Dart Classes and Objects – Complete Beginner to Advanced Guide - Learn FLUTTER | Revochamp

Name: Dart Classes and Objects – Complete Beginner to Advanced Guide
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Author: Revochamp

What is a Class in Dart?

A class is a blueprint used to create objects. It defines properties (variables) and behaviors (methods). In Dart, everything is treated as an object, making classes a core concept of the language.

Basic Class Example

A class is defined using the class keyword. It contains fields (data) and methods (functions).

DARTRead-only

class Person {
  String name = '';
  int age = 0;

  void sayHello() {
    print('Hello, my name is $name');
  }
}

class Person {
  String name = '';
  int age = 0;

  void sayHello() {
    print('Hello, my name is $name');
  }
}

Creating Objects (Instances)

Objects are created using constructors. The new keyword is optional in Dart.

DARTRead-only

void main() {
  var person = Person();
  person.name = 'Alice';
  person.age = 30;
  person.sayHello();
}

void main() {
  var person = Person();
  person.name = 'Alice';
  person.age = 30;
  person.sayHello();
}

Constructors in Dart

Constructors initialize objects. If not defined, Dart automatically provides a default constructor.

Parameterized Constructor

DARTRead-only

class Person {
  String name;
  int age;

  Person(this.name, this.age);

  void introduce() {
    print('I am $name and I am $age years old');
  }
}

class Person {
  String name;
  int age;

  Person(this.name, this.age);

  void introduce() {
    print('I am $name and I am $age years old');
  }
}

Named Constructors

Named constructors allow multiple ways to create objects.

DARTRead-only

class Person {
  String name;
  int age;

  Person(this.name, this.age);

  Person.guest() {
    name = 'Guest';
    age = 18;
  }

  Person.fromJson(Map<String, dynamic> json)
      : name = json['name'],
        age = json['age'];
}

class Person {
  String name;
  int age;

  Person(this.name, this.age);

  Person.guest() {
    name = 'Guest';
    age = 18;
  }

  Person.fromJson(Map<String, dynamic> json)
      : name = json['name'],
        age = json['age'];
}

Using the `this` Keyword

this refers to the current instance of the class and is used to avoid naming conflicts.

DARTRead-only

class Point {
  int x, y;

  Point(int x, int y) {
    this.x = x;
    this.y = y;
  }
}

class Point {
  int x, y;

  Point(int x, int y) {
    this.x = x;
    this.y = y;
  }
}

Getters and Setters (Important Fix)

Use private variables with getters and setters to control access and validation.

DARTRead-only

class Rectangle {
  double _width;
  double height;

  Rectangle(this._width, this.height);

  double get area => _width * height;

  set width(double value) {
    if (value > 0) {
      _width = value;
    } else {
      throw ArgumentError('Width must be positive');
    }
  }
}

class Rectangle {
  double _width;
  double height;

  Rectangle(this._width, this.height);

  double get area => _width * height;

  set width(double value) {
    if (value > 0) {
      _width = value;
    } else {
      throw ArgumentError('Width must be positive');
    }
  }
}

Static Members

DARTRead-only

class MathUtils {
  static const double pi = 3.14159;

  static int add(int a, int b) => a + b;
}

class MathUtils {
  static const double pi = 3.14159;

  static int add(int a, int b) => a + b;
}

Encapsulation in Dart

Dart uses library-level privacy. Variables starting with _ are private to the file.

DARTRead-only

class BankAccount {
  String _accountNumber;
  double _balance = 0;

  BankAccount(this._accountNumber);

  void deposit(double amount) {
    if (amount > 0) _balance += amount;
  }

  double get balance => _balance;
}

class BankAccount {
  String _accountNumber;
  double _balance = 0;

  BankAccount(this._accountNumber);

  void deposit(double amount) {
    if (amount > 0) _balance += amount;
  }

  double get balance => _balance;
}

Complete Real-World Example

DARTRead-only

class Student {
  String name;
  int age;
  final int id;
  static int _nextId = 1000;

  Student(this.name, this.age) : id = _nextId++;

  Student.anonymous()
      : name = 'Anonymous',
        age = 0,
        id = _nextId++;

  void introduce() {
    print('Student #$id: $name, $age years old');
  }

  String get description => 'ID $id: $name ($age)';
}

class Student {
  String name;
  int age;
  final int id;
  static int _nextId = 1000;

  Student(this.name, this.age) : id = _nextId++;

  Student.anonymous()
      : name = 'Anonymous',
        age = 0,
        id = _nextId++;

  void introduce() {
    print('Student #$id: $name, $age years old');
  }

  String get description => 'ID $id: $name ($age)';
}

Best Practices

- Always use constructor shorthand when possible
- Use _ for private fields
- Prefer final for immutable data
- Use named constructors for clarity
- Avoid heavy logic inside constructors
- Keep classes small and focused

class Car { String brand; int year; Car(this.brand, this.year); void describe() { print('$brand car from $year'); } } void main() { var car = Car('Toyota', 2023); car.describe(); }

Test Your Knowledge

of 4

Which keyword is used to define a class in Dart?

class

object

struct

define

of 4

What does `this` refer to in Dart?

Parent class

Current instance

Static context

Constructor only

of 4

What is the purpose of a constructor?

Delete objects

Initialize objects

Print values

Run loops

of 4

What will be the output? class A { static int count = 0; A() { count++; } } void main() { A(); A(); print(A.count); }

Error

Frequently Asked Questions

Can a class be private in Dart?

Dart doesn't have private keywords. Instead, library‑level privacy is achieved by prefixing an identifier with _. If you want a class to be private to a file, place it in its own file and don't export it.

What is the difference between a class and an object?

A class is a blueprint (definition), while an object is an instance created from that blueprint. For example, Person is a class; var alice = Person('Alice', 30) creates an object alice.

Can a class extend multiple classes?

No, Dart supports single inheritance – a class can extend only one superclass. However, you can implement multiple interfaces and use mixins to reuse code from multiple sources.

What is the purpose of a constructor?

A constructor initializes a new instance of a class. It sets initial values for fields and performs any setup required before the object is used.

How do you make a variable read‑only from outside the class?

Declare it as final (if set once) or make it private (_var) and provide only a getter (no setter).

class Person { String name; int age; Person(this.name, this.age); Person.guest() { name = 'Guest'; age = 18; } Person.fromJson(Map<String, dynamic> json) : name = json['name'], age = json['age']; }

class Rectangle { double _width; double height; Rectangle(this._width, this.height); double get area => _width * height; set width(double value) { if (value > 0) { _width = value; } else { throw ArgumentError('Width must be positive'); } } }

class BankAccount { String _accountNumber; double _balance = 0; BankAccount(this._accountNumber); void deposit(double amount) { if (amount > 0) _balance += amount; } double get balance => _balance; }

class Student { String name; int age; final int id; static int _nextId = 1000; Student(this.name, this.age) : id = _nextId++; Student.anonymous() : name = 'Anonymous', age = 0, id = _nextId++; void introduce() { print('Student #$id: $name, $age years old'); } String get description => 'ID $id: $name ($age)'; }

class Car { String brand; int year; Car(this.brand, this.year); void describe() { print('$brand car from $year'); } } void main() { var car = Car('Toyota', 2023); car.describe(); }

Test Your Knowledge

of 4

Which keyword is used to define a class in Dart?

class

object

struct

define

of 4

What does `this` refer to in Dart?

Parent class

Current instance

Static context

Constructor only

of 4

What is the purpose of a constructor?

Delete objects

Initialize objects

Print values

Run loops

of 4

What will be the output? class A { static int count = 0; A() { count++; } } void main() { A(); A(); print(A.count); }

Error

Frequently Asked Questions

Can a class be private in Dart?

What is the difference between a class and an object?

A class is a blueprint (definition), while an object is an instance created from that blueprint. For example, Person is a class; var alice = Person('Alice', 30) creates an object alice.

Can a class extend multiple classes?

No, Dart supports single inheritance – a class can extend only one superclass. However, you can implement multiple interfaces and use mixins to reuse code from multiple sources.

What is the purpose of a constructor?

A constructor initializes a new instance of a class. It sets initial values for fields and performs any setup required before the object is used.

How do you make a variable read‑only from outside the class?

Declare it as final (if set once) or make it private (_var) and provide only a getter (no setter).

Dart Classes and Objects – Complete Beginner to Advanced Guide

Dart Classes and Objects – Complete Beginner to Advanced Guide

What is a Class in Dart?

Basic Class Example

Creating Objects (Instances)

Constructors in Dart

Parameterized Constructor

Named Constructors

Using the `this` Keyword

Getters and Setters (Important Fix)

Static Members

Encapsulation in Dart

Complete Real-World Example

Best Practices

Try it yourself

Test Your Knowledge

Frequently Asked Questions

Need help?

Dart Classes and Objects – Complete Beginner to Advanced Guide

Dart Classes and Objects – Complete Beginner to Advanced Guide

What is a Class in Dart?

Basic Class Example

Creating Objects (Instances)

Constructors in Dart

Parameterized Constructor

Named Constructors

Using the `this` Keyword

Getters and Setters (Important Fix)

Static Members

Encapsulation in Dart

Complete Real-World Example

Best Practices

Try it yourself

Test Your Knowledge

Frequently Asked Questions

Need help?

What is a Class in Dart?

Basic Class Example

Creating Objects (Instances)

Constructors in Dart

Parameterized Constructor

Named Constructors

Using the this Keyword

Getters and Setters (Important Fix)

Static Members

Encapsulation in Dart

Complete Real-World Example

Best Practices

Try it yourself

Test Your Knowledge

Frequently Asked Questions

What is a Class in Dart?

Basic Class Example

Creating Objects (Instances)

Constructors in Dart

Parameterized Constructor

Named Constructors

Using the this Keyword

Getters and Setters (Important Fix)

Static Members

Encapsulation in Dart

Complete Real-World Example

Best Practices

Try it yourself

Test Your Knowledge

Frequently Asked Questions

Using the `this` Keyword

Using the `this` Keyword