Dart Object Oriented For Beginner : Expense Manager Case Study Part 2

Lesson 2: Creating Your First Class

Duration: 30 minutes

App Feature: 💰 Building the Expense Class

What You'll Build: Create an Expense class to represent a single expense

Prerequisites: Complete Lesson 1

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What We'll Learn Today

By the end of this lesson, you'll be able to:

• ✅ Write your first Dart class
• ✅ Understand properties (instance variables)
• ✅ Create objects from your class
• ✅ Use constructors to initialize objects
• ✅ Add methods to make objects do things
• ✅ Work with multiple expense objects

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Review: What is a Class?

Remember from Lesson 1:

• A class is a blueprint/template
• An object is an actual instance created from that class
• Example: Expense class → your coffee purchase object

Today we're going to actually write this code ourselves!

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Part 1: Your First Class (The Basic Structure)

Let's start with the simplest possible Expense class:

class Expense {
  // Properties (also called instance variables or fields)
  String description;
  double amount;
  String category;
  DateTime date;
}

Breaking It Down:

class Expense {

• class keyword tells Dart we're creating a new class
• Expense is the name (always start with capital letter!)
• { opens the class body

  String description;
  double amount;
  String category;
  DateTime date;

• These are properties - data that each expense will store
• String = text (description, category)
• double = decimal number (amount like 4.50)
• DateTime = date and time

}

• Closes the class body

🎯 Try It Yourself:

Create a file called expense.dart and type this class. Pay attention to:

• Capital E in Expense
• Semicolons ; after each property
• Proper indentation (2 or 4 spaces)

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Part 2: Creating Objects (Instances)

Now that we have our class, let's create actual expense objects!

void main() {
  // Create an object using the class
  var coffee = Expense();

  // Set the properties
  coffee.description = 'Morning coffee';
  coffee.amount = 4.50;
  coffee.category = 'Food';
  coffee.date = DateTime.now();

  // Print the details
  print('I spent \$${coffee.amount} on ${coffee.description}');
}

Understanding the Code:

var coffee = Expense();

• var = variable (Dart figures out the type)
• coffee = variable name (our choice)
• Expense() = calling the class to create a new object
• The () is important - it creates a new instance

coffee.description = 'Morning coffee';

• Use dot notation (.) to access properties
• Set each property one by one

DateTime.now()

• Gets the current date and time
• Perfect for tracking when you made the expense

🎯 Try It Yourself:

Add this code below your class and run it. You should see:

I spent $4.50 on Morning coffee

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Part 3: The Problem with Our Current Code

Our code works, but there's a problem. Try running this:

void main() {
  var expense = Expense();
  print('Amount: ${expense.amount}');
}

What happens? You'll get an error!

Error: Non-nullable instance field 'amount' must be initialized.

This is because we created an expense but didn't set its properties. Dart doesn't know what values to use!

We need a better way to create objects...

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Part 4: Introducing Constructors

A constructor is a special method that runs when you create an object. It initializes (sets up) the object with data.

Basic Constructor:

class Expense {
  String description;
  double amount;
  String category;
  DateTime date;

  // Constructor - same name as the class
  Expense(String desc, double amt, String cat, DateTime dt) {
    description = desc;
    amount = amt;
    category = cat;
    date = dt;
  }
}

void main() {
  // Now we MUST provide values when creating an object
  var coffee = Expense('Coffee', 4.50, 'Food', DateTime.now());
  print('${coffee.description}: \$${coffee.amount}');
}

Understanding Constructors:

Expense(String desc, double amt, String cat, DateTime dt) {

• Constructor has the same name as the class
• Takes parameters (the values we need)
• No return type (not even void)

  description = desc;
  amount = amt;

• Inside the constructor, we assign parameter values to properties

var coffee = Expense('Coffee', 4.50, 'Food', DateTime.now());

• Now we pass values when creating the object
• Order matters! (description, amount, category, date)

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Part 5: Dart's Shortcut Constructor (The Better Way!)

Dart has a shortcut that makes constructors much cleaner:

class Expense {
  String description;
  double amount;
  String category;
  DateTime date;

  // Shortcut constructor using 'this'
  Expense(this.description, this.amount, this.category, this.date);
}

This does the same thing as before, but in one line!

• this.description means "set the description property to the value passed in"
• Much cleaner and easier to read
• This is the style most Dart developers use

🎯 Try It Yourself:

Update your Expense class to use the shortcut constructor and create 3 different expenses:

void main() {
  var coffee = Expense('Coffee', 4.50, 'Food', DateTime.now());
  var lunch = Expense('Lunch at cafe', 12.75, 'Food', DateTime.now());
  var netflix = Expense('Netflix subscription', 15.99, 'Entertainment', DateTime.now());

  print('${coffee.description}: \$${coffee.amount}');
  print('${lunch.description}: \$${lunch.amount}');
  print('${netflix.description}: \$${netflix.amount}');
}

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Part 6: Adding Methods (Making Objects DO Things)

So far our objects just hold data. Let's make them do something useful!

A method is a function inside a class that can work with the object's data.

class Expense {
  String description;
  double amount;
  String category;
  DateTime date;

  Expense(this.description, this.amount, this.category, this.date);

  // Method to print expense details
  void printDetails() {
    print('━━━━━━━━━━━━━━━━━━━━');
    print('📝 Description: $description');
    print('💵 Amount: \$$amount');
    print('📁 Category: $category');
    print('📅 Date: ${date.toString().split(' ')[0]}');
    print('━━━━━━━━━━━━━━━━━━━━');
  }
}

void main() {
  var coffee = Expense('Morning coffee', 4.50, 'Food', DateTime.now());

  // Call the method
  coffee.printDetails();
}

Understanding Methods:

void printDetails() {

• void = doesn't return anything
• printDetails = method name (use camelCase)
• () = no parameters needed

print('💵 Amount: \$$amount');

• Inside methods, we can directly access properties
• No need for this.amount, just amount
• \$ escapes the dollar sign so it prints literally

coffee.printDetails();

• Call methods using dot notation
• Don't forget the ()!

🎯 Try It Yourself:

Add the printDetails() method and create 3 expenses. Call printDetails() on each:

void main() {
  var coffee = Expense('Coffee', 4.50, 'Food', DateTime.now());
  var uber = Expense('Uber to work', 8.20, 'Transport', DateTime.now());
  var movie = Expense('Movie ticket', 12.00, 'Entertainment', DateTime.now());

  coffee.printDetails();
  uber.printDetails();
  movie.printDetails();
}

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Part 7: Methods That Return Values

Methods can also calculate and return values:

class Expense {
  String description;
  double amount;
  String category;
  DateTime date;

  Expense(this.description, this.amount, this.category, this.date);

  void printDetails() {
    print('━━━━━━━━━━━━━━━━━━━━');
    print('📝 $description');
    print('💵 \$$amount');
    print('📁 $category');
    print('📅 ${date.toString().split(' ')[0]}');
    print('━━━━━━━━━━━━━━━━━━━━');
  }

  // Method that returns a boolean
  bool isMajorExpense() {
    return amount > 100;
  }

  // Method that returns a String
  String getSummary() {
    return '$description: \$$amount [$category]';
  }

  // Method that returns a formatted amount
  String getFormattedAmount() {
    return '\$${amount.toStringAsFixed(2)}';
  }
}

void main() {
  var laptop = Expense('New laptop', 899.99, 'Electronics', DateTime.now());
  var coffee = Expense('Coffee', 4.50, 'Food', DateTime.now());

  // Using methods that return values
  print(laptop.getSummary());
  print('Is major expense? ${laptop.isMajorExpense()}');
  print('Formatted: ${laptop.getFormattedAmount()}');

  print('');

  print(coffee.getSummary());
  print('Is major expense? ${coffee.isMajorExpense()}');
}

Output:

New laptop: $899.99 [Electronics]
Is major expense? true
Formatted: $899.99

Coffee: $4.50 [Food]
Is major expense? false

Understanding Return Values:

bool isMajorExpense() {
  return amount > 100;
}

• bool = this method returns true or false
• return sends the value back
• Can use the returned value in conditions or print it

String getSummary() {
  return '$description: \$$amount [$category]';
}

• String = returns text
• Builds a nice formatted string
• Return value can be stored in a variable or printed directly

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Part 8: Complete Example

Here's our complete Expense class with everything we've learned:

class Expense {
  // Properties
  String description;
  double amount;
  String category;
  DateTime date;

  // Constructor
  Expense(this.description, this.amount, this.category, this.date);

  // Method: Print detailed information
  void printDetails() {
    print('━━━━━━━━━━━━━━━━━━━━');
    print('📝 Description: $description');
    print('💵 Amount: ${getFormattedAmount()}');
    print('📁 Category: $category');
    print('📅 Date: ${getFormattedDate()}');
    if (isMajorExpense()) {
      print('⚠️  This is a major expense!');
    }
    print('━━━━━━━━━━━━━━━━━━━━');
  }

  // Method: Check if major expense
  bool isMajorExpense() {
    return amount > 100;
  }

  // Method: Get one-line summary
  String getSummary() {
    String emoji = isMajorExpense() ? '🔴' : '🟢';
    return '$emoji $description: ${getFormattedAmount()} [$category]';
  }

  // Method: Get formatted amount with 2 decimals
  String getFormattedAmount() {
    return '\$${amount.toStringAsFixed(2)}';
  }

  // Method: Get formatted date (without time)
  String getFormattedDate() {
    return date.toString().split(' ')[0];
  }
}

void main() {
  print('🏦 MY EXPENSES\n');

  // Create multiple expenses
  var expenses = [
    Expense('Monthly rent', 1200.00, 'Bills', DateTime.now()),
    Expense('Groceries', 67.50, 'Food', DateTime.now()),
    Expense('Coffee', 4.50, 'Food', DateTime.now()),
    Expense('New phone', 799.99, 'Electronics', DateTime.now()),
    Expense('Gas', 45.00, 'Transport', DateTime.now()),
  ];

  // Print summary for each
  print('SUMMARY:');
  for (var expense in expenses) {
    print(expense.getSummary());
  }

  print('\n');

  // Print details for major expenses
  print('MAJOR EXPENSES (>$100):');
  for (var expense in expenses) {
    if (expense.isMajorExpense()) {
      expense.printDetails();
      print('');
    }
  }

  // Calculate total
  double total = 0;
  for (var expense in expenses) {
    total += expense.amount;
  }
  print('💰 TOTAL SPENT: \$${total.toStringAsFixed(2)}');
}

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🎯 Practice Exercises

Now it's your turn to practice! Complete these exercises to solidify your understanding:

Exercise 1: Basic Class Creation (Easy)

Create a Category class with:

• Properties: name (String), icon (String), color (String)
• Constructor that takes all three properties
• Method: getLabel() that returns "icon name" (e.g., "🍔 Food")

Solution:

class Category {
  String name;
  String icon;
  String color;

  Category(this.name, this.icon, this.color);

  String getLabel() {
    return '$icon $name';
  }
}

void main() {
  var food = Category('Food', '🍔', 'green');
  var transport = Category('Transport', '🚗', 'blue');

  print(food.getLabel());
  print(transport.getLabel());
}

Exercise 2: Adding Methods (Medium)

Add these methods to your Expense class:

1. isThisMonth() - returns true if expense is from current month
2. isFood() - returns true if category is "Food"
3. getDaysAgo() - returns how many days ago this expense was made

Hints:

• Use DateTime.now() to get current date
• Use date.difference(otherDate).inDays to calculate days
• Use date.month and date.year to check if same month

Solution:

bool isThisMonth() {
  DateTime now = DateTime.now();
  return date.year == now.year && date.month == now.month;
}

bool isFood() {
  return category == 'Food';
}

int getDaysAgo() {
  DateTime now = DateTime.now();
  return now.difference(date).inDays;
}

Exercise 3: Real-World Scenario (Hard)

Create expenses for your last week and:

1. Print all expenses from this month
2. Calculate total food expenses
3. Find the largest expense
4. Count how many major expenses (>$100)

Challenge yourself before looking at the solution!

Solution:

void main() {
  var expenses = [
    Expense('Rent', 1200.00, 'Bills', DateTime(2025, 10, 1)),
    Expense('Lunch', 15.50, 'Food', DateTime(2025, 10, 5)),
    Expense('Coffee', 4.50, 'Food', DateTime(2025, 10, 6)),
    Expense('Groceries', 89.30, 'Food', DateTime(2025, 10, 7)),
    Expense('Uber', 12.00, 'Transport', DateTime(2025, 10, 8)),
  ];

  // 1. Print this month's expenses
  print('THIS MONTH\'S EXPENSES:');
  for (var expense in expenses) {
    if (expense.isThisMonth()) {
      print(expense.getSummary());
    }
  }

  // 2. Total food expenses
  double foodTotal = 0;
  for (var expense in expenses) {
    if (expense.category == 'Food') {
      foodTotal += expense.amount;
    }
  }
  print('\nTotal food: \${foodTotal.toStringAsFixed(2)}');

  // 3. Find largest expense
  Expense? largest;
  for (var expense in expenses) {
    if (largest == null || expense.amount > largest.amount) {
      largest = expense;
    }
  }
  print('Largest: ${largest?.getSummary()}');

  // 4. Count major expenses
  int majorCount = 0;
  for (var expense in expenses) {
    if (expense.isMajorExpense()) {
      majorCount++;
    }
  }
  print('Major expenses: $majorCount');
}

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Common Mistakes & How to Fix Them

Mistake 1: Forgetting this in Constructor

// ❌ Wrong
Expense(description, amount, category, date);

// ✅ Correct
Expense(this.description, this.amount, this.category, this.date);

Mistake 2: Wrong Method Call

// ❌ Wrong - no parentheses
coffee.printDetails;

// ✅ Correct
coffee.printDetails();

Mistake 3: Using Class Name Instead of Object

// ❌ Wrong - Expense is the class, not an object
print(Expense.amount);

// ✅ Correct - coffee is the object
var coffee = Expense('Coffee', 4.50, 'Food', DateTime.now());
print(coffee.amount);

Mistake 4: Forgetting Return Statement

// ❌ Wrong - no return
String getSummary() {
  '$description: \$$amount';
}

// ✅ Correct
String getSummary() {
  return '$description: \$$amount';
}

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Key Concepts Review

Before moving to Lesson 3, make sure you understand:

✅ Classes are blueprints - define structure with properties and methods

✅ Objects are instances - created using the class

✅ Constructors initialize objects - use this.property shortcut

✅ Properties hold data - accessed with dot notation

✅ Methods define behavior - can be void or return values

✅ Use proper naming - classes PascalCase, methods camelCase

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Self-Check Questions

Test your understanding:

1. What's the difference between a property and a method?

Answer:

• Property = data (like description, amount)
• Method = behavior/action (like printDetails(), getSummary())

2. Why do we use constructors?

Answer:
To initialize objects with data when we create them, ensuring all required values are set.

3. What does this mean in a constructor?

Answer:
this refers to the current object being created. this.amount means "the amount property of this specific expense object".

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What's Next?

In Lesson 3: Constructors Deep Dive, we'll learn:

• Named constructors for easier object creation
• Optional parameters
• Default values
• Factory constructors
• Making our Expense class even more flexible!

Example preview:

// Instead of this:
var expense = Expense('Coffee', 4.50, 'Food', DateTime.now());

// We'll be able to do this:
var expense = Expense.quick('Coffee', 4.50, 'Food');
var bill = Expense.monthly('Rent', 1200.00);

Much cleaner, right? See you in Lesson 3! 🚀

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Additional Resources

• Dart Language Tour: https://dart.dev/guides/language/language-tour#classes
• Practice: Try creating other classes like Budget, User, or Transaction
• Next Step: Move on to Lesson 3 when you're comfortable with this material

Remember: Don't rush! Make sure you can complete the exercises before moving forward. Practice is key! 💪