Dart與Java的語法區別

1. 主函數

　　(1) 沒有public static

　　(2) 命令參數List<String> args

void main() {
}

2. 可以在class外定義變量，方法等

3. 沒有public, private, protected關鍵字

4. 創建對象，new可選

5. 獲取值${variableValue}, ${expression}

6. Class中屬性默認public，若聲明私有，只需在屬性名前加_

class Bicycle {
  int cadence;
  int _speed = 0;
  int get speed => _speed;
  int gear;

  Bicycle(this.cadence, this.gear);

  void applyBrake(int decrement) {
    _speed -= decrement;
  }

  void speedUp(int increment) {
    _speed += increment;
  }

  @override
  String toString() => 'Bicycle: $_speed mph';
}

void main() {
  var bike = Bicycle(2, 1);
  print(bike);
}

7. getter/setter方法

//返回值類型/get/外部可訪問屬性/方法體
int get speed => _speed

//set/外部可訪問屬性(值)/方法體
set speed(int value) => _speed = value;

8. 未初始化的變量值均為null

9. 構造函數簡化，避免重載

import 'dart:math';

class Rectangle {

  Point origin;
  num width;
  num height;

  Rectangle({this.origin = const Point(0, 0), this.width = 0, this.height = 0});

  @override
  String toString() => 'Origin: (${origin.x}, ${origin.y}), width: $width, height: $height';
}

void main() {
  print(Rectangle(origin: const Point(3,4), width: 10, height: 20));
  print(Rectangle(origin: const Point(10,20)));
  print(Rectangle(width: 20));
  print(Rectangle());
}

10. 一個dart文件中可以定義多個class，也可以僅僅定義方法

//factory.dart
import 'shapes.dart';

Shape shapeFactory(String type){
  if(type == 'circle') return Circle(2);
  if(type == 'square') return Square(2);
  throw 'Can\'t create $type';
}

11. 字符串可用單引號或者雙引號

12. 工廠構造函數 => 在抽象類中定義工廠函數，規定子類實例化方式

abstract class Shape {
  //factory constructor
 factory Shape(String type){
    if(type == 'circle') return Circle(10);
    if(type == 'square') return Square(10);
    throw 'can\'t create $type';
  }
  num get area;
}

13. Dart中沒有interface關鍵字，每個類都可以做接口

import 'shapes.dart';

class CircleMock implements Circle {
  num area;
  num radius;
}

// 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) => 'Hi $who, I\'m $_name';
}

// An implementation of the Person interface.
class Impostor implements Person {
  @override
  String get _name => this._name;

  @override
  String greet(String who) => 'Hi $who, do you know who I am?';
}

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

main() {
  print(greetBob(Person('chris')));
  print(greetBob(Impostor()));
}

14. 同一對象級聯操作

通過..符號可以在同一對象上級聯操作。

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

15. 函數無需聲明可能拋出的異常類型

16. 捕獲異常

try {
  // ···
} on Exception catch (e) {
  print('Exception details:\n $e');
} catch (e, s) {
  print('Exception details:\n $e');
  print('Stack trace:\n $s');
}

17. 在運行時，Dart保留泛型類型信息

  var names = List<String>();
  names.addAll(['Seth', 'Kathy', 'Lars']);
  print(names is List<String>); // true