What is the difference between inheritance and composition?
Inheritance models an is a relationship, where a child class is a kind of the parent, and it reuses the parent by extending it. Composition models a has a relationship, where one class holds another class as a field and uses its behaviour. Modern advice is to favour composition because it is more flexible and creates looser coupling than deep inheritance.
The simple test: is a versus has a
Ask yourself whether the relationship is an is a or a has a. A Dog is an Animal, so inheritance fits. A Car has an Engine, so composition fits. Getting this test right is most of the answer.
Inheritance example
class Animal { void eat() {} }
class Dog extends Animal { } // Dog is an AnimalComposition example
class Engine { void start() {} }
class Car {
private Engine engine = new Engine(); // Car has an Engine
void start() { engine.start(); }
}Why teams often prefer composition
- Flexibility: you can swap the inner object for another type at run time.
- Loose coupling: the outer class depends on behaviour, not on a rigid parent hierarchy.
- Avoids fragile base class problems where a parent change breaks many children.
The phrase interviewers love is favour composition over inheritance. Say inheritance for a clear is a relationship and composition for a has a relationship, then explain that composition keeps designs flexible. That single sentence signals real experience.
Common follow up questions
Related interview questions
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