100 Questions to Master OOP in Dart

This is Part 11 of the Object-Oriented Dart series — the part where you stop reading and start proving it. The previous ten parts taught the concepts; this question bank turns them into mastery.

How to use this bank:

• There are 100 questions, grouped by topic and tagged [Basic], [Medium], or [Advanced], and marked (Theory) or (Coding).
• Each one has a Hint — the logic, approach, or key idea, deliberately not a full solution. The struggle before the hint is where the learning happens, so try each one cold first.
• For (Coding) questions, write and run real Dart (dartpad.dev is perfect — no setup). Code you don't run is code you don't understand yet.
• Don't binge. Do 10–15 a day, revisit the matching part when one stumps you, and re-attempt anything you needed the hint for a week later.

If you can answer all 100 without hints — explaining the why, not just the what — you genuinely understand OOP in Dart. Let's go.

Every hint points back to the part that covers it, so you always know where to review. Pen and paper for theory; DartPad for coding.

Section A — Foundations: classes, objects, this (Q1–10)

Q1. [Basic] (Theory) In your own words, what is the difference between a class, an object, and an instance?

Q2. [Basic] (Theory) Dart says "everything is an object." Give two concrete things you can do because of that which you couldn't in a language with primitive types.

Q3. [Basic] (Coding) Model a Laptop class with brand, ram, and price. Create three laptops in a list and print the brand of each.

Q4. [Basic] (Coding) Add a describe() method to Laptop that returns a one-line summary string using all three fields.

Q5. [Medium] (Theory) When is the this keyword required rather than optional? Give the one situation where omitting it causes a silent bug.

Q6. [Medium] (Theory) Why is if (x is Animal) preferred over if (x.runtimeType == Animal)? Name the two distinct advantages.

Q7. [Medium] (Coding) Given List<Object> items mixing Laptops and Strings, print only the laptops' summaries and skip everything else.

Q8. [Advanced] (Theory) Predict and explain:

var a = Laptop('Dell', 16, 1000);
var b = a;
b.ram = 32;
print(a.ram); // ?

Q9. [Advanced] (Coding) Write a function Laptop copyOf(Laptop other) that returns a new, independent laptop with the same values, then prove mutating the copy doesn't affect the original.

Q10. [Advanced] (Theory) What does it mean that a method "automatically operates on whichever instance you called it on"? Tie your answer to how this is passed.

Section B — Constructors (Q11–22)

Q11. [Basic] (Theory) What do you get for free if you declare a class with no constructor, and what happens to that freebie the moment you add one of your own?

Q12. [Basic] (Coding) Write a User class with a required named name and an optional named isAdmin defaulting to false. Build one admin and one normal user.

Q13. [Basic] (Theory) Explain the difference between a named parameter and a named constructor. They sound the same — they're not.

Q14. [Medium] (Coding) Give User a User.guest() named constructor (name 'Guest', not admin). Then rewrite it as a redirecting constructor.

Q15. [Medium] (Coding) Build a Temperature class with a final celsius and a Temperature.fromFahrenheit(double f) constructor that converts in an initializer list.

Q16. [Medium] (Theory) Why can't an initializer list access this, and why is that perfectly fine for setting up final fields?

Q17. [Medium] (Coding) Add an assert to a Person(name, age) constructor that rejects negative ages, and show where in the constructor it goes.

Q18. [Advanced] (Coding) Make an immutable Color (final r, g, b) with a const constructor. Then prove that two const Color(255, 0, 0) values are identical, but two non-const ones are not.

Q19. [Advanced] (Coding) Write a Currency class so that Currency('USD') returns the same cached instance every time. Which constructor type, and what's the supporting machinery?

Q20. [Advanced] (Coding) Create an abstract class Shape with a factory Shape(String kind) returning a Circle or Square based on the string. Why can a factory do this when a generative constructor can't?

Q21. [Advanced] (Theory) You write var p = ImmutablePoint(1, 2) (no const) with a const constructor. Is p a compile-time constant? Are two such objects identical? Explain.

Q22. [Advanced] (Theory) Why is this forbidden inside a factory constructor, and how does that very restriction connect to what makes factories useful?

Section C — Encapsulation: privacy, getters/setters, statics (Q23–33)

Q23. [Basic] (Theory) How do you make a member private in Dart? What scope is it private to — and why does that surprise people from Java?

Q24. [Basic] (Coding) Take class Game { int score = 0; } and encapsulate it so score can be read but only increased through an addPoints method that rejects negatives.

Q25. [Basic] (Coding) Give a Rectangle(width, height) read-only area and perimeter getters. No stored fields for those two.

Q26. [Medium] (Theory) What's the difference between a getter and a regular method, and what's the decision rule for choosing one over the other?

Q27. [Medium] (Coding) Write a Temperature with a fahrenheit getter and setter, where setting fahrenheit updates the underlying _celsius. Show t.fahrenheit = 212 makes t.celsius become 100.

Q28. [Medium] (Theory) Two classes A and B sit in the same file. Can A read B's _secret? What about across two files? Why?

Q29. [Medium] (Coding) A Team holds a private List<String> _members. Expose them for reading without letting callers mutate the internal list. Show that team.members.add(...) throws.

Q30. [Medium] (Coding) Track how many Widget objects have ever been created, exposed as a read-only class-level count.

Q31. [Advanced] (Theory) Explain final vs. late final for a field. What new failure mode does late introduce, and when does it occur?

Q32. [Advanced] (Coding) Make a Config whose settings are produced by an expensive loader that runs only on first access — and never if settings is never read. Prove the lazy behavior with a print statement.

Q33. [Advanced] (Theory) A static method can't see instance fields or this. Explain why, and derive from that the rule for when a method should be static.

Section D — Methods, operators, cascades, equality (Q34–45)

Q34. [Basic] (Theory) What does the operator keyword let you do, and why should you use it sparingly?

Q35. [Basic] (Coding) Create a Vector(x, y) and overload + and - so a + b and a - b return new vectors. Add a readable toString.

Q36. [Basic] (Coding) Use a cascade to configure a StringBuffer so it ends up containing [1, 2, 3], then print it.

Q37. [Medium] (Theory) By default, what does == compare for two objects of your custom class? Why is that often not what you want?

Q38. [Medium] (Coding) Give a Money(cents) value equality: two Money(500) should be == and behave correctly in a Set. What two things must you override, and together?

Q39. [Medium] (Theory) A class overrides == but not hashCode. Describe the concrete, observable bug this causes in a Set or Map.

Q40. [Medium] (Theory) Why must an overridden == take an Object parameter rather than your own class type?

Q41. [Medium] (Coding) Make a Version(major, minor) implement Comparable<Version> so a list sorts correctly with ..sort().

Q42. [Advanced] (Theory) Explain exactly what a cascade (..) returns, and why that's what enables chaining. How does ?.. differ?

Q43. [Advanced] (Coding) Make a Multiplier(factor) whose instances are callable like functions: Multiplier(3)(10) returns 30, and [1,2,3].map(triple) works.

Q44. [Advanced] (Coding) Make a Money type that supports +, -, <, value equality, and Comparable, then sort a list of them. (Combine four Part-4 ideas.)

Q45. [Advanced] (Theory) Contrast a "callable object" (with a call method) against just storing a closure in a field. When is the callable object the better design?

Section E — Inheritance & polymorphism (Q46–58)

Q46. [Basic] (Theory) What relationship does inheritance model, and what's the keyword? Give a clean "is-a" example and a clean non-example.

Q47. [Basic] (Coding) Make Vehicle with start(), and a Car extends Vehicle that adds honk(). Show a Car can do both.

Q48. [Basic] (Theory) What are the two distinct uses of super?

Q49. [Medium] (Coding) Employee extends Person, where Person has name and Employee adds salary. Forward name to the parent using a super parameter.

Q50. [Medium] (Coding) Override describe() in Bird so it prints the parent's line first and then "It can fly." Which pattern is this?

Q51. [Medium] (Theory) @override is optional. Give the concrete bug it catches, with an example of how the bug happens.

Q52. [Medium] (Coding) Build a List<Shape> with a Circle and a Rectangle, then print each area through the common Shape type. Name the mechanism that makes the right area() run.

Q53. [Medium] (Theory) Inheritance vs. composition: give the deciding question and one example of each.

Q54. [Advanced] (Theory) In polymorphism, does the variable's declared type or the object's runtime type decide which implementation runs? What does the other one decide?

Q55. [Advanced] (Theory) Predict and explain:

class A {
  String who() => 'A';
  String greet() => 'Hello from ${who()}';
}
class B extends A {
  @override
  String who() => 'B';
}
print(B().greet()); // ?

Q56. [Advanced] (Coding) Implement a Shape base with a polymorphic toString() that calls an overridden area(). Add Circle and Square, put them in a list, and print each — plus the total area with fold.

Q57. [Advanced] (Theory) When would you use covariant, and what safety trade-off does it make?

Q58. [Advanced] (Theory) What is noSuchMethod, and name a real-world tool category that relies on it.

Section F — Abstract classes & interfaces (Q59–67)

Q59. [Basic] (Theory) What does marking a class abstract prevent, and when is that the right call?

Q60. [Basic] (Coding) Turn class Animal { String makeSound() => ''; } into an abstract class with an abstract makeSound(), then implement Cat and Dog.

Q61. [Medium] (Theory) Dart has no interface keyword for declaring interfaces. So where do interfaces come from, and how do you write a "pure" one?

Q62. [Medium] (Coding) Add a concrete introduce() to an abstract Animal that prints "I say (sound)" using the abstract makeSound(). Show a subclass making it work.

Q63. [Medium] (Coding) Define Flyer (fly()) and Swimmer (swim()) interfaces, and a Duck that is both.

Q64. [Medium] (Theory) List the three biggest differences between extends and implements.

Q65. [Advanced] (Theory) When you implements a concrete class with ten method bodies, do you inherit those bodies? Explain why this makes implements tedious there, and what you'd use instead to reuse code.

Q66. [Advanced] (Coding) Design a plugin system: an abstract Plugin interface (name getter, run()), two concrete plugins, and a runAll(List<Plugin>). Explain why adding a new plugin requires zero changes to runAll.

Q67. [Advanced] (Coding) Write an abstract class Logger with a factory Logger(String type) that returns a ConsoleLogger or SilentLogger. The caller should never name the concrete classes. Which two ideas combine here?

Section G — Mixins (Q68–76)

Q68. [Basic] (Theory) What problem do mixins solve that neither extends nor implements can, and what keyword applies one?

Q69. [Basic] (Coding) Define a Logger mixin with a log(String) method and apply it to two unrelated classes, Server and Client.

Q70. [Medium] (Coding) Create Walks and Barks mixins and a Dog that mixes in both. Then make a SmartPhone that extends Device with Camera, GPS.

Q71. [Medium] (Theory) What two things does an on clause do for a mixin?

Q72. [Medium] (Coding) Write a Stats mixin usable only on a Character (which has int level) that adds levelUp(). Show that class Rock with Stats {} won't compile.

Q73. [Medium] (Coding) Write a Json mixin with a concrete printJson() that depends on an abstract Map<String, dynamic> toJson() the host class must implement.

Q74. [Advanced] (Theory) Predict the output and explain via linearization:

mixin X { String tag() => 'X'; }
mixin Y { String tag() => 'Y'; }
class Z with X, Y {}
print(Z().tag()); // ?

Q75. [Advanced] (Theory) Why can't a regular mixin declare a non-default constructor? What do you reach for if you need a type that's both constructible and mixable?

Q76. [Advanced] (Coding) Model a game with abstract class Entity (name, health) and two mixins Damageable and Healable, both on Entity. A Player composes both. Show damage/heal in action and explain why a Monster extends Entity with Damageable reuses the same logic.

Section H — Enums (Q77–84)

Q77. [Basic] (Theory) What problems do enums solve compared to using Strings or ints for a fixed set of options? Name three.

Q78. [Basic] (Coding) Define a Suit enum and print every value with its index and name.

Q79. [Medium] (Theory) Why is a switch over an enum safer without a default clause? What does adding default cost you?

Q80. [Medium] (Coding) Make an enhanced Coin enum where each value carries its cents (penny=1 … quarter=25), plus a double get dollars. Don't forget the structural rules.

Q81. [Medium] (Coding) Create a LogLevel enum (debug, info, warning, error) with a shouldLog(LogLevel min) that returns whether this level is at least as severe as min.

Q82. [Advanced] (Coding) Build an Operation enum (add, subtract, multiply, divide) where each value carries a symbol and has an apply(a, b) method using an exhaustive switch (this).

Q83. [Advanced] (Coding) Make a Priority enum implement Comparable<Priority> so a list of priorities sorts low→high.

Q84. [Advanced] (Theory) An enum can implements and use with, but cannot extends. Explain the asymmetry.

Section I — Extensions & extension types (Q85–91)

Q85. [Basic] (Theory) What does an extension method let you do that you otherwise couldn't, and what does this refer to inside one?

Q86. [Basic] (Coding) Write an extension on String adding a capitalized getter (uppercases the first letter, handles empty strings).

Q87. [Medium] (Coding) Add an extension on int with times(void Function() action) that runs the action that many times, so 3.times(() => print('hi')) works.

Q88. [Medium] (Coding) Write a generic extension on List<T> with T? get firstOrNull.

Q89. [Medium] (Theory) Why does an extension method fail when called on a dynamic variable? Explain "statically resolved."

Q90. [Advanced] (Theory) Contrast dispatch of an overridden method vs. an extension method. If you call an extension through a supertype reference, which version runs — and why does that mean extensions aren't a substitute for real overrides?

Q91. [Advanced] (Coding) Use an extension type to make UserId and ProductId (both over int) into distinct compile-time types so the compiler refuses to mix them — at zero runtime cost. What's the key runtime property?

Section J — Class modifiers, sealed classes & capstone (Q92–100)

Q92. [Basic] (Theory) In one line each: what do base, interface, and final (the class modifiers) permit and forbid from another library?

Q93. [Medium] (Theory) What does sealed do to a class, and what single capability does it unlock that the other modifiers don't?

Q94. [Medium] (Coding) Model sealed class Animal with Dog, Cat, Bird and a String sound(Animal) using an exhaustive switch (no default). Then add Fish and observe what the compiler says.

Q95. [Medium] (Coding) Extend Q94 so Dog has a name and the matching case returns "Rex says Woof" using an object pattern that destructures the field.

Q96. [Medium] (Theory) Choose the modifier: (a) a Money value type nobody should ever subtype; (b) a PaymentGateway others implement but never extend. Justify each.

Q97. [Advanced] (Theory) Compare modelling a closed set of cases with an enum vs. a sealed class. Give the rule of thumb for when each wins.

Q98. [Advanced] (Coding — Capstone) Build a sealed class Result<T> with Loading, Success(data), Failure(message), and a render(Result<String>) that pattern-matches all three with no default. Explain why a UI literally cannot forget the error state.

Q99. [Advanced] (Coding — Capstone) Write a tiny expression evaluator: sealed class Expr with Num(value), Add(left, right), Mul(left, right), and a recursive int eval(Expr). Evaluate (2 + 3) * 4. Then add a Sub node and note what the compiler forces.

Q100. [Advanced] (Coding — Grand Capstone) Design a small library management system that uses as many series concepts as you can justify: an abstract LibraryItem (or sealed for exhaustive reporting), concrete Book/Magazine/DVD, value equality on an ISBN (consider an extension type), a Borrowable mixin, an enum for ItemStatus, encapsulated private collections, polymorphic describe(), and an exhaustive switch to compute late fees per item type. No starter code — architect it yourself.

Where to go from here

If you worked through all 100 — especially the coding ones, run and not just read — you now have a working command of object-oriented Dart, from the smallest class to sealed hierarchies with exhaustive pattern matching.

A few suggestions to lock it in:

• Re-attempt anything you needed a hint for, a week later, cold. Spaced repetition is how concepts move from "I recognize it" to "I can wield it."
• Build the grand capstone (Q100) for real. A finished mini-project teaches more than fifty isolated snippets.
• Read good Dart source. The Flutter framework and popular pub.dev packages are master classes in these exact patterns — now you can read them fluently.

That brings the Object-Oriented Dart series to a close — eleven parts, from "what is a class?" to a 100-question gauntlet. Head back to Part 1 any time you want to review, and thanks for going the distance. Now go build something — and make it object-oriented.