Type annotations in Dart allow you to explicitly declare the type of a variable, function parameter, or return value. This is part of Dart’s static typing system, which helps catch errors at compile time by ensuring that variables are used consistently with their declared types.

Type Annotations

Type annotations specify the data type of a variable or a function’s parameters and return type. This provides clarity and helps the compiler understand how the data should be handled.

1. Variable Declarations

When declaring variables, you can explicitly annotate their types:

int age = 25;
String name = 'Alice';
bool isValid = true;

Here:

• int specifies that age must hold an integer value.
• String specifies that name must hold a string.
• bool specifies that isValid must hold a boolean value.

2. Function Parameters and Return Types

You can annotate the types of a function’s parameters and return value:

int add(int a, int b) {
  return a + b;
}

In this example:

• The function add takes two int parameters (a and b) and returns an int.

Another example with a void return type (indicating no return value):

void printMessage(String message) {
  print(message);
}

Here, printMessage takes a String parameter and returns nothing (void).

Type Inference

While Dart is statically typed, it also has a powerful type inference system. This means that if you don’t explicitly declare a type, Dart can often infer it from the context.

Variable Declarations with Inference

You can use var to let Dart infer the type based on the assigned value:

dartCopy codevar age = 25; // Dart infers 'int'
var name = 'Alice'; // Dart infers 'String'
var isValid = true; // Dart infers 'bool'

Although var is used, Dart infers the type (int, String, bool) and these variables cannot later hold a value of a different type.

Final and Const Variables

With final and const variables, type inference also works:

dartCopy codefinal age = 25; // Dart infers 'int'
const pi = 3.14; // Dart infers 'double'

Here:

• final means the variable can be set only once and cannot be reassigned.
• const is similar to final but also makes the value a compile-time constant.

Combining Type Annotations and Inference

You can mix explicit annotations with type inference, depending on your needs:

dartCopy codeint age = 25; // Explicit annotation
var name = 'Alice'; // Inferred annotation

Why Use Type Annotations?

• Clarity: They make your code more readable and clear about the intended data types.
• Safety: They help catch errors at compile time, reducing runtime errors.
• Flexibility: When you don’t need strict typing, type inference keeps your code concise while still being type-safe.