Something is wrong with my code that works with Fi...

# getting-started

[nilTheDev]

Something is wrong with my code that works with File IO.

fun main() {
    val strs = listOf(
        "line1",
        "line2"
    )
    val writer = File("src/main/resources/logging/temp.txt")
        .writer()

    strs.forEach { writer.appendLine(it) }
    writer.close()
}

The temp.txt file is empty when I run this program.

After that the contents of the temp.txt file looks something like this,

line1
line2

This is a perfectly reasonable output. However, things go wrong when I run a slightly modified version of the code for the second time.

fun main() {
    val strs = listOf(
        "line3",
        "line4"
    )
    val writer = File("src/main/resources/logging/temp.txt")
        .writer()

    strs.forEach { writer.appendLine(it) }
    writer.close()
}

This is the new version. But this time the temp.txt file wasn't empty. It had the output of the first program saved into it.

However, after running the new version the temp.txt file looks like this,

line3
line4

The contents written by the first program are completely wiped out.

Isn't the temp.txt file supposed to look like this?

line1
line2
line3
line4

If not then what's even the point of using append over write? Isn't append expected to write new content after the previously added content instead of wiping out and starting afresh?

Or, am I missing something?

[Klitos Kyriacou]

What is happening is that you're calling the writer() extension to java.io.File. This extension creates a new java.io.FileWriter which it constructs without the append argument, so that this FileWriter truncates the file. The append method you're calling comes from the FileWriter's Appendable interface - nothing to do with appending to the file.

[nilTheDev]

@Klitos Kyriacou I see. But what is a proper way of appending to a text file?

[Paul Griffith]

One of many options is to create your writer with StandardOpenOption.APPEND as an `OpenOption`:

val writer = Path("C:/Users/pgriffith/Downloads/test.txt")
    .writer(options = arrayOf(StandardOpenOption.APPEND))

[nilTheDev]

@Paul Griffith That seems a reasonable way to do that. But it feels kind of java-ish. Is there any idiomatic way to do it in Kotlin?

[Tomasz Krakowiak]

Well... Isn't Kotlin java-ish itself? ^^

[ephemient]

these IO extensions are Java-only to begin with…

[Paul Griffith]

Kotlin doesn't have multiplatform file IO (yet), so you're going to be calling a Java API to read a file no matter what. The writer extension is already significantly nicer than the Files static method you'd use in plain Java

[ephemient]

you can define your own with an append: Boolean parameter

[ephemient]

also, you should probably use .use { } (equivalent to try-with-resources in Java) to auto-close at the end of the block, and consider using .buffered

[Paul Griffith]

Heh, actually, if you use Path you've actually already got an even nicer API - path.appendLines()

public inline fun Path.appendLines(lines: Iterable<CharSequence>, charset: Charset = Charsets.UTF_8): Path {

[Paul Griffith]

➕ for more explicit use of a buffered writer if this is going to end up a large file/you're writing many lines, though