Understanding Dart Programming Basics

# CHAPTER 3

Understanding Dart Programming Basics #

1. Introduction #

2. Learning Objectives #

• Declare variables using var, int, String, final, and const.

• Understand Dart's strict Null Safety features.

• Write functions to execute reusable blocks of code.

• Control flow using if/else statements and for loops.

• Store collections of data using List and Map.

• Create custom data structures using Classes (OOP).

3. Variables and Data Types #

void main() {
  // Explicitly defining the type
  int age = 25;
  double price = 19.99;
  String name = "Alice";
  bool isStudent = true;

  // Type inference (Dart knows this is a String)
  var city = "New York"; 

  print("My name is $name and I live in $city.");
}

final vs const: If a variable will never change, use final or const.

• final username = "Bob"; (Can be set once, but evaluated at runtime).

• const pi = 3.1415; (Must be a hardcoded compile-time constant).

4. Null Safety #

String username; // ERROR: Must be initialized!
String? maybeName; // Adding the '?' means it is ALLOWED to be null.
maybeName = null; // Perfectly fine.

5. Functions #

// Returns an integer, takes two integers as parameters
int calculateArea(int width, int height) {
  return width * height;
}

// Arrow function (shorthand for single-line returns)
int multiply(int a, int b) => a * b;

void main() {
  int area = calculateArea(5, 10);
  print("Area is $area"); // Output: Area is 50
}

6. Control Flow (Conditions and Loops) #

void main() {
  int score = 85;

  // Conditions
  if (score >= 90) {
    print("Grade: A");
  } else if (score >= 80) {
    print("Grade: B");
  } else {
    print("Grade: F");
  }

  // Loops (print numbers 1 through 5)
  for (int i = 1; i <= 5; i++) {
    print("Count: $i");
  }
}

7. Collections: Lists and Maps #

void main() {
  // LIST (Ordered collection)
  List<String> fruits = ["Apple", "Banana", "Mango"];
  fruits.add("Orange");
  print(fruits[0]); // Prints Apple

  // MAP (Key-Value pairs)
  Map<String, int> userAges = {
    "Alice": 25,
    "Bob": 30,
  };
  print(userAges["Bob"]); // Prints 30
}

8. Object-Oriented Programming (Classes) #

// Defining a Blueprint (Class)
class User {
  String name;
  int age;

  // Constructor (runs when creating the object)
  User({required this.name, required this.age});

  // A method inside the class
  void sayHello() {
    print("Hi, I am $name, and I am $age years old.");
  }
}

void main() {
  // Instantiating the object
  User user1 = User(name: "Sarah", age: 28);
  user1.sayHello();
}

9. Common Mistakes #

• Forgetting the Semicolon: Unlike JavaScript or Python, Dart strictly requires a semicolon ; at the end of every statement.

• Ignoring the required keyword: In the class example above, if a parameter is wrapped in {} (named parameters), you must mark it as required or make it nullable ?.

10. Best Practices #

• Use var for local variables: To keep your code clean, use var for local variables inside functions where the type is obvious (e.g., var name = "Alice";). Use explicit types (like String) for class properties and function return types.

11. Practice Exercises #

1. 1. Write a Dart function that takes a String name as a parameter and returns a greeting.

1. 2. Create a List of integers, and write a for loop that prints each integer multiplied by 2.

12. MCQs with Answers #

13. Interview Questions #

• Q: Explain the concept of "Sound Null Safety" in modern Dart. How does it prevent runtime exceptions (NullPointerExceptions) compared to older languages like Java?

• Q: Contrast the final keyword with the const keyword in Dart. Provide an example of when you would use each.

• Q: Describe how named parameters work in Dart functions or classes, and explain the usage of the required keyword.

14. FAQs #

15. Summary #

16. Next Chapter Recommendation #