Library class
- Kotlin course
- Intermediate
- Lesson 60/116
- 6 min
Set up collection management with the Library class.
The Library class
The brain of the library is the Library class. It holds two mutable lists for books and members: val books = mutableListOf<Book>() and val members = mutableListOf<Member>().
To add elements you write fun addBook(book: Book) and fun addMember(member: Member). Both work with a single line like books.add(book). This way the library manages its book and member collections; checkout and search will build on these lists in later lessons.
addBook and addMember
The add method appends an element to a list. The Library class wraps it twice: fun addBook(book: Book) { books.add(book) } and fun addMember(member: Member) { members.add(member) }.
From the outside you call lib.addBook(Book(...)) while the list itself stays hidden. This is the encapsulation principle: even though the list variable is val, its contents are managed through the add methods. To print the list use books.forEach { println(it) }.
Practice Define the Library class
Define the Book and Member data classes as in the previous lesson. Then define class Library with val books = mutableListOf<Book>() and val members = mutableListOf<Member>(). Add fun addBook(book: Book) and fun addMember(member: Member). In main add two books and print them with books.forEach { println(it) }.
Starter code
data class Book(val title: String, val author: String, val isbn: String)
data class Member(val name: String, val id: String)
// Define the Library class: books and members lists, addBook and addMember methods
fun main() {
val lib = Library()
// Add two books with addBook and print books with forEach
}Sample solution
data class Book(val title: String, val author: String, val isbn: String)
data class Member(val name: String, val id: String)
class Library {
val books = mutableListOf<Book>()
val members = mutableListOf<Member>()
fun addBook(book: Book) { books.add(book) }
fun addMember(member: Member) { members.add(member) }
}
fun main() {
val lib = Library()
lib.addBook(Book("War and Peace", "Tolstoy", "978-1"))
lib.addBook(Book("Anna Karenina", "Tolstoy", "978-2"))
lib.books.forEach { println(it) }
}Expected output
Book(title=War and Peace, author=Tolstoy, isbn=978-1) Book(title=Anna Karenina, author=Tolstoy, isbn=978-2)
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the book list
class Library {
val books = mutable____<Book>()
}class Library {
val books = mutableListOf<Book>()
}Explanation: mutableListOf creates a mutable book list.
2 Match the Library parts
| Term | Meaning |
|---|---|
addBook |
Adds a book |
addMember |
Adds a member |
books |
Book list |
members |
Member list |
3 What does mutableListOf<Book>() produce?
- Fixed-size array
- Read-only list
- Single-element set
- Mutable list Correct answer
Explanation: mutableListOf returns a mutable list that supports add and remove.