Hướng dẫn và ví dụ phương thức trong Dart

return_type method_name(arguments)    {  
     // statement(s)  
}

class Cat {
  void sayHello(String name) {
    print("Hello $name");
  }
}
void main() {
  Cat tom = new Cat(); // Create a Cat object.  
  tom.sayHello("Jerry"); // Call the method through the object.
}

class Rectangle {
  int width;
  int height;
  Rectangle(this.width, this.height); // Constructor

  int getArea() {
    return width * height;
  }
  void changeWidthHeight(int newWidth, int newHeight)  {
      width = newWidth;
      height = newHeight;
  }
  void showMe()  {
     print('I am a rectangle, width: $width, height: $height');
  }
}
void main() {
   Rectangle rec = Rectangle(5, 10); // Create an object.
   
   rec.showMe();  // Call the method.
   var area = rec.getArea();  // Call the method.
   print('Area: $area');
   print(' --- Change width and height --- ');
   rec.changeWidthHeight(25, 15); // Set newWidth, newHeight
   rec.showMe(); // Call the method.  
   area = rec.getArea(); // Call the method.
   print('Area: $area');
}

static return_type method_name(arguments)    {  
     // statement(s)  
}

class MyUtility {
  static int sum(int a, int b)  {
    return a + b;
  }
  static int minus(int a, int b)  {
    return a - b;
  }
}
void main() {
   var result = MyUtility.sum(100, 50);
   print('Sum Result: $result');
   result = MyUtility.minus(100, 50);
   print('Minus Result: $result');
}

class Triangle {
  late double side1;
  late double side2;
  late double side3;
  static const String DEFAULT_COLOR  = 'blue'; // Static field

  Triangle(double s1, double s2, double s3) {
    bool valid = isValidSides(s1, s2, s3); // Check if all sides are valid.
    if(!valid) {
      throw ArgumentError('Invalid Sides!'); // Throw an Error.
    }
     side1 = s1;
     side2 = s2;
     side3 = s3;
  }
  static bool isValidSides(double s1, double s2, double s3)  {
     if(s1 < 0 || s2 < 0 || s3 < 0)  {
       return false;
     }
     return s1 + s2 > s3 && s1 + s3 > s2 && s2 + s3 > s1;
  }
  bool isEquilateralTriangle() {
      return side1 == side2 && side2 == side3;
  }
}
void main() {
   bool valid = Triangle.isValidSides(6, 8, 10);
   print('Are sides 6, 8 and 10 valid to make a triangle? $valid');
   var triangle = Triangle(3.0, 4.0, 5.0);
   // Check if the triangle is equilateral triangle.
   var check = triangle.isEquilateralTriangle();
   print('Is Equilateral Triangle? $check');
}

abstract return_type method_name(arguments);

abstract class Person {
  void sayHello(); // An abstract method.
}
class EnglishPerson extends Person {
  void sayHello() {
    print("Hello");
  }
}
class RussianPerson extends Person {
   void sayHello() {
      print("Привет");
   }
}
void main() {
    Person enPerson = EnglishPerson();
    enPerson.sayHello();
    Person ruPerson = RussianPerson();
    ruPerson.sayHello();
}

// Non-static method with optional arguments:
return_type method_name(typeM argM, typeN argN,[typeP? argP, typeQ? argQ]){  
     // statement(s)  
}
// Static method with optional arguments:
static return_type method_name(typeM argM, typeN argN,[typeP? argP, typeQ? argQ]){   
     // statement(s)  
}
// Abstract method with optional arguments:
return_type method_name(typeM argM, typeN argN,[typeP? argP, typeQ? argQ]);

// Non-static method with optional arguments:
return_type method_name(typeM argM, typeN argN,
                        [typeP? argP = defaultValueP, typeQ? argQ]){  
     // statement(s)  
}
// Static method with optional arguments:
static return_type method_name(typeM argM, typeN argN,
                        [typeP? argP = defaultValueP, typeQ? argQ]){   
     // statement(s)  
}
// Abstract method with optional arguments:
return_type method_name(typeM argM, typeN argN,
                        [typeP? argP = defaultValueP, typeQ? argQ]);

class MyUtility {
  static String concat(String s1, String s2, [String? s3]) {
    if (s3 != null) {
      return s1 + s2 + s3;
    }
    return s1 + s2;
  }
  static double sum(double v1, double v2, [double? v3, double? v4]) {
    return v1 + v2 + (v3 ?? 0) + (v4 ?? 0);
  }
}
void main() {
  String result1 = MyUtility.concat('One', 'Two');
  print('result1: $result1');
  String result2 = MyUtility.concat('One', 'Two', 'Three');
  print('result2: $result2');
  double value1 = MyUtility.sum(1, 2, 3, 4);
  print('value1: $value1');
  double value2 = MyUtility.sum(1, 2, 3);
  print('value2: $value2');
  double value3 = MyUtility.sum(1, 2);
  print('value3: $value3');
}

class Team {
   List<String> members = [];

   void addMembers(String member1, [String? member2, String? member3]) {
      members.add(member1);
      if(member2!= null) {
         members.add(member2);
      }
      if(member3!= null) {
         members.add(member3);
      }
   }
   void printAllMembers()  {
      print(' --- All members: --- ');
      for(var member in members) {
          print(member);
      }
   }
}  
void main() {
  var team = Team(); // Create Team object.
  team.addMembers('Tom', 'Jerry');
  team.printAllMembers();
}

// Non-static method with optional named parameters:
return_type method_name(typeM argM, typeN argN,
                  {typeP paramP = defaultValueP, typeQ paramQ = defaultValueQ}){  
     // statement(s)  
}
// Static method with optional arguments:
static return_type method_name(typeM argM, typeN argN,
                  {typeP paramP = defaultValueP, typeQ paramQ = defaultValueQ}){  
     // statement(s)  
}
// Abstract method with optional arguments:
return_type method_name(typeM argM, typeN argN,
                  {typeP paramP = defaultValueP, typeQ paramQ = defaultValueQ});

class StringUtility {
  static String concatAndTrim(String s1, String s2, {bool trimLeft = true, bool trimRight = true}) {
    var s = s1 + s2;
    if (trimLeft && trimRight) {
      return s.trim();
    } else if (trimLeft) {
      return s.trimLeft();
    } else if (trimRight) {
      return s.trimRight();
    }
    return s;
  }
}
void main() {
  var s1 = '  One  ';
  var s2 = '  Two  ';
  var result1 = StringUtility.concatAndTrim(s1, s2); // trim left and right
  print('result1: -->$result1<-- (Trim left and right)');

  var result2 = StringUtility.concatAndTrim(s1, s2, trimLeft: false); // trim right only
  print('result2: -->$result2<-- (Trim right only)');

  var result3 = StringUtility.concatAndTrim(s1, s2, trimRight: false); // trim left only
  print('result3: -->$result3<-- (Trim left only)' );

  var result4 = StringUtility.concatAndTrim(s1, s2, trimLeft: false, trimRight: false); // no trim
  print('result4: -->$result4<!-- (No Trim)');
}

class Animal {
  void sayAnything() {
    print('<Nothing>');
  }
}
class Cat extends Animal {
  @override
  void sayAnything() { // Override method from parent class.
    print('Meow');
  }
}
void main() {
  Animal animal = Cat();
  animal.sayAnything();
}

class BoldFormatter {
  String formatText(String text) {
    return '<b>' + text + '</b>';
  }
}
class BoldItalicFormatter extends BoldFormatter {
  @override
  String formatText(String text) { // Override method from parent class.
    var boldText = super.formatText(text); // Call super method.
    return '<i>' + boldText + '</i>';
  }
}
void main() {
  var formatter = BoldItalicFormatter();
  var formattedText = formatter.formatText('Hello');
  print(formattedText);
}

import 'package:meta/meta.dart';

class Foo {
   @nonVirtual
   void bar() {
     print("bar from Foo!");
   }
}
class SubFoo extends Foo {
  @override
   void bar()  {
      print("bar from SubFoo");
   }
}