Instance vs Static Members

Part of the Object Oriented Programming section of Coddy's Dart journey — lesson 17 of 110.

In Dart classes, members (variables and methods) can belong to either individual objects or to the class itself. Understanding this distinction is fundamental to organizing your code effectively.

Instance members belong to each object you create. Every object gets its own copy of instance variables, and instance methods operate on that specific object's data:

class Dog {
  String name;  // Instance variable
  
  Dog(this.name);
  
  void bark() {  // Instance method
    print('$name says woof!');
  }
}

Dog dog1 = Dog('Buddy');
Dog dog2 = Dog('Max');

dog1.bark();  // Buddy says woof!
dog2.bark();  // Max says woof!

Each Dog object has its own name. Changing one dog's name doesn't affect the other.

Static members belong to the class itself, not to any particular object. They're shared across all instances and accessed using the class name:

class Dog {
  String name;
  static int totalDogs = 0;  // Static variable
  
  Dog(this.name) {
    totalDogs++;
  }
}

Dog dog1 = Dog('Buddy');
Dog dog2 = Dog('Max');

print(Dog.totalDogs);  // 2

The totalDogs variable is shared - there's only one copy regardless of how many Dog objects exist. You access it through the class name Dog.totalDogs, not through an instance.

Challenge

Easy

Let's build a visitor counter system for a museum that tracks both individual exhibit visits and overall museum statistics.

You'll organize your code into two files:

exhibit.dart: Define an Exhibit class that represents a museum exhibit. Each exhibit should have:
- An instance variable name (String) for the exhibit's name
- An instance variable visitorCount (int) starting at 0 to track how many people visited this specific exhibit
- A static variable totalVisitors (int) starting at 0 to track total visits across all exhibits
- A constructor that takes the exhibit name
- A recordVisit() method that increments both the exhibit's visitorCount and the class's totalVisitors
- A displayStats() method that prints the exhibit's information
main.dart: Import your exhibit class and create two exhibits:
- A 'Dinosaur Bones' exhibit
- An 'Ancient Egypt' exhibit
Record 3 visits to the Dinosaur Bones exhibit, then record 2 visits to the Ancient Egypt exhibit. After all visits, call displayStats() on each exhibit in the order they were created, then print the total visitors across all exhibits.

The displayStats() method should print in this exact format:

[name]: [visitorCount] visitors

For the total visitors line, print:

Total museum visitors: [totalVisitors]

Access the static variable using the class name Exhibit.totalVisitors.

Expected output:

Dinosaur Bones: 3 visitors
Ancient Egypt: 2 visitors
Total museum visitors: 5

Cheat sheet

Class members in Dart can be either instance members (belonging to individual objects) or static members (belonging to the class itself).

Instance members are unique to each object. Every object has its own copy of instance variables:

class Dog {
  String name;  // Instance variable
  
  Dog(this.name);
  
  void bark() {  // Instance method
    print('$name says woof!');
  }
}

Static members are shared across all instances and accessed using the class name with the static keyword:

class Dog {
  String name;
  static int totalDogs = 0;  // Static variable
  
  Dog(this.name) {
    totalDogs++;
  }
}

// Access static members through the class name
print(Dog.totalDogs);  // 2

Static variables maintain a single shared value regardless of how many objects are created.

Try it yourself

import 'exhibit.dart';

void main() {
  // TODO: Create a 'Dinosaur Bones' exhibit
  
  // TODO: Create an 'Ancient Egypt' exhibit
  
  // TODO: Record 3 visits to the Dinosaur Bones exhibit
  
  // TODO: Record 2 visits to the Ancient Egypt exhibit
  
  // TODO: Call displayStats() on each exhibit in order
  
  // TODO: Print total museum visitors using Exhibit.totalVisitors
  // Format: Total museum visitors: [totalVisitors]
}