      },
    );
  }
}