What is dependency injection and why is it useful?
Reported in Criteo European engineering loops. Java/Spring staple connecting OOP design to testable service architecture.
Interview scenario
Often asked in Criteo loops at European offices (London, Berlin, Amsterdam, Paris, Stockholm, Dublin, and remote EU). Prepare a clear spoken answer plus key trade-offs.
Model answer
Try answering aloud first
Cover trade-offs, structure, and a concrete example before revealing the baseline response.
How to frame this at Criteo: Connect your answer to measurable impact, clarity of thought, and trade-offs the team cares about. Below is a strong baseline response you can adapt with your own project examples.
Dependency injection (DI) supplies an object's collaborators from outside rather than constructing them internally (new EmailService() inside UserService). Inversion of Control container (Spring) creates and wires beans.
Benefits: testability (inject mocks), flexibility (swap implementations via config), separation of concerns, and clearer constructors showing required dependencies.
Injection styles: constructor (preferred—immutable, explicit), setter, field (@Autowired—harder to test). Prefer interface-based dependencies.
Relate to SOLID—Dependency Inversion Principle: depend on abstractions. Mention lifecycle scopes (singleton, prototype), @Configuration classes, and avoiding service locator anti-pattern.
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