• Explain the difference between inheritance and composition

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 Book {
    private String title;
    private String author;
    
    Book(String title, String author) {
        this.title = title;
        this.author = author;
    }
}

public class Library {
    
    // A library has an array of books
    Book[] books = new Book[10];
    
}

This is a very simplified example, but it shows that we know a library has several books. So we can represent this in code by using the Book objects in the composition of a Library.

As we can see, 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.

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.