File Handling in Kotlin

# CHAPTER 22

File Handling in Kotlin #

1. Chapter Introduction #

2. Learning Objectives #

• Import java.io.File.

• Write text to a new file (writeText).

• Append text to an existing file (appendText).

• Read the entire contents of a file (readText).

• Read a file line-by-line for large datasets (forEachLine).

3. Writing to a File #

import java.io.File

fun main() {
    val fileName = "settings.txt"
    val file = File(fileName)
    
    // Writes the text and creates the file if it doesn't exist
    file.writeText("Theme=Dark\nVolume=80")
    
    println("File written successfully!")
}

4. Appending to a File #

import java.io.File

fun main() {
    val file = File("logs.txt")
    
    // Adds text to the end of the file
    file.appendText("Error: Connection timeout at 10:00 AM\n")
    file.appendText("Error: Database locked at 10:05 AM\n")
    
    println("Logs appended!")
}

5. Reading a File #

import java.io.File

fun main() {
    val file = File("settings.txt")
    
    // Check if the file exists before trying to read it!
    if (file.exists()) {
        val contents = file.readText()
        println("File Contents:\n$contents")
    } else {
        println("File not found!")
    }
}

6. Reading Large Files (Line by Line) #

import java.io.File

fun main() {
    val file = File("logs.txt")
    
    if (file.exists()) {
        // Reads one line into memory, processes it, and throws it away
        file.forEachLine { line ->
            println("READING: $line")
        }
    }
}

7. Mini Project: Notes Manager #

import java.io.File

fun main() {
    val file = File("my_notes.txt")
    
    print("Enter a note to save: ")
    val note = readln()
    
    try {
        // Append the note and add a newline
        file.appendText("$note\n")
        println("Note saved successfully!")
        
        println("\n--- ALL NOTES ---")
        println(file.readText())
    } catch (e: Exception) {
        println("Critical Error saving file: ${e.message}")
    }
}

8. Common Mistakes #

• Relative Path Confusion: If you just write File("data.txt"), where does it save? It saves in the "Current Working Directory", which is usually the root folder of your IntelliJ Project. If you run the code on a different machine, the path might break.

• Forgetting try-catch: File I/O (Input/Output) is highly volatile. The hard drive might be full, or the user might lack permission. Always wrap critical file operations in a try-catch block.

9. Best Practices #

• Use forEachLine for CSVs: If you are reading a database dump or a CSV file, always default to forEachLine or bufferedReader().useLines {} to keep your memory footprint extremely small.

10. Exercises #

1. 1. Write a program that asks the user for their Name and Age.

1. 2. Format a string: "User: [Name], Age: [Age]".

1. 3. Use appendText() to save it to users.txt.

11. MCQs with Answers #

12. Interview Questions #

• Q: Compare .readText() and .forEachLine { }. When would you use one over the other?

• Q: Explain why file operations must be handled with care regarding Exceptions.

13. Summary #

14. Next Chapter Recommendation #