Interview questions: Dart

How to answer

Interviewers want understanding, not memorised lines. A strong structure: define it in one sentence, say why it exists, give a two-line example (ideally from your own project), and mention one pitfall. Practise aloud; clear explanation is a skill you can train.

Q1. final versus const?

Model answer: "Both create variables that cannot be reassigned. final is set once at run time, for example final now = DateTime.now(). const must be known at compile time and makes the value deeply immutable and canonical, so identical const objects are shared. In Flutter, const widgets are created once and can be skipped during rebuilds."

Q2. Explain null safety and its operators.

Model answer: "Since Dart 3, null safety is sound and always on: String cannot be null, String? can. I handle nullable values with ?. for safe access, ?? for defaults, ??= to assign if null, and flow analysis after a null check. ! asserts non-null and throws if wrong, so I avoid it. late defers initialisation, and required forces named parameters."

Q3. Future versus Stream?

Model answer: "A Future delivers one value or error later, such as an HTTP response. A Stream delivers many values over time, such as Firestore snapshots or live scores. I await Futures and listen to Streams or use await for; in Flutter I use FutureBuilder and StreamBuilder, and I cancel subscriptions in dispose."

Q4. How does async/await work if Dart is single-threaded?

Model answer: "Each isolate runs an event loop on one thread. await pauses only the current function and returns control to the event loop, which keeps processing other events like taps and frames. When the awaited Future completes, the function resumes. Waiting does not block, but heavy CPU work still does, so I use Isolate.run or compute for that."

Q5. What is an isolate?

Model answer: "An isolate is Dart's unit of concurrency: its own memory and event loop. Isolates do not share memory and communicate by messages, so there are no data races. I use Isolate.run for heavy work like parsing a huge JSON, so the UI isolate keeps rendering smoothly."

Q6. Mixin versus inheritance versus interface?

Model answer: "extends inherits code from one parent (is-a). implements promises the interface of a class without inheriting code, useful for fakes in tests. A mixin with with adds reusable behaviour to unrelated classes, and a class can use several mixins; SingleTickerProviderStateMixin is a Flutter example."

Q7. What are factory constructors and extension methods?

Model answer: "A factory constructor runs code before deciding what to return: it can validate input, return a cached instance or a subtype; fromJson constructors are often factories. Extension methods add methods to existing types like String or BuildContext without changing them; they are resolved statically."

Q8. Dart 3: records, patterns and sealed classes?

Model answer: "Records group a few values without a class, like returning (each, left) from splitBill. Patterns destructure values in switch, if-case and declarations, and can check JSON shapes safely. A sealed class has a fixed set of subtypes in its library, so a switch over it is exhaustive; I use it for UI states like Loading, Success and Error."

Common interview mistakes

• Saying async makes code run on another thread. It does not.
• Confusing final and const, especially for objects.
• Answering without an example.
• Claiming ! is fine "because the value is never null". Show a safe alternative.