java - Writing and returning a File in Kotlin using a buffer - Stack Overflow

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I'm working on a function to process some data and output the results as a csv file. I am using a 'use' block to wrap the function and ensure all the resources are closed. the problem I am running into is that I want the function to return the newly created File to the caller. The use block has the ability to return a value, however since it is being called with a bufferedWriter, there is no ability to actually return the File object from inside the block. Can anyone offer suggestions on how to solve this?

I want to do something like:

fun List<Contact>.buildCsv():File
{
    return File("people.csv").bufferedWriter(Charsets.ISO_8859_1).use { out ->

            out.write("a,b,c,d\n")
            this.forEach {
                out.write(it.toString())
            }
        }

}

but that code does not compile.

I'm working on a function to process some data and output the results as a csv file. I am using a 'use' block to wrap the function and ensure all the resources are closed. the problem I am running into is that I want the function to return the newly created File to the caller. The use block has the ability to return a value, however since it is being called with a bufferedWriter, there is no ability to actually return the File object from inside the block. Can anyone offer suggestions on how to solve this?

I want to do something like:

fun List<Contact>.buildCsv():File
{
    return File("people.csv").bufferedWriter(Charsets.ISO_8859_1).use { out ->

            out.write("a,b,c,d\n")
            this.forEach {
                out.write(it.toString())
            }
        }

}

but that code does not compile.

• java
• kotlin
• file
• bufferedwriter
• closeable

Share Improve this question Follow edited Mar 28 at 6:08 David Soroko 9,17133 gold badges4444 silver badges5858 bronze badges asked Mar 26 at 20:33 pbuchheitpbuchheit 1,70122 gold badges2828 silver badges6363 bronze badges 3

• 2 You can't return a File in Java. It's not what you think. The functionality of that class is really to manipulate paths and the like. It really has little to do with the actual content of a file system. If you really want to return a 'file' you would do well to think of it as returning a stream to the actual content (and all that entails) – g00se Commented Mar 26 at 20:44
• I am not very familiar with Kotlin, but I would use a library over just trying to create CSV content the way you are doing it. If things get complicated, your output will pay the price. – SedJ601 Commented Mar 26 at 21:12
• I'd recommend you don't tie yourself to Files and hence the file system. It gets messy when unit testing. (You do write unit tests, right?) Instead of a File, pass an OutputStream or a StreamWriter. In a test you can pass one that collects what gets written in a byte array or string. In real life, you can still pass a stream that writes to a file. But this decouples I/O from the logic in your buildCsv function. – Robert Commented Mar 26 at 22:44

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2 Answers 2

Sorted by: Reset to default 3

You can use some builtin scope functions such as apply, also, etc.

An example:

fun List<Contact>.buildCsv() = File("people.csv").apply {
    bufferedWriter(Charsets.ISO_8859_1).use { out ->
        out.write("a,b,c,d\n")
        forEach { out.write("$it") }
    }
}

How about:

fun List<Contact>.buildCsv(): File {
    val file = File("people.csv")

    file.bufferedWriter(Charsets.ISO_8859_1).use { out ->
        out.write("a,b,c,d\n")
        this.forEach {
            out.write(it.toString())
        }
    }

    return file
}

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