Inheritance vs Composition Interview Questions

Inheritance is is-a: CheckedReport is a Report. Composition is has-a: a Report has a Formatter. I prefer composition when behavior varies independently — PDF versus JSON output — because subclassing every combination becomes a tree I cannot maintain. I inject a Serializer instead of XmlReport and JsonReport subclasses. Inheritance is still right for a true taxonomy and when I need runtime polymorphism on a stable interface. 'Favor composition over inheritance' for me means default to strategy objects, not 'never extend a class'.

A BaseController that grew twenty helpers. Changing the base broke children. I would compose CsvExporter and AuthFilter. Fragile base class is the name I use. I still inherit for true is-a like Exception. I would not inherit Stack from ArrayList.

I inject behavior. OrderService gets a PricingEngine, not a GodService parent. I keep inheritance for frameworks that require it. Default: prefer composition. Exception: genuine is-a with a stable base.

Yes, because I call a.foo() or b.foo() explicitly. Java forbids multiple class inheritance partly for diamonds. Interface default methods can still clash and I override. Composition is the practical escape hatch.

If the base is stable and LSP holds, yes. Square as a Rectangle breaks LSP when you set width. I would make Rectangle immutable or model Square separately. Slogan is-a is not enough. LSP is the test.