Composition and inheritance are both reuse techniques, but they model different relationships.

Inheritance

Inheritance models an is-a relationship.

class Report {}
class MonthlyReport extends Report {}

A monthly report is a report.

Use inheritance when:

• The child truly is a parent type.
• The parent behavior is stable.
• Substitution works without surprises.
• Runtime polymorphism is needed.

Composition

Composition models a has-a relationship.

class ReportService {
    private PdfRenderer renderer;
}

A report service has a renderer.

Use composition when:

• You want to assemble behavior from smaller objects.
• The relationship is a capability or dependency.
• You want easier testing and replacement.
• Inheritance would create a fragile hierarchy.

Composition Example

class OrderService {
    private final PaymentGateway paymentGateway;
    private final EmailService emailService;
}

The service uses these collaborators; it does not inherit from them.

Comparison

Prefer Composition?

"Prefer composition over inheritance" means do not use inheritance only for code reuse. If the relationship is not truly is-a, composition is usually safer.

Interview Notes

• Inheritance is good for true is-a hierarchies.
• Composition is better for has-a dependencies and replaceable behavior.
• Wrong inheritance can violate LSP.
• Backend services usually use composition heavily through dependency injection.