[Dart] Understand Classes and Inheritance in Dart

We will look at how we can create classes and explore some various features. Dart adopts a single-inheritance model, meaning that you can only extend a single class. We will therefore extend our class example by creating an Employee subclass of a parent Person class.

A normal class:

class Person {
  String name;
  int age;
 
  Person(name, age) {
    this.name = name;
    this.age = age;
  }
}

We can also use short hand syntax:

class Person {
  String name;
  int age;
 
  Person(this.name, this.age);
}

We can have optional arguements, override operator, and getter, setter:

class Person {
  // private name
  String _name;
  int age;
  String occupaction;
 
  // optional {this.occupation}
  Person(this._name, this.age, {this.occupaction});
  // optional [this.occupaction]
  Person.fromJson(Map json, [this.occupaction]) {
    _name = json['name'];
    age = json['age'];
  }
  
  // override == operator, define a custom one
  bool operator ==(dynamic b) => _name == b.name && age == b.age && occupaction == b.occupation;
  
  String get name => _name;
  void set name(String updateName) => _name = updateName;
  
  speak() {
    print("My name is &_name. I'm $age years old.");
  }
}

Use:

void main() {
  Person johnny = Person('Johnny', 42, occupaction: 'WD')
   ..speak(); // My name is &_name. I'm 42 years old.
  Person jane = Person.fromJson({'name': 'Johnny', 'age': 42, 'occupation': 'WD'});
  print(jane == johnny); // true
}

Here '..speak()' the same as:

  Person johnny = Person('Johnny', 42, occupation: 'Pilot')
  johnny.speak()

--- 

void main() {
  Person johnny = Person('Johnny', 42, occupation: 'Pilot')
    ..speak()
    ..name = 'Big Johnny'
    ..speak();
  
  print(johnny.name);
  print(johnny.occupation);
  
  Person jane = Person.fromJson({'name': 'Jane', 'age': 39}, 'Web Developer');
  jane.speak();
  print(jane.occupation);
  
  print(johnny == jane);
  Person jane2 = Person('Jane', 39, occupation: 'Web Developer');
  print(jane == jane2);
  
  var bob = Employee('Bob', 23, DateTime.now());
  bob.speak();
}

class Employee extends Person {
  Employee(String name, int age, this.joinDate): super(name, age);
  
  final DateTime joinDate;
  
  @override
  speak() {
    print('My name is $name. I joined on $joinDate');
  }
}

class Person {
  Person(this._name, this.age, {this.occupation});
  Person.fromJson(Map json, [this.occupation]) {
    _name = json['name'];
    age = json['age'];
  }
  
  String _name;
  int age;
  String occupation;
  
  String get name => _name;
  void set name(String updatedName) => _name = updatedName;
  
  // If overriding the == operator, you should also override the Object's `hashCode` getter
  // Learn more at https://www.dartlang.org/guides/libraries/library-tour#implementing-map-keys
  bool operator ==(dynamic b) => _name == b.name && age == b.age && occupation == b.occupation;
  
  speak() {
    print("My name is $_name. I'm $age years old.");
  }
  
//   void _hiddenMethod() {
//     print('This method is hidden');
//   }
}