Dependency injection interview lab

Rapid answers

@Binds versus @Provides?

Use @Binds for mapping an injectable implementation to an abstraction without executing construction logic. Use @Provides when calling a builder/factory, creating third-party types, or applying custom construction.

Does @Singleton mean one forever?

It means one instance per SingletonComponent/component instance. A new graph creates another; process death creates another.

@ViewModelScoped versus @ActivityRetainedScoped?

ViewModelScoped shares within one ViewModel component. ActivityRetainedScoped can be shared across multiple ViewModels belonging to the same retained Activity lifetime.

Scenario: user data survives logout

Cause: user-specific repository/session was application-scoped or a child container was never discarded. Create explicit authenticated-session lifetime, clear durable sensitive caches by policy, and ensure consumers cannot retain old references.

Scenario: two OkHttp clients resolve ambiguously

Add semantic qualifiers at provider and consumer sites. Audit transitive consumers; do not “fix” by deleting one binding or using fragile string names.

Scenario: startup regressed after Hilt migration

Generated wiring is unlikely the whole cause. Trace provider/constructor work, inspect eager roots, delay optional SDK initialization, and compare release Macrobenchmark/startup traces before and after.

System design: white-label commerce

• base app graph: database, network, auth/session owner
• product flavor qualifiers: endpoint/branding/payment capabilities
• feature multibindings: payment handlers and validators
• Activity/graph/ViewModel scopes: UI flow state only
• Workers: Hilt worker factory with stable IDs
• tests: production variant graph compilation, qualifier assertions, plugin presence, scope identity, release endpoint safeguards
• observability: record plugin failures and graph-config version without secrets

Review checklist

• Constructors expose real dependencies.
• Composition root is the only graph-access boundary.
• Scope matches ownership and cleanup lifetime.
• Qualifiers express semantics.
• Assisted values are true runtime inputs.
• Multibinding has ordering/failure/discovery policy.
• Unit tests bypass full graph where possible.
• Variant and integration tests validate production wiring.
• Provider work is measured for startup.
• Long-lived bindings do not retain UI objects.

Interview framing

Start with dependency visibility and lifetimes. Then explain graph key, binding source, scope identity, runtime inputs, replacement strategy, failure modes, and measured cost.

Exercises

1. Draw Hilt component ownership for checkout plus background retry.
2. Refactor a locator-based legacy feature incrementally.
3. Design a DI test pyramid for a multi-flavor app.