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Sound null safety interview questions

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

6 questions: 2 warm-ups and 4 with model answers · Levels: Fresher to senior

Warm-up questions

Short questions with short answers, each answered in full.

Fresher

What does the ! operator do, and why is it called a code smell?

Answered in the lesson Sound null safety: nullable types and operators
Fresher

When should you prefer a nullable type over a late variable?

Answered in the lesson Type promotion, late and definite assignment

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 screen crashes with "Null check operator used on a null value" for some users but not others. The code reads user profile fields with ! throughout, such as user.phone! and user.address!.city!. Walk through how you would find the crash and remove the ! operators without changing behaviour for users whose fields are set.
What it tests

Knowing that ! throws only when a field is actually null, how to reproduce with a sparse profile, and the safe replacements: ??, promotion, ?., if-case and null-aware elements.

First step

Reproduce with a user whose optional fields are null, then read the stack trace to the exact ! that threw. The ones that never fire are on fields that happen to be always set for the users you tested.
Follow-ups
- Why does it crash for only some users?
- How do you decide between ?? , promotion and ?. for each field?
- How would you stop new ! operators from creeping back in?
From the lesson Sound null safety: nullable types and operators
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In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Mid to senior

A teammate used a language whose nullability was only a warning and asks what "sound" buys that is worth the extra ? and ! in the code. Explain what soundness guarantees, one thing it lets the compiler do, and one class of bug it removes, with a concrete example.
What it tests

Knowing that soundness means a non-nullable type is never null at run time, that it lets the compiler elide null checks, and that it turns a class of null-dereference crashes into compile errors.

First step

Contrast a guarantee with a warning: a warning can be ignored and the null still arrives, while a sound type system proves the null cannot be there. Then name what each side can and cannot rely on.
Follow-ups
- What can the compiler optimise because the guarantee is sound?
- Where does ! poke a hole in the guarantee, and whose responsibility is it then?
- How does data from outside, such as JSON, re-enter the nullable world?
From the lesson Sound null safety: nullable types and operators
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Mid to senior

A class has a public field String? status, and a method checks if (status != null) then calls status.trim(). The compiler reports that status cannot be promoted. Explain why a local variable would promote but this field does not, and give the fixes you would choose between.
What it tests

Knowing that fields can change or be overridden by a getter so the checked value may differ from the used value, and the fixes: a local copy, a private final field (promotable since 3.2), or an if-case.

First step

Ask what could change status between the check and the call. A local cannot change underneath you; a public field could be reassigned, or a subclass could make it a getter that returns a new value each read.
Follow-ups
- Why does copying the field into a local fix it?
- When does making the field private and final help, and since which version?
- Does marking the field late change anything about promotion?
From the lesson Type promotion, late and definite assignment
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Mid to senior

A Session class declares late User user; and late String token; so the rest of the code can use them without null checks. Logged-out sessions never set them, and the app crashes with LateInitializationError when a guest opens a profile screen. Explain the trade-off the team made with late, and how you would redesign Session.
What it tests

Knowing that late trades a compile-time guarantee for a run-time one, that a legitimately-absent value should be nullable, and how to split always-present from maybe-absent fields.

First step

For each late field ask whether a logged-out session can exist without it. If yes, late is the wrong tool: the absence is legitimate and belongs in the type as nullable.
Follow-ups
- What exactly did late promise that logged-out sessions broke?
- Which fields stay late or become constructor-initialised, and which become nullable?
- How do readers change when a field becomes nullable?
From the lesson Type promotion, late and definite assignment
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes

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

Warm-up questions

What does the ! operator do, and why is it called a code smell?

When should you prefer a nullable type over a late variable?

Questions with model answers

A teammate used a language whose nullability was only a warning and asks what "sound" buys that is worth the extra ? and ! in the code. Explain what soundness guarantees, one thing it lets the compiler do, and one class of bug it removes, with a concrete example.

A class has a public field String? status, and a method checks if (status != null) then calls status.trim(). The compiler reports that status cannot be promoted. Explain why a local variable would promote but this field does not, and give the fixes you would choose between.