writeText

Learn to write files with File.writeText.

writeText: Write to a file

writeText(text) writes the given string to the file. If the file does not exist it is created; if it exists, its content is completely overwritten. So the old content is lost. To append to the end use appendText (next lesson).

val f = File("member.txt")
f.writeText("Alex")
f.writeText("Sam")   // file now contains only "Sam"

The second writeText overwrites the first; the file ends up with only the last content. If you do not want this behavior use appendText. writeText uses UTF-8 encoding.

Creating a new file

writeText automatically creates a new file if it does not exist. If the parent directories do not exist it throws an exception; so it is safe to create them first with f.parentFile?.mkdirs(). If the file already exists only the content changes; the file metadata (permissions etc.) is preserved.

val f = File("data/output.txt")
f.parentFile?.mkdirs()
f.writeText("ready")

Here if the data directory does not exist it is created first, then the file is written. ?.mkdirs() is a safe call; if parentFile is null it does nothing. This pattern is common in programs that produce file output.

Exercise Write a two-line file and read it

Create val f = File("not.txt"). Write two lines with f.writeText("line 1\nline 2") (use \n inside the string). Read it back with println(f.readText()). Add the import java.io.File import.

Show example solution

fun main() {
    val f = File("not.txt")
    f.writeText("line 1\nline 2")
    println(f.readText())
}

Test yourself

What happens if writeText is called on an existing file?
- Appends to the end
- Throws an exception
- Overwrites the content
- Ignores the call
Show answer

Overwrites the content — writeText erases the old content and writes the new one.
Match the writeText concepts

writeText(text)

Overwrites
overwrite

Erases old content
UTF-8

Encoding
appendText

Appends (not overwrite)

Write to the file

f.____("new content")

Show answer

f.writeText("new content")

Next lesson: readLines

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