Dart

late String description;

void main() {
  description = 'Feijoada!';
  print(description);
}

final name = 'Bob';               // Without a type annotation
final String nickname = 'Bobby';

const bar = 1000000;              // Unit of pressure (dynes/cm2)
const double atm = 1.01325 * bar; // Standard atmosphere

var list = [10, 20, 30];              // 创建 List，类型推导得到的类型为 List<int>
assert(list.length == 3);
assert(list[0] == 10);                // 下标从 0 开始，即首元素的下标是 0
assert(list[1] == 20);

list.add(40);                         // 使用 add 往 list 中增加一个元素

list[1] = 10;                         // 直接修改 List 中数据
assert(list[1] == 10);

var list = [1, 2, 3];
var list2 = [0, ...list];
assert(list2.length == 4);

var list;
var list2 = [0, ...?list];            // list 为 null，也不会报错
assert(list2.length == 1);

var nav = [
  'Home',
  'Furniture',
  'Plants',
  if (promoActive) 'Outlet'
];

var listOfInts = [1, 2, 3];
var listOfStrings = [
  '#0',
  for (var i in listOfInts) '#$i'
];
assert(listOfStrings[0] == '#0');
assert(listOfStrings[1] == '#1');
assert(listOfStrings[2] == '#2');
assert(listOfStrings[3] == '#3');

var halogens = {'fluorine', 'chlorine', 'bromine', 'iodine', 'astatine'};

var names = <String>{};
// Set<String> names = {};   // This works, too.
// var names = {};           // Creates a map, not a set.

names.add('fluorine');       // 往 set 中增加元素
names.addAll(halogens);      // 往 set 中增加多个元素

var gifts = {                         // 类型推导为 Map<String, String>
  // Key:    Value
  'first': 'partridge',
  'second': 'turtledoves',
  'fifth': 'golden rings'
};

var nobleGases = {                    // 类型推导为 Map<int, String>
  2: 'helium',
  10: 'neon',
  18: 'argon',
};

var gifts = Map<String, String>();
gifts['first'] = 'partridge';          // Add a key-value pair
gifts['second'] = 'turtledoves';
gifts['fifth'] = 'golden rings';

var nobleGases = Map<int, String>();
nobleGases[2] = 'helium';
nobleGases[10] = 'neon';
nobleGases[18] = 'argon';

//int a = null;         // 从 Dart 2.12 起，这是不合法的。int 不能为 null
int? a = null;          // 类型后加个 ? 表示，这个变量可能为 null

a = 42;
int b = a!;             // ! 是 Null assertion operator，从 int? 类型的 a 得到了 int 类型的 b

querySelector('#confirm')    // Get an object.
  ?..text = 'Confirm'        // Use its members.
  ..classes.add('important')
  ..onClick.listen((e) => window.alert('Confirmed!'));

var button = querySelector('#confirm');
button?.text = 'Confirm';
button?.classes.add('important');
button?.onClick.listen((e) => window.alert('Confirmed!'));

void func1({String arg1, bool arg2}) {
  print(arg1);
  print(arg2);
}

main() {
  func1(arg1: "abc", arg2: true);     // 调用时，需要指定参数的名称 arg1，arg2
  func1();                            // 如果指定参数的话，函数体内会为 null
}

String say(String from, String msg, [String device]) { // 参数 device 在中括号中，是可选的
  var result = '$from says $msg';
  if (device != null) {
    result = '$result with a $device';
  }
  return result;
}

main() {
  assert(say('Bob', 'Howdy') == 'Bob says Howdy');
  assert(say('Bob', 'Howdy', 'smoke signal') == 'Bob says Howdy with a smoke signal');
}

/// Sets the [bold] and [hidden] flags ...
void enableFlags({bool bold = false, bool hidden = false}) {...}

// bold will be true; hidden will be false.
enableFlags(bold: true);

([[Type] param1[, …]]) {
  codeBlock;
};

(employee as Person).firstName = 'Bob';

querySelector('#confirm')      // Get an object.
  ..text = 'Confirm'           // Use its members.
  ..classes.add('important')
  ..onClick.listen((e) => window.alert('Confirmed!'));

var button = querySelector('#confirm');
button.text = 'Confirm';
button.classes.add('important');
button.onClick.listen((e) => window.alert('Confirmed!'));

if (isRaining()) {
  you.bringRainCoat();
} else if (isSnowing()) {
  you.wearJacket();
} else {
  car.putTopDown();
}

var message = StringBuffer('Dart is fun');
for (var i = 0; i < 5; i++) {
  message.write('!');
}

var collection = [0, 1, 2];
for (var x in collection) {
  print(x); // 0 1 2
}

while (!isDone()) {
  doSomething();
}

do {
  printLine();
} while (!atEndOfPage());

var command = 'OPEN';

switch (command) {
  case 'CLOSED':
    executeClosed();
    break;
  case 'PENDING':
    executePending();
    break;
  case 'APPROVED':
    executeApproved();
    break;
  case 'DENIED':
    executeDenied();
    break;
  case 'OPEN':
    executeOpen();
    break;
  default:
    executeUnknown();
}

class Square extends Shape {}

class Circle extends Shape {}

class Shape {
  Shape();

  factory Shape.fromTypeName(String typeName) {   // 这个构造函数返回子类型
    if (typeName == 'square') return Square();
    if (typeName == 'circle') return Circle();

    print('I don\'t recognize $typeName');
    throw Error();
  }
}

class Rectangle {
  double left, top, width, height;

  Rectangle(this.left, this.top, this.width, this.height);

  // Define two calculated properties: right and bottom.
  double get right => left + width;
  set right(double value) => left = value - width;
  double get bottom => top + height;
  set bottom(double value) => top = value - height;
}

void main() {
  var rect = Rectangle(3, 4, 20, 15);
  assert(rect.left == 3);
  rect.right = 12;
  assert(rect.left == -8);
}

// This class is declared abstract and thus
// can't be instantiated.
abstract class AbstractContainer {              // 这是一个抽象类
  // Define constructors, fields, methods...

  void updateChildren();                        // Abstract method.
}

// A person. The implicit interface contains greet().
class Person {
  // In the interface, but visible only in this library.
  final _name;

  // Not in the interface, since this is a constructor.
  Person(this._name);

  // In the interface.
  String greet(String who) => 'Hello, $who. I am $_name.';
}

// An implementation of the Person interface.
class Impostor implements Person {                          // Impostor 实现了 Person 接口
  get _name => '';

  String greet(String who) => 'Hi $who. Do you know who I am?';
}

String greetBob(Person person) => person.greet('Bob');

void main() {
  print(greetBob(Person('Kathy')));
  print(greetBob(Impostor()));
}

class Television {
  void turnOn() {
    _illuminateDisplay();
    _activateIrSensor();
  }
  // ···
}

class SmartTelevision extends Television {  // SmartTelevision 是 Television 的子类
  void turnOn() {
    super.turnOn();                         // super 关键字表示其父类
    _bootNetworkInterface();
    _initializeMemory();
    _upgradeApps();
  }
  // ···
}