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The Boring Flutter·#4

Dart Exercises: Part 1

July 1, 2025·8 min read
  • dart
  • flutter
  • exercises
  • practice

Prequel

you have read the theory. variables, null safety, collections, OOP, async. you know what the words mean.

but reading code and writing code are different things. the neurons that let you produce dart are not built by reading explanations. they are built by getting stuck, figuring it out, and running it. that is what these exercises do.

10 exercises. each one targets something from the last two posts. they start simple and get harder, in that order. every concept you solve here compounds directly into real flutter work.

no answers in this post. look things up, make mistakes, make it run.

Exercise 1 - Variables

covers var, explicit type annotations, final, const, .runtimeType, nullable types

directions

  • declare at least one variable using each of: var, an explicit type, final, const. use at least four types: int, double, String, bool
  • try to reassign a final variable and a const variable. note what happens and put the error in a comment
  • declare a nullable String? and a non-nullable String. assign null to the nullable one. try assigning null to the non-nullable one and note what the compiler says
  • print the runtime type of each variable using .runtimeType

expected output

42 is int
3.14 is double
Dart is String
true is bool
name is final, cannot be reassigned
PI is const, cannot be reassigned
Nullable string: null
Non-nullable string: Hello

Exercise 2 - Control Flow

covers if/else, switch, for, while, break, continue

directions

  • create a list of integers from 1 to 20
  • use a for loop with continue to print only even numbers, skipping odd ones
  • write a switch statement that classifies a number as "small" (1 to 5), "medium" (6 to 10), or "large" (11 to 20). run it on three different values
  • use a while loop to find the first number greater than 15 that is divisible by both 3 and 4. break when you find it

expected output

Even numbers: 2 4 6 8 10 12 14 16 18 20
3 → small
8 → medium
17 → large
First match (>15, div by 3&4): 24

Exercise 3 - Functions

covers named parameters, optional positional parameters, default values, arrow functions, higher-order functions

directions

  • write a function greet that takes a required name and a named optional title (default: "Dev"). it should return a greeting string
  • write an arrow function square that returns the square of a number
  • write a function describe with a required param and an optional positional param [String suffix = '!']
  • write a higher-order function applyTwice that takes a function and a value, applies the function twice, and returns the result
  • call applyTwice with square and the value 3

expected output

Hello, Dev Rakhul!
Hello, Engineer Arjun!
Square of 5: 25
described: Cool Dart!
applyTwice(square, 3): 81

Exercise 4 - Strings

covers string interpolation, multi-line strings, .toUpperCase(), .contains(), .split(), .trim(), .replaceAll(), .startsWith(), raw strings

directions

  • build a full name from firstName and lastName using string interpolation
  • write a multi-line string using """ with three lines, any quote or text about code you want
  • demonstrate each of these in a print: .toUpperCase(), .contains(), .split(), .trim(), .replaceAll(), .startsWith()
  • print a raw string r'\n is not a newline here' and then a regular string that actually uses \n
  • reverse a string without using any built-in reverse method

expected output

Full name: Ada Lovelace
--- Quote ---
Code is poetry
written in logic
read by machines
---
Upper: ADA LOVELACE
Contains 'Love': true
Split by ' ': [Ada, Lovelace]
Trimmed: "Dart"
Replaced: Hello World
Raw: \n is not a newline here
With escape:
new line here
Reversed: ecalevoL adA

Exercise 5 - Collections

covers List, Map, Set, the spread operator ..., collection-if, collection-for

directions

  • create a List<String> of 5 programming languages. add one, remove one, sort it, then iterate with index
  • create a Map<String, int> of language to year created. add a new entry, check if a key exists, loop through all entries and print them
  • create two Set<int>s and compute their union, intersection, and difference
  • use collection-if to conditionally include an element in a new list
  • use collection-for to build a list of uppercased strings from an existing list
  • use the spread operator ... to merge two lists into one

expected output

Languages (sorted): [C, Dart, Go, Python, Rust]
After add/remove: [C, Dart, Python, Rust]
Map entry — Dart: 2011
Has 'Go': false
Union: {1, 2, 3, 4, 5}
Intersection: {3}
Difference: {1, 2}
Conditional list: [dart, flutter, firebase]
Uppercased: [DART, FLUTTER, FIREBASE]
Merged: [C, Dart, Python, Go, Java]

Exercise 6 - Classes

covers instance variables, private fields, constructors, named constructors, getters, computed properties

directions

  • create a BankAccount class with owner (String) and a private _balance (double)
  • add a default constructor and a named constructor BankAccount.empty(owner) that initializes the balance to zero
  • add deposit(amount) and withdraw(amount) methods. withdraw should print an error message if funds are insufficient and leave the balance unchanged
  • add a getter balance that exposes the private _balance
  • add a computed getter isRich that returns true if the balance is above 10000
  • create both types of accounts, run through deposits and withdrawals, and print the state after each step

expected output

Account: Rakhul | Balance: ₹5000.0
After deposit: ₹8000.0
After withdraw: ₹5500.0
Insufficient funds!
Balance unchanged: ₹5500.0
Is rich: false
Empty account — Arjun | Balance: ₹0.0

Exercise 7 - Null Safety

covers ?, ??, ??=, !, null-aware access ?., null-aware cascade ?..

directions

  • declare a nullable String? city. print it before and after assigning a value
  • use ?? to print the city or "Unknown City" as a fallback when it is null
  • use ??= to assign "Mumbai" only if city is null. then try ??= again with a different value and confirm it did not change
  • create a nullable List<String>?. use ?.length to safely access its length before and after giving it a value
  • create a simple class with a method that returns a string. make a nullable instance of it and call the method safely using ?.
  • use the null assertion operator ! on a known non-null nullable variable. add a comment in your code explaining what would happen at runtime if you used ! on an actual null value

expected output

city is: null
city with fallback: Unknown City
After ??= 'Mumbai': Mumbai
Assigning again with ??=... still: Mumbai
Nullable list length: null
After init, length: 3
Safe method call result: HELLO
Null assertion result: 42

Exercise 8 - Inheritance

covers abstract, extends, @override, implements, polymorphism

directions

  • write an abstract class Shape with an abstract method area() and a concrete method describe() that prints "I am a [runtimeType]"
  • create Circle and Rectangle that extend Shape and implement area()
  • create an interface (an abstract class) Drawable with a method draw()
  • make Rectangle implement Drawable as well as extend Shape
  • create a List<Shape> with one Circle and one Rectangle, loop through it, and call both .area() and .describe() on each shape

expected output

I am a Circle
Circle area: 78.54
I am a Rectangle
Rectangle area: 24.0
Rectangle is being drawn on canvas
Polymorphic loop:
  Circle → area: 78.54
  Rectangle → area: 24.0

Exercise 9 - Mixins

covers mixin, with, the on constraint, mixin composition

directions

  • create a mixin Logger with a method log(String message) that prints [LOG] message
  • create a mixin Validator with a method isValidEmail(String email) that returns a bool
  • create a class UserService with no parent class. give it both mixins using with
  • create a base class Model with an id field and a name field
  • create a mixin Serializable with an on Model constraint. give it a toJson() method that reads from the Model fields and returns a Map
  • create a class that extends Model and uses Serializable. create an instance and call toJson() on it

expected output

[LOG] UserService initialized
Email valid (test@dart.dev): true
Email valid (notanemail): false
[LOG] Validating user input...
User JSON: {id: 1, name: Rakhul}

Exercise 10 - Async

covers Future, async/await, .then(), .catchError(), Future.wait, try/catch

directions

  • write Future<String> fetchUser(int id) using Future.delayed to simulate a 2-second network call. it should return "User: $id" for valid ids, or throw an exception if id is negative
  • call it with async/await and print the result
  • call it again using .then() and .catchError() instead
  • write two separate async functions fetchPosts() and fetchComments(), each with a short simulated delay
  • run both in parallel using Future.wait and print both results once they are done
  • call fetchUser with a negative id inside a try/catch block and handle the error

expected output

Fetching user...
User: 42
Then-style: User: 7
Fetching posts and comments in parallel...
Done: [Posts loaded, Comments loaded]
Error caught: Invalid user ID

That's Part 1

ten exercises, ten concepts. variables, control flow, functions, strings, collections, classes, null safety, inheritance, mixins, async.

if you got through all of them, you can write dart. not just understand it when someone else wrote it. actually write it.

part 2 goes deeper. sealed classes, streams, isolates, and the patterns you will hit in real flutter apps.