• Explain the difference between composition and inheritance

Composition and inheritance represent two types of relationships between classes. Composition and inheritance both promote code reuse, but in different ways.

Composition is when a class has references to other classes as members. We can think of composition as having a "has-a" relationship:

• A library has a book.
• A house has a kitchen.
• A car has a steering wheel

Consider the following example:

public class SteeringWheel {
    private double radius;
    private boolean heated;
    ...
}

public class Engine {
    private String type;
    private int cylinders;
    ...
}

public class Car {
    private SteeringWheel steeringWheel;
    private Engine engine;
    ...
}

This is a very simplified example, but it shows that we know a car has a steering wheel, an engine, and possibly other parts. We can represent this in code by defining a Car class to have fields referencing a SteeringWheel and an Engine object.

Composition differs from inheritance since inheritance is associated with an "is-a" relationship:

• A cat is an animal.
• Football is a sport.
• A car is a vehicle.

public class Sport {
    ...
}

public class Football extends Sport {
    ...
}

We've seen with polymorphism that inheritance promotes substitutability. Since Football inherits from Sport, a Football object can be used anywhere a Sport object is expected since football "is-a" sport.

When deciding on whether to use composition or inheritance, it is important to remember the "is-a" versus "has-a" relationships as they serve different purposes.

Why don't we just have Car inherit from SteeringWheel rather than using composition? Because a car is not a type of steering wheel, a car "has-a" steering wheel as a part.

Why don't we just have Football use composition to contain a Sport object? Because football "is-a" type of sport, and we can use an instance of Football anywhere a Sport object is needed.