LearnCore Built-In Types: int, double, num, bool, String

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Lesson 04 of 59

5 mins read

Core Built-In Types: int, double, num, bool, String

In Plain English

Dart features a clean primitive type hierarchy. All numbers inherit from the abstract `num` superclass: `int` (arbitrary precision in VM, 64-bit integers) and `double` (64-bit IEEE 754 floating point). `bool` represents strict booleans (`true` or `false`), and `String` represents UTF-16 code unit sequences.

The Mental Model

"`num` is a parent category "Currency", while `int` is whole dollar bills and `double` is coins with decimal change."

Deep Dive: How It Works

`int` & `double`: Both inherit from `num`. You can declare `num x = 1; x = 1.5;` to accept either.

Type Conversions: `int.parse("42")`, `double.parse("3.14")`, and `value.toString()`.

Strict Boolean Logic: Unlike JS or C, non-boolean values (like `1` or `"true"`) are NOT truthy. Only the literal `true` satisfies `if` conditions.

Syntax Blueprint

int age = 30;
double price = 19.99;
num rating = 4.8; // Accepts int or double
bool isActive = true;
String code = "DART";

Core built-in type declarations in Dart.

Core Rules to Remember

num Superclass: Both `int` and `double` are subtypes of `num`.

Explicit Parsing: Use `int.parse()` or `int.tryParse()` (safe fallback) to parse strings.

Strict Booleans: No truthy/falsy coercion; conditions must evaluate to `bool`.

Common Beginner Traps & How to Fix Them

Mistake:Using `if (userCount)` or `if (items.length)` as a boolean check.

Why it happens: JavaScript and C treat non-zero integers as truthy; Dart requires explicit boolean expressions.

How to fix: Write `if (userCount > 0)` or `if (items.isNotEmpty)`.

Pro Tip: Always prefer `int.tryParse(input)` over `int.parse(input)` when processing user input to prevent unhandled `FormatException` crashes.

Live Interactive Example

Hit Run Code to see it live

Working with Numbers and Type Conversions

Python 3.12

Output Console

Click "Run Code" to view the rendered output.

How it works: Parsed string input to integer, computed floating point arithmetic, and evaluated boolean honors status.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Parse and Calculate Average

Parse the string `"45"` to an int `a` and `"55"` to an int `b`.
Compute the double `avg = (a + b) / 2`.
Print `Average: $avg`.

Sandbox Output

Click "Run & Check" to test your solution.

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