Variables, Types and Null Safety in Dart

Welcome to your first step in learning Dart! In this chapter, we will cover the core basics of syntax, variable declarations, type inference, mutability controls, and the modern sound Null Safety system.

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1. Declarations and Type Inference

In Dart, everything you assign to a variable is an object. This includes numbers, functions, and even null. Dart is a strongly-typed language, but it offers powerful type inference through the var keyword.

Explicit Typing vs. Type Inference

variables.dart

void main() {
  // 1. Explicit typing
  String courseName = "Flutter for Mobile Entornos";
  int studentCount = 35;
  double passingGrade = 3.0;
  bool isLabDay = true;

  // 2. Type inference (Dart infers the types automatically)
  var topic = "Introduction to Dart"; // inferred as String
  var weeks = 16;                     // inferred as int
  
  // topic = 100; // Error! Dart does not allow changing the variable type after inference.
  
  print("Course: $courseName | Topic: $topic");
}

The dynamic Escape Hatch

If you explicitly need a variable to store values of different types dynamically at runtime, you can declare it using dynamic. Use this sparingly as it turns off compile-time type checking.

dynamic_example.dart

void main() {
  dynamic variable = "Hello Icesi";
  print(variable); // Prints: Hello Icesi

  variable = 42; // Allowed!
  print(variable); // Prints: 42
}

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2. Mutability Controls: final and const

To enforce read-only variables, Dart provides two keywords: final and const. While they look similar, they have a key difference regarding when they are evaluated.

Keyword	Evaluation Time	Common Use Case
final	Run-time	API responses, system times, databases.
const	Compile-time	Fixed mathematical constants, UI paddings, styling configurations.

final_vs_const.dart

void main() {
  // final: can only be set once, but the value is evaluated at runtime
  final DateTime accessTime = DateTime.now(); 

  // const: must be known and constant during compile time
  const double gravity = 9.80665;
  // const DateTime compileTime = DateTime.now(); // Error! DateTime.now() is evaluated at runtime.

  print("Access time: $accessTime | Gravity: $gravity");
}

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3. Sound Null Safety

Dart uses sound null safety. This means variables cannot contain null unless you explicitly declare them as nullable. This prevents the classic "Null Pointer Exception" crashes before code runs.

• Non-nullable (default): String name = "Luis"; (cannot be assigned null).
• Nullable: String? nickname; (can be assigned null, default is null if not initialized).

Null Safety Operators

• Conditional access (?.): Returns null if the object is null, instead of crashing.
• Null-assertion (!): Forces a nullable variable to be treated as non-nullable. Throws an exception at runtime if it is indeed null.
• Null-coalescing (??): Provides a fallback value if the left expression evaluates to null.

null_operators.dart

void main() {
  String? courseDescription;

  // 1. Conditional access (?.): avoids crashes if the variable is null
  print(courseDescription?.length); // Prints: null

  // 2. Null-coalescing (??): assigns default value if null
  String finalDescription = courseDescription ?? 'No description available';
  print(finalDescription); // Prints: No description available

  // 3. Null-assertion (!): only use when you are 100% sure it is not null
  courseDescription = 'Flutter class';
  print(courseDescription.length); // Works
}

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Practical Exercises

Test your understanding by resolving the following challenges on variables, mutability, and Null Safety.

Exercise 1: Identify Compilation Errors

Analyze the following code snippet and determine which lines will cause compilation errors and why.

exercise1.dart

void main() {
  var counter = 10;
  counter = '15';
  
  final piValue = 3.14;
  piValue = 3.1416;
  
  const double gravity = 9.8;
  const currentHour = DateTime.now().hour;
  
  String? username;
  print(username!.length);
}

View detailed solution

The code contains four compilation errors and one potential runtime exception:

1. Line 3 (counter = '15';): Compilation error. Dart inferred counter as int. You cannot assign a String to it.
2. Line 6 (piValue = 3.1416;): Compilation error. piValue is final and can only be assigned once.
3. Line 9 (const currentHour = DateTime.now().hour;): Compilation error. const variables must be compile-time constants. DateTime.now() is resolved at runtime. Change to final.
4. Line 12 (print(username!.length);): Runtime error. Although it compiles because the developer forces it with !, it will throw a NullThrownError at runtime because username is null.

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Self-Assessment

Test your knowledge of Dart variables, types, and Null Safety with this interactive quiz.

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