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Generics and the type system interview questions

Interview questions on generics and the type system: short warm-ups, then what each harder question tests, the first step of a good answer and its follow-ups.

7 questions: 3 warm-ups and 4 with model answers · Levels: Fresher to senior

Warm-up questions

Short questions with short answers, each answered in full.

Fresher

In Dart, is <int>[] a List<num>, and could a Java program ask the same question?

Answered in the lesson Generics: classes, methods and bounds
Fresher

What does the declaration List tags = ['new', 'sale']; really declare?

Answered in the lesson Generics: classes, methods and bounds
Fresher

What does it mean that Dart's type system is sound, and which checks still happen while a program runs?

Answered in the lesson The Dart type system: dynamic, Object?, Never

Questions with model answers

What each question tests, the first step of a good answer and the follow-ups to expect.

Mid to senior

A service turns JSON into model objects through a registry: register<T>(fromJson) stores a decoder, and decode<T>(body) looks one up for T. Explain why this works in Dart but needs another shape in Java, how you key the registry, and what decode<Object> or an unregistered type should do.
What it tests

Knowing that Dart keeps type arguments at run time, so T itself can key a map, why erased Java code hands over a Class object instead, and how a lookup by exact type behaves for supertypes and nullable types.

First step

Start from what decode<T> knows while it runs: in Dart the T of each call is a real Type value, so the registry can be a map from Type to decoder, filled by register<T> and read by decode<T>. Then ask what an exact Type lookup does for Object, for a subtype and for T?.
Follow-ups
- Why can T be a map key in Dart, and what does Java code use instead?
- What should decode<Object>, decode<User?> or an unregistered type do?
- How do you give the caller a T back without a cast at every call site?
From the lesson Generics: classes, methods and bounds
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In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 4 minutes
Mid to senior

A 300-file Dart app predates its lint setup and has about 400 raw List and Map annotations. The team wants no_raw_types to fail CI. Plan the rollout so that CI stays green while the fixes land, explain how you choose each missing type argument, and say where a wrong choice will show up.
What it tests

Knowing that no_raw_types reports infos until its severity is raised, that dart fix offers no fix for it, how ignore comments stage a rollout, and that a wrong type argument turns into compile errors or run-time TypeErrors.

First step

Measure before changing anything: turn the lint on at its default info severity, run dart analyze to count the raw types file by file, and note that dart analyze still exits 0 at that point. The count and the file list become the plan.
Follow-ups
- How do you stop new raw types from appearing while the old ones are fixed?
- How do you choose between List<String>, List<Object> and List<dynamic> for one declaration?
- What does a wrong type argument break, and which tests find it?
From the lesson Generics: classes, methods and bounds
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 4 minutes
Mid to senior

An app reads API responses as Map<String, dynamic> in about 40 widgets. A backend release sends some numbers as strings, such as "qty": "2", and three screens crash far from the network code. Design a decoding layer where a format change fails in one place, naming the field, and decide what to do with "2".
What it tests

Knowing that reads from dynamic are implicit casts checked at run time, how to validate JSON once at the boundary into typed classes, how to report the failing field, and when tolerating a changed format is right.

First step

Find where the casts happen: every final int x = json['...'] in a widget is an unchecked promise. Move them into one fromJson per model, called right after jsonDecode, so the widgets only ever see typed objects and the first wrong value stops in that one function.
Follow-ups
- What does the fromJson for an order line check, and what error does it throw?
- Should the app accept "2" as 2, and who decides?
- Which analyzer settings stop new implicit casts from reaching the widgets?
From the lesson The Dart type system: dynamic, Object?, Never
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 4 minutes
Mid to senior

A settings library exposes dynamic setting(String key), and callers write setting('timeout') + 5 and setting('hosts').first. For the next major version you want it to return Object? instead. What breaks at the call sites, how do callers adapt, and what typed API would you offer next to it?
What it tests

Knowing that Object? forbids unchecked member use where dynamic allowed it, how callers migrate with type tests or casts, and how a generic accessor such as T setting<T>(String key) can check the value at run time.

First step

List what callers do with the result today: arithmetic, collection access, string calls. Each of those compiles on dynamic and becomes a compile error on Object?, which is the point: every place that assumed a type now has to say which type it expects.
Follow-ups
- Why does changing dynamic to Object? break callers at compile time, and why is that good?
- How would T setting<T>(String key) check its value, and what does it do on a mismatch?
- How do you ship the change without breaking every caller in one release?
From the lesson The Dart type system: dynamic, Object?, Never
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 4 minutes

All Dart (3.13) interview questions · The Dart (3.13) track

Warm-up questions

In Dart, is <int>[] a List<num>, and could a Java program ask the same question?

What does the declaration List tags = ['new', 'sale']; really declare?

What does it mean that Dart's type system is sound, and which checks still happen while a program runs?

Questions with model answers

A service turns JSON into model objects through a registry: register<T>(fromJson) stores a decoder, and decode<T>(body) looks one up for T. Explain why this works in Dart but needs another shape in Java, how you key the registry, and what decode<Object> or an unregistered type should do.

A 300-file Dart app predates its lint setup and has about 400 raw List and Map annotations. The team wants no_raw_types to fail CI. Plan the rollout so that CI stays green while the fixes land, explain how you choose each missing type argument, and say where a wrong choice will show up.

A settings library exposes dynamic setting(String key), and callers write setting('timeout') + 5 and setting('hosts').first. For the next major version you want it to return Object? instead. What breaks at the call sites, how do callers adapt, and what typed API would you offer next to it?