Introduction to OOP in Dart

Object-Oriented Programming (OOP) is a programming paradigm that organizes software design around objects rather than functions or logic. In modern software development, OOP is widely adopted because it promotes reusability, modularity, scalability, and maintainability. Dart, the programming language developed by Google and used extensively in Flutter development, is a fully object-oriented language. This makes it an excellent choice for building complex applications with clean, maintainable code.

In this article, we will explore OOP concepts in Dart, understand why Dart supports OOP, see practical examples, and discuss best practices for object-oriented programming in Dart.

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1. What is Object-Oriented Programming?

Object-Oriented Programming revolves around the concept of objects, which are instances of classes. A class serves as a blueprint for creating objects and defines their properties (fields) and behaviors (methods).

The four main principles of OOP are:

1. Encapsulation – Hiding internal state and exposing only necessary functionality.
2. Abstraction – Representing complex systems in simplified models.
3. Inheritance – Reusing code by creating a hierarchy of classes.
4. Polymorphism – Allowing objects to take multiple forms and behave differently in different contexts.

Dart fully supports all four principles, making it a modern object-oriented language.

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2. Why Dart Supports Object-Oriented Programming

Dart was designed with OOP in mind. Here are the reasons why Dart supports OOP:

2.1 Everything is an Object

In Dart, everything is an object, including numbers, functions, and even null. This uniformity simplifies programming because developers can consistently use methods and properties on any data type.

Example:

void main() {
  int number = 10;
  print(number.isEven); // true
  print(number.toString()); // "10"
  
  String name = "Alice";
  print(name.length); // 5
}

• Here, both int and String are objects.
• You can access their methods and properties directly, which is a core OOP feature.

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2.2 Reusability Through Classes

Dart allows developers to define classes to create objects with shared properties and behaviors. This promotes code reuse and modularity.

Example:

class Car {
  String brand;
  int year;
  
  Car(this.brand, this.year);
  
  void display() {
print('Brand: $brand, Year: $year');
  }
}

void main() {
  Car car1 = Car('Toyota', 2020);
  car1.display(); // Brand: Toyota, Year: 2020
  
  Car car2 = Car('Honda', 2019);
  car2.display(); // Brand: Honda, Year: 2019
}

• Car is a class (blueprint).
• car1 and car2 are objects (instances).
• Methods like display() encapsulate behavior.

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2.3 Encapsulation

Encapsulation means restricting access to internal object data and exposing only what is necessary. Dart uses private variables (prefix _) and getters/setters to implement encapsulation.

Example:

class BankAccount {
  String _accountNumber; // private variable
  double _balance;
  
  BankAccount(this._accountNumber, this._balance);
  
  double get balance => _balance; // getter
  
  void deposit(double amount) {
if (amount > 0) _balance += amount;
  }
  
  void withdraw(double amount) {
if (amount <= _balance) _balance -= amount;
  }
}

void main() {
  BankAccount account = BankAccount('123456', 1000);
  account.deposit(500);
  account.withdraw(200);
  print(account.balance); // 1300
}

• _accountNumber is private.
• Direct access from outside the class is restricted.
• deposit and withdraw provide controlled access to internal state.

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2.4 Abstraction

Abstraction hides implementation details while exposing essential functionality. In Dart, abstraction is achieved using abstract classes and interfaces.

Abstract Class Example:

abstract class Shape {
  void draw(); // abstract method
}

class Circle extends Shape {
  @override
  void draw() {
print('Drawing a Circle');
  }
}

class Square extends Shape {
  @override
  void draw() {
print('Drawing a Square');
  }
}

void main() {
  Shape circle = Circle();
  circle.draw(); // Drawing a Circle
  
  Shape square = Square();
  square.draw(); // Drawing a Square
}

• Shape defines what a shape should do.
• Circle and Square provide specific implementations.
• Users interact with the abstract Shape interface without worrying about concrete details.

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2.5 Inheritance

Inheritance allows a class to reuse properties and methods of another class. Dart supports single inheritance, meaning a class can extend only one superclass.

Example:

class Vehicle {
  void start() {
print('Vehicle started');
  }
}

class Car extends Vehicle {
  void honk() {
print('Car honks');
  }
}

void main() {
  Car car = Car();
  car.start(); // Vehicle started
  car.honk();  // Car honks
}

• Car inherits start() from Vehicle.
• This reduces code duplication and promotes modularity.

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2.6 Polymorphism

Polymorphism allows objects to take multiple forms. In Dart, polymorphism is achieved through method overriding and interfaces.

Example of Method Overriding:

class Animal {
  void sound() {
print('Some generic sound');
  }
}

class Dog extends Animal {
  @override
  void sound() {
print('Bark');
  }
}

void main() {
  Animal myAnimal = Dog();
  myAnimal.sound(); // Bark
}

• myAnimal is of type Animal but behaves as Dog.
• This demonstrates runtime polymorphism.

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2.7 Interfaces in Dart

Every class in Dart can act as an interface. A class can implement multiple interfaces using the implements keyword.

Example:

class Printer {
  void printDocument() {
print('Printing document');
  }
}

class Scanner {
  void scanDocument() {
print('Scanning document');
  }
}

class MultiFunctionDevice implements Printer, Scanner {
  @override
  void printDocument() {
print('MFD Printing document');
  }
  
  @override
  void scanDocument() {
print('MFD Scanning document');
  }
}

void main() {
  MultiFunctionDevice device = MultiFunctionDevice();
  device.printDocument(); // MFD Printing document
  device.scanDocument();  // MFD Scanning document
}

• MultiFunctionDevice implements both Printer and Scanner.
• Interfaces allow Dart to achieve multiple polymorphism.

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3. Classes and Objects in Dart

3.1 Class Structure

A Dart class can contain:

1. Fields – Variables representing the object state.
2. Methods – Functions representing behavior.
3. Constructors – Special methods to initialize objects.
4. Getters/Setters – Encapsulation tools.
5. Static Members – Shared across all instances.

Example:

class Person {
  String name;
  int age;
  
  // Constructor
  Person(this.name, this.age);
  
  // Method
  void introduce() {
print('Hi, I am $name and I am $age years old.');
  }
  
  // Getter
  String get info => '$name, Age: $age';
}

void main() {
  Person p = Person('Alice', 25);
  p.introduce();       // Hi, I am Alice and I am 25 years old.
  print(p.info);       // Alice, Age: 25
}

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3.2 Constructors in Dart

Constructors initialize object properties. Dart supports:

1. Default constructors
2. Named constructors
3. Const constructors
4. Factory constructors

Named Constructor Example:

class Point {
  int x, y;
  
  Point(this.x, this.y);
  
  // Named constructor
  Point.origin() {
x = 0;
y = 0;
  }
}

void main() {
  Point p1 = Point(5, 10);
  Point p2 = Point.origin();
  print('p1: (${p1.x}, ${p1.y})'); // p1: (5, 10)
  print('p2: (${p2.x}, ${p2.y})'); // p2: (0, 0)
}

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3.3 Static Members

Static members belong to the class, not the instance.

class Calculator {
  static double pi = 3.14159;
  
  static double square(double num) => num * num;
}

void main() {
  print(Calculator.pi);           // 3.14159
  print(Calculator.square(5));    // 25
}

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4. Why OOP in Dart is Important

• Reusability: Classes and objects allow code reuse.
• Maintainability: Modular code is easier to manage.
• Scalability: Applications can grow without rewriting core logic.
• Readability: Clear structure improves understanding.
• Flutter Integration: Flutter heavily relies on OOP with widgets as classes.

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5. Best Practices for OOP in Dart

1. Use private variables for encapsulation with _.
2. Prefer named constructors for clarity.
3. Favor composition over inheritance to reduce tight coupling.
4. Keep classes focused on a single responsibility.
5. Use interfaces for flexibility when multiple implementations are possible.
6. Document methods and properties to improve readability.

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6. Real-World Example: Flutter Widgets

In Flutter, everything is a widget (an object). Consider a Container widget:

Container(
  width: 100,
  height: 100,
  color: Colors.blue,
  child: Text('Hello'),
);

• Container is a class.
• Its properties (width, height, color) are fields.
• The child widget is an object of another class (Text).

OOP makes Flutter highly modular and reusable.