File I/O
- Kotlin course
- Intermediate
- Lesson 115/116
- 6 min
Save and load data with File.
File saving for Library
This lesson adds file save and load methods to Library. save(path) writes each book's format output line by line to the file; load(path) reads the file and returns its content as a string. So the library data becomes persistent in a file.
fun save(path: String) {
val content = books.joinToString("\n") { it.format() }
File(path).writeText(content)
}
fun load(path: String): String = File(path).readText()joinToString joins each book's format output with \n. writeText writes the result to the file; readText reads all content back. The File class comes from java.io.File; so import java.io.File is needed. For large libraries, line-by-line reading may be preferred instead.
The save and load pattern
save and load work together: save writes data to the file, load reads it back. This is useful to make data persist between program runs. The file path is given as a string; a relative path is resolved against the working directory.
val lib = Library()
lib.add(Book("Crime and Punishment", "Dostoyevsky", 1866, Fiction))
lib.save("library.txt")
println(lib.load("library.txt"))Here a book is added, written to the file with save, then read back with load. The output is the format output that was written to the file. This pattern is common in small data-driven applications. For more complex data, JSON or CSV formats may be preferred; a topic for later.
Practice Add save and load methods
The Book, Category and Library definitions are ready (Library has add and listAll). Add two methods to the Library class: fun save(path: String) whose body joins content with books.joinToString("\n") { it.format() } and writes with File(path).writeText(content). fun load(path: String): String whose body returns File(path).readText(). In main create val lib = Library(); add two books; call lib.save("library.txt"); print println(lib.load("library.txt")). Add the import java.io.File import at the top of the file.
Starter code
import java.io.File
data class Book(val title: String, val author: String, val year: Int, val category: Category) {
fun format(): String = "$title - $author ($year) [${category.label()}]"
}
sealed class Category {
abstract fun label(): String
}
object Fiction : Category() {
override fun label() = "Fiction"
}
object NonFiction : Category() {
override fun label() = "Non-fiction"
}
class Library {
private val books = mutableListOf<Book>()
fun add(b: Book) { books.add(b) }
fun listAll(): List<Book> = books.toList()
}
// Add save(path: String) to Library: write each book's format output line by line
// Add load(path: String): String: read the file and return its content
// main: add two books, call save("library.txt"), read with load and println
fun main() {
// write here
}Sample solution
import java.io.File
data class Book(val title: String, val author: String, val year: Int, val category: Category) {
fun format(): String = "$title - $author ($year) [${category.label()}]"
}
sealed class Category {
abstract fun label(): String
}
object Fiction : Category() {
override fun label() = "Fiction"
}
object NonFiction : Category() {
override fun label() = "Non-fiction"
}
class Library {
private val books = mutableListOf<Book>()
fun add(b: Book) { books.add(b) }
fun listAll(): List<Book> = books.toList()
fun save(path: String) {
val content = books.joinToString("\n") { it.format() }
File(path).writeText(content)
}
fun load(path: String): String {
return File(path).readText()
}
}
fun main() {
val lib = Library()
lib.add(Book("Crime and Punishment", "Dostoyevsky", 1866, Fiction))
lib.add(Book("Sapiens", "Harari", 2011, NonFiction))
lib.save("library.txt")
println(lib.load("library.txt"))
}Expected output
Crime and Punishment - Dostoyevsky (1866) [Fiction] Sapiens - Harari (2011) [Non-fiction]
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does the save method write to the file?
- Only the first book
- The book count
- An empty string
- Each book's format output, line by line Correct answer
Explanation: save joins each book's format output with joinToString and writes it.
2 Write to the file
File(path).____(content)File(path).writeText(content)Explanation: writeText writes the given string to the file.
3 Match the save/load parts
| Term | Meaning |
|---|---|
fun save(path) |
Write method |
fun load(path) |
Read method |
joinToString |
Join |
writeText |
Writes to file |