📁 File I/O

📖 What is File I/O?

File I/O (Input/Output) is the process of reading and writing files. It's essential for data storage, configuration management, logging, and more!

💡 Reading Files

Reading Entire File

import java.io.File

fun main() {
    // Read entire file as string
    val content = File("data.txt").readText()
    println(content)

    // Read file as list of lines
    val lines = File("data.txt").readLines()
    for (line in lines) {
        println(line)
    }
}

Safe Reading

import java.io.File

fun readFileSafe(filename: String): String? {
    return try {
        File(filename).readText()
    } catch (e: Exception) {
        println("Failed to read file: ${e.message}")
        null
    }
}

fun main() {
    val content = readFileSafe("data.txt")
    if (content != null) {
        println("Content: $content")
    } else {
        println("Unable to read file")
    }
}

Line-by-Line Processing

import java.io.File

fun main() {
    // Memory efficient (good for large files)
    File("data.txt").forEachLine { line ->
        println(line)
    }

    // useLines - automatically closes
    val lineCount = File("data.txt").useLines { lines ->
        lines.count()
    }
    println("Total lines: $lineCount")
}

✏️ Writing Files

Overwriting

import java.io.File

fun main() {
    // Write text
    File("output.txt").writeText("Hello\nKotlin!")

    // Write line by line
    val lines = listOf("First line", "Second line", "Third line")
    File("output.txt").writeText(lines.joinToString("\n"))
}

Appending

import java.io.File

fun main() {
    val file = File("log.txt")

    // Append text
    file.appendText("Log 1\n")
    file.appendText("Log 2\n")
    file.appendText("Log 3\n")
}

Safe Writing

import java.io.File

fun writeFileSafe(filename: String, content: String): Boolean {
    return try {
        File(filename).writeText(content)
        true
    } catch (e: Exception) {
        println("Failed to write file: ${e.message}")
        false
    }
}

fun main() {
    if (writeFileSafe("data.txt", "Hello Kotlin!")) {
        println("File saved successfully")
    }
}

🎯 Practical Examples

Simple Notebook

import java.io.File

class SimpleNotebook(private val filename: String) {
    private val file = File(filename)

    fun write(content: String) {
        file.writeText(content)
        println("Saved successfully")
    }

    fun append(content: String) {
        file.appendText("$content\n")
        println("Appended successfully")
    }

    fun read(): String {
        return if (file.exists()) {
            file.readText()
        } else {
            "File not found"
        }
    }

    fun clear() {
        file.writeText("")
        println("Content cleared")
    }
}

fun main() {
    val notebook = SimpleNotebook("mynotes.txt")

    notebook.write("First note")
    notebook.append("Second note")
    notebook.append("Third note")

    println("\n=== Notebook Content ===")
    println(notebook.read())
}

Logger

import java.io.File
import java.time.LocalDateTime
import java.time.format.DateTimeFormatter

class Logger(private val filename: String) {
    private val file = File(filename)
    private val formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss")

    fun log(message: String) {
        val timestamp = LocalDateTime.now().format(formatter)
        file.appendText("[$timestamp] $message\n")
    }

    fun info(message: String) = log("INFO: $message")
    fun error(message: String) = log("ERROR: $message")
    fun warn(message: String) = log("WARN: $message")

    fun readLogs(): String {
        return if (file.exists()) {
            file.readText()
        } else {
            "No logs"
        }
    }
}

fun main() {
    val logger = Logger("app.log")

    logger.info("App started")
    logger.warn("Low memory")
    logger.error("Connection failed")

    println(logger.readLogs())
}

CSV File Processing

import java.io.File

data class Person(val name: String, val age: Int, val city: String)

object CsvHandler {
    fun writeCsv(filename: String, people: List<Person>) {
        val csv = StringBuilder()
        csv.append("Name,Age,City\n")

        for (person in people) {
            csv.append("${person.name},${person.age},${person.city}\n")
        }

        File(filename).writeText(csv.toString())
    }

    fun readCsv(filename: String): List<Person> {
        val file = File(filename)
        if (!file.exists()) return emptyList()

        val lines = file.readLines()
        if (lines.isEmpty()) return emptyList()

        return lines.drop(1).map { line ->
            val parts = line.split(",")
            Person(parts[0], parts[1].toInt(), parts[2])
        }
    }
}

fun main() {
    val people = listOf(
        Person("John Doe", 25, "Seoul"),
        Person("Jane Smith", 30, "Busan"),
        Person("Bob Lee", 28, "Daegu")
    )

    // Save CSV
    CsvHandler.writeCsv("people.csv", people)
    println("CSV saved successfully")

    // Read CSV
    val loaded = CsvHandler.readCsv("people.csv")
    println("\n=== CSV Content ===")
    loaded.forEach { println(it) }
}

Configuration File Management

import java.io.File

class Config(private val filename: String) {
    private val settings = mutableMapOf<String, String>()

    init {
        load()
    }

    private fun load() {
        val file = File(filename)
        if (file.exists()) {
            file.forEachLine { line ->
                val parts = line.split("=")
                if (parts.size == 2) {
                    settings[parts[0].trim()] = parts[1].trim()
                }
            }
        }
    }

    fun save() {
        val content = settings.entries.joinToString("\n") {
            "${it.key}=${it.value}"
        }
        File(filename).writeText(content)
    }

    fun set(key: String, value: String) {
        settings[key] = value
    }

    fun get(key: String): String? {
        return settings[key]
    }

    fun getAll(): Map<String, String> {
        return settings.toMap()
    }
}

fun main() {
    val config = Config("app.config")

    // Save settings
    config.set("host", "localhost")
    config.set("port", "8080")
    config.set("timeout", "3000")
    config.save()

    // Read settings
    println("Host: ${config.get("host")}")
    println("Port: ${config.get("port")}")

    println("\n=== All Settings ===")
    config.getAll().forEach { (key, value) ->
        println("$key = $value")
    }
}

📂 File/Directory Management

File Information

import java.io.File

fun main() {
    val file = File("data.txt")

    println("Exists: ${file.exists()}")
    println("Is file: ${file.isFile}")
    println("Is directory: ${file.isDirectory}")
    println("Size: ${file.length()} bytes")
    println("Absolute path: ${file.absolutePath}")
    println("Name: ${file.name}")
    println("Parent folder: ${file.parent}")
}

Directory Operations

import java.io.File

fun main() {
    // Create directory
    val dir = File("mydata")
    dir.mkdir()

    // List files
    dir.listFiles()?.forEach { file ->
        println("${file.name} (${if (file.isDirectory) "folder" else "file"})")
    }

    // Find all files recursively
    dir.walk().forEach { file ->
        println(file.absolutePath)
    }
}

File Copy/Delete

import java.io.File

fun main() {
    val source = File("source.txt")
    val dest = File("dest.txt")

    // Copy
    source.copyTo(dest, overwrite = true)

    // Move
    source.renameTo(File("moved.txt"))

    // Delete
    dest.delete()

    // Delete entire directory
    File("mydata").deleteRecursively()
}

🤔 FAQ

Q1. What if the file doesn't exist?

A: Exception handling is required!

fun readFileSafe(filename: String): String {
    val file = File(filename)

    if (!file.exists()) {
        return "File not found"
    }

    return try {
        file.readText()
    } catch (e: Exception) {
        "Read failed: ${e.message}"
    }
}

Q2. What about large files?

A: Process line by line!

// ❌ Risk of out of memory
val content = File("huge.txt").readText()

// ✅ Line-by-line processing
File("huge.txt").forEachLine { line ->
    processLine(line)
}

Q3. What about path separators?

A: Use File.separator!

// ❌ Different on each OS
val path = "data/files/text.txt"

// ✅ Works on all OS
val file = File("data", "files").resolve("text.txt")
// Or
val path2 = listOf("data", "files", "text.txt").joinToString(File.separator)

🎬 Conclusion

Store and manage data with file I/O!

Key Takeaways:
✅ Read files with readText()
✅ Write files with writeText()
✅ Process line-by-line with forEachLine()
✅ Check file existence with exists()
✅ Exception handling is essential

Next Step: Learn about pattern matching in Regular Expressions!

📖 What is File I/O?​

💡 Reading Files​

Reading Entire File​

Safe Reading​

Line-by-Line Processing​

✏️ Writing Files​

Overwriting​

Appending​

Safe Writing​

🎯 Practical Examples​

Simple Notebook​

Logger​

CSV File Processing​

Configuration File Management​

📂 File/Directory Management​

File Information​

Directory Operations​

File Copy/Delete​

🤔 FAQ​

Q1. What if the file doesn't exist?​

Q2. What about large files?​

Q3. What about path separators?​

🎬 Conclusion​