#Lecture 52 — OOP Fundamentals

A map can hold a student's name and age, but Dart cannot check it: a misspelled key gives null and every value loses its type. A class describes the record once, and Dart checks every use of it.

Contents

Prerequisites · Connection · Objectives · Lesson · Code Examples · Common Mistakes · Best Practices · Mini Challenge · Practice Questions · Answers · Takeaways

Every error message in this lecture is real. Each one was produced by running the code shown on Dart 3.12.2, so you will recognize it when you meet it yourself.

#Prerequisites

Students should already understand, from Lectures 001–051:

• variables, types and naming conventions (Lectures 004–007)
• that every value in Dart is an Object (Lecture 011)
• functions with parameters and return values (Lectures 030–033)
• lists, for-in loops and maps (Lectures 039–050)
• records stored as List<Map<…>> (Lecture 051)

#Connection With Previous Lectures

In Lecture 051 we stored records in maps. Here is a student:

void main() {
  var ali = {'name': 'Ali', 'age': 20};
  print(ali['name']);
  print(ali['nmae']);
}

Ali
null

The typo 'nmae' is not an error — just a silent null. And because the map mixes a String and an int, Dart types every value as Object, so even simple arithmetic fails:

void main() {
  var ali = {'name': 'Ali', 'age': 20};
  // print(ali['age'] + 1); // ✗ the value's type is Object?, not int
}

Error: The operator '+' isn't defined for the type 'Object?'.

We need a way to say "a student has a String name and an int age" — once — and have Dart check every use. That is what a class does.

#Learning Objectives

By the end of this lecture you will be able to:

• Explain what an object is, in terms of state and behaviour
• Explain the difference between a class and an object
• Write a small class with fields and a method
• Create several objects from one class and change each one separately
• Call a method on an object and explain which object's data it uses
• Explain why a class is safer than a map for a record
• Name the four pillars of object-oriented programming
• Recognize the errors Dart gives for common class mistakes

#1. You Have Been Using Objects All Along

An object is a value that bundles data with actions that work on that data. Every string and list you have used is one.

void main() {
  var name = 'Sara';
  print(name.length);
  print(name.toUpperCase());

  var marks = [80, 75];
  marks.add(90);
  print(marks.length);
}

4
SARA
3

The string knows its own length and can make an uppercase copy. The list knows its elements and can add one. The dot . reaches into an object for its data or its actions.

#2. State and Behaviour

Every object has two sides:

• State — the data it holds right now (a list's elements, an account's balance).
• Behaviour — what it can do (add an element, deposit money).

Object-oriented programming (OOP) is the habit of modelling a program as objects like these, each looking after its own state.

#3. A Class Is the Blueprint

A class describes a kind of object: which data it holds and what it can do. An object created from a class is called an instance of it.

Analogy — a class is the architect's blueprint for a house; an object is a house built from it. One blueprint, many houses — and painting one house does not repaint the others.

String and List are classes written by the Dart team. 'Sara' is one instance of String, and runtimeType (Lecture 010) names the class of any value:

void main() {
  print('Sara'.runtimeType);
  print([80, 75].runtimeType);
}

String
List<int>

Now we will write a class of our own.

#4. Your First Class

A class is written with the keyword class, a name in UpperCamelCase, and a body in braces. Variables inside a class are called fields:

class Student {
  String name = '';
  int age = 0;
}

void main() {
  var ali = Student();
  print(ali.name);
  print(ali.age);
}

0

Student() — the class name followed by () — creates a new object. The fields start with their default values, so name is the empty string (the blank first line) and age is 0. Classes are written at the top level, outside main().

#5. Giving an Object Its Own Data

Use the dot to read and change an object's fields. Each object has its own copy of every field:

class Student {
  String name = '';
  int age = 0;
}

void main() {
  var ali = Student();
  ali.name = 'Ali';
  ali.age = 20;

  var sara = Student();
  sara.name = 'Sara';
  sara.age = 18;

  print('${ali.name} is ${ali.age}');
  print('${sara.name} is ${sara.age}');
}

Ali is 20
Sara is 18

One class, two objects, two separate sets of data. Setting all fields one line at a time is clumsy; Lecture 054 — Constructors gives an object its values at the moment it is created.

#6. Adding Behaviour: a Method

A function written inside a class is called a method. A method can use the object's fields directly by name:

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

  void introduce() {
    print('Hi, I am $name and I am $age.');
  }
}

void main() {
  var ali = Student();
  ali.name = 'Ali';
  ali.age = 20;
  ali.introduce();
}

Hi, I am Ali and I am 20.

ali.introduce() runs the method for ali, so name inside it means ali.name.

#7. A Method That Changes State

Methods can change the object's fields and return values, just like the functions of Lecture 030:

class BankAccount {
  String owner = '';
  int balance = 0;

  void deposit(int amount) {
    balance = balance + amount;
  }

  bool canAfford(int price) {
    return balance >= price;
  }
}

void main() {
  var account = BankAccount();
  account.owner = 'Sara';
  account.deposit(500);
  account.deposit(250);
  print('${account.owner}: ${account.balance}');
  print(account.canAfford(1000));
}

Sara: 750
false

The data (balance) and the rules that change it (deposit) now live together. Anyone who wants to know how a balance changes looks in one place: the class.

#8. Why a Class Beats a Map

Back to the problems from the start. With a class, a typo is a compile-time error instead of a silent null:

class Student {
  String name = '';
  int age = 0;
}

void main() {
  var ali = Student();
  // print(ali.nmae); // ✗ there is no field called nmae
}

Error: The getter 'nmae' isn't defined for the type 'Student'.

Each field keeps its own type, so ali.age + 1 just works — and a wrong value is caught:

class Student {
  String name = '';
  int age = 0;
}

void main() {
  var ali = Student();
  print(ali.age + 1);
  // ali.age = 'twenty'; // ✗ age is an int
}

1

Error: A value of type 'String' can't be assigned to a variable of type 'int'.

Maps are still the right choice for lookups by key (Lecture 051). For a record with fixed fields, use a class.

#9. The Four Pillars of OOP

Object-oriented programming rests on four ideas. You only need their names today — each one has its own lectures:

The BankAccount above already hints at encapsulation: deposit is the "proper" way to change balance. Right now nothing stops someone writing account.balance = 1000000; Lecture 057 shows how to forbid that.

#10. A Real-World Example

A class with a list field and a method that calculates from it, used for several students:

class Student {
  String name = '';
  List<int> marks = [];

  double average() {
    var total = 0;
    for (final mark in marks) {
      total += mark;
    }
    return total / marks.length;
  }
}

void main() {
  var ali = Student();
  ali.name = 'Ali';
  ali.marks = [80, 75, 85];

  var sara = Student();
  sara.name = 'Sara';
  sara.marks = [92, 88, 95];

  for (final student in [ali, sara]) {
    print('${student.name}: ${student.average()}');
  }
}

Ali: 80.0
Sara: 91.66666666666667

Compare Lecture 051's Map<String, List<int>>: the same data, but now each student is one typed object that can calculate its own average. A List<Student> holds them like any other value.

#Code Examples

#Example 1 — A Class with Two Fields

class Book {
  String title = 'Untitled';
  int pages = 0;
}

void main() {
  var book = Book();
  print(book.title);
  print(book.pages);
}

Untitled
0

#Example 2 — Changing Fields

class Book {
  String title = 'Untitled';
  int pages = 0;
}

void main() {
  var book = Book();
  book.title = 'Dart Basics';
  book.pages = 240;
  print('${book.title}: ${book.pages} pages');
}

Dart Basics: 240 pages

#Example 3 — Two Objects Are Independent

class Counter {
  int count = 0;
}

void main() {
  var a = Counter();
  var b = Counter();
  a.count = 5;
  print(a.count);
  print(b.count);
}

5
0

#Example 4 — A Method That Prints

class Dog {
  String name = '';

  void bark() {
    print('$name says woof');
  }
}

void main() {
  var dog = Dog();
  dog.name = 'Rex';
  dog.bark();
}

Rex says woof

#Example 5 — A Method That Returns a Value

class Rectangle {
  double width = 0;
  double height = 0;

  double area() {
    return width * height;
  }
}

void main() {
  var room = Rectangle();
  room.width = 4;
  room.height = 3.5;
  print(room.area());
}

14.0

#Example 6 — A Method That Changes a Field

class Counter {
  int count = 0;

  void increment() {
    count++;
  }
}

void main() {
  var clicks = Counter();
  clicks.increment();
  clicks.increment();
  clicks.increment();
  print(clicks.count);
}

3

#Example 7 — Student Report

class Student {
  String name = '';
  int mark = 0;

  bool passed() {
    return mark >= 50;
  }
}

void main() {
  var zayan = Student();
  zayan.name = 'Zayan';
  zayan.mark = 64;
  print('${zayan.name} passed: ${zayan.passed()}');
}

Zayan passed: true

#Example 8 — Shopping Cart

class Cart {
  List<int> prices = [];

  void add(int price) {
    prices.add(price);
  }

  int total() {
    var sum = 0;
    for (final price in prices) {
      sum += price;
    }
    return sum;
  }
}

void main() {
  var cart = Cart();
  cart.add(120);
  cart.add(250);
  cart.add(90);
  print('Items: ${cart.prices.length}');
  print('Total: ${cart.total()}');
}

Items: 3
Total: 460

#Example 9 — Bank Account

class BankAccount {
  int balance = 0;

  void deposit(int amount) {
    balance += amount;
  }

  void withdraw(int amount) {
    if (amount <= balance) {
      balance -= amount;
    } else {
      print('Not enough money');
    }
  }
}

void main() {
  var account = BankAccount();
  account.deposit(1000);
  account.withdraw(300);
  account.withdraw(900);
  print('Balance: ${account.balance}');
}

Not enough money
Balance: 700

The rule "you cannot withdraw more than you have" lives inside the class, next to the data it protects.

#Example 10 — Thermometer

class Thermometer {
  double celsius = 0;

  double fahrenheit() {
    return celsius * 9 / 5 + 32;
  }
}

void main() {
  var lahore = Thermometer();
  lahore.celsius = 35;
  print('${lahore.celsius} °C = ${lahore.fahrenheit()} °F');
}

35.0 °C = 95.0 °F

#Common Beginner Mistakes

1. Forgetting the parentheses

class Student {
  String name = '';
}

void main() {
  var ali = Student; // no () — this is the class itself, not a new object
  // ali.name = 'Ali'; // ✗
}

Error: The setter 'name' isn't defined for the type 'Type'.

Student() creates an object; Student alone names the class.

2. A field without a starting value

class Student {
  // String name; // ✗ a String cannot start as null
}

Error: Field 'name' should be initialized because its type 'String' doesn't allow null.

The same rule as top-level variables in Lecture 004. Give it a default for now; Lecture 054 shows how a constructor supplies the value instead.

3. Calling a method on the class instead of an object

class Student {
  String name = '';

  void introduce() {
    print('Hi, I am $name');
  }
}

void main() {
  // Student.introduce(); // ✗ which student?
}

Error: Member not found: 'Student.introduce'.

Create an object first and call the method on it: Student().introduce().

4. Writing a class inside main()

void main() {
  // class Student {} // ✗ classes live at the top level
}

Error: 'class' can't be used as an identifier because it's a keyword.

Inside a function, Dart reads class as an ordinary name and gets confused. Move the class outside main().

5. Misspelling a field name

ali.nmae gives Error: The getter 'nmae' isn't defined for the type 'Student'. — a good error: the same typo in a map would have given a silent null.

#Best Practices

• Name classes in UpperCamelCase (BankAccount) and fields and methods in lowerCamelCase (balance, deposit).
• One class should describe one thing.
• Name fields with nouns (balance) and methods with verbs (deposit, calculateTotal).
• Put the rules that change a field in a method of the same class, not scattered in main().
• Use a class for records with fixed fields, and a map for lookups by key.

#Mini Challenge

Write a Player class with a name (default '') and a score (default 0), and a method addPoints(int points). Create Ali, give him 10 points and then 15 more, and print:

Ali: 25

Try it yourself before opening the solution.

class Player {
  String name = '';
  int score = 0;

  void addPoints(int points) {
    score += points;
  }
}

void main() {
  var ali = Player();
  ali.name = 'Ali';
  ali.addPoints(10);
  ali.addPoints(15);
  print('${ali.name}: ${ali.score}');
}

Ali: 25

#Practice Questions

Beginner

1. For a car, name two pieces of state and two behaviours.
2. Write a class City with a field name (default ''). Create one, set its name to 'Lahore', and print it.
3. Create two City objects named 'Lahore' and 'Karachi' and print both names.
4. What does Student mean on its own, and what does Student() mean?

Application

5. Write a class Circle with a double radius = 0 and a method double area() that returns 3.14 * radius * radius. Print the area for radius 10.
6. Write a class Lamp with bool isOn = false and a method toggle() that flips it. Toggle a lamp three times and print isOn.
7. Write a class Product with name and price, and a method int priceFor(int quantity). Create tea at 120 and print the price for 3.
8. Name the four pillars of OOP and the lecture where each is studied.

Challenge

9. Write a class Wallet with int balance = 0 and a method bool pay(int amount) that subtracts the amount and returns true only if there is enough money, otherwise returns false without changing anything. Deposit 500 by setting balance, then pay 200 and then 400, printing each result and the final balance.
10. Write a class Student with a name and List<int> marks, and a method int highest(). Create Sara with marks [72, 95, 88] and print Sara: 95.

#Practice Answers

#Question 1 — Solution

State: colour, speed, fuel level. Behaviour: accelerate, brake, refuel. (Any two of each.)

#Question 2 — Solution

class City {
  String name = '';
}

void main() {
  var city = City();
  city.name = 'Lahore';
  print(city.name);
}

Lahore

#Question 3 — Solution

class City {
  String name = '';
}

void main() {
  var a = City();
  var b = City();
  a.name = 'Lahore';
  b.name = 'Karachi';
  print(a.name);
  print(b.name);
}

Lahore
Karachi

#Question 4 — Solution

Student names the class — the blueprint. Student() creates a new object from it.

#Question 5 — Solution

class Circle {
  double radius = 0;

  double area() {
    return 3.14 * radius * radius;
  }
}

void main() {
  var circle = Circle();
  circle.radius = 10;
  print(circle.area());
}

314.0

#Question 6 — Solution

class Lamp {
  bool isOn = false;

  void toggle() {
    isOn = !isOn;
  }
}

void main() {
  var lamp = Lamp();
  lamp.toggle();
  lamp.toggle();
  lamp.toggle();
  print(lamp.isOn);
}

true

#Question 7 — Solution

class Product {
  String name = '';
  int price = 0;

  int priceFor(int quantity) {
    return price * quantity;
  }
}

void main() {
  var tea = Product();
  tea.name = 'tea';
  tea.price = 120;
  print(tea.priceFor(3));
}

360

#Question 8 — Solution

Encapsulation (Lecture 057), inheritance (Lecture 059), polymorphism (Lecture 065) and abstraction (Lectures 062 and 063).

#Question 9 — Solution

class Wallet {
  int balance = 0;

  bool pay(int amount) {
    if (amount > balance) {
      return false;
    }
    balance -= amount;
    return true;
  }
}

void main() {
  var wallet = Wallet();
  wallet.balance = 500;
  print(wallet.pay(200));
  print(wallet.pay(400));
  print(wallet.balance);
}

true
false
300

#Question 10 — Solution

class Student {
  String name = '';
  List<int> marks = [];

  int highest() {
    var best = marks[0];
    for (final mark in marks) {
      if (mark > best) {
        best = mark;
      }
    }
    return best;
  }
}

void main() {
  var sara = Student();
  sara.name = 'Sara';
  sara.marks = [72, 95, 88];
  print('${sara.name}: ${sara.highest()}');
}

Sara: 95

#Quick Revision

1. What two things does every object have?
2. What is the difference between a class and an object?
3. What is a field? What is a method?
4. What does Student() do?
5. If two objects come from the same class, does changing one change the other?
6. Why is ali.nmae better than ali['nmae']?
7. Name the four pillars of OOP.

#Key Takeaways

• An object bundles state (data) with behaviour (actions).
• A class is the blueprint; each object made from it is an instance.
• Strings and lists are objects too — String and List are classes.
• Fields are variables in a class; methods are functions in a class.
• ClassName() creates a new object; the dot reads fields and calls methods.
• Every object has its own copy of the fields, and a method works on the object it is called on.
• A class gives each field its own type and turns typos into compile-time errors.
• The four pillars — encapsulation, inheritance, polymorphism, abstraction — each get their own lectures.

#What's Next?

Lecture 53 — Creating Classes & Objects. You have seen the idea and a first class. Next we look closely at how classes and objects work in Dart: fields, methods, creating instances, setting fields with the cascade operator, and what really happens when two variables refer to the same object.