Java-to-Kotlin translation bug

Question

Using the latest Kotlin 1.3.10 plugin from the latest IntelliJ IDEA 2018.3, if I translate the following Java code to Kotlin:

    BufferedReader br = Files.newBufferedReader(Paths.get("filename"));
    String line;
    while ((line = br.readLine()) != null)
        System.out.println(line);

I get this:

    val br = Files.newBufferedReader(Paths.get("filename"))
    var line: String
    while ((line = br.readLine()) != null)
        println(line)

The while ((line = br.nextLine()) != null) is idiomatic Java, but its automatic translation results in illegal syntax because assignments in Kotlin are not expressions.

Is this a known bug in the Java-to-Kotlin translator?
What's the Kotlin idiomatic syntax equivalent to the above Java code?

I suppose that declaration at an upper level is the only way — nyarian
– nyarian, Commented Nov 22, 2018 at 11:17
Partly a duplicate of stackoverflow.com/questions/36879236/… — Klitos Kyriacou
– Klitos Kyriacou, Commented Nov 22, 2018 at 11:24
It is a known bug, please vote for youtrack.jetbrains.com/issue/KT-6294 — Alexey Belkov
– Alexey Belkov, Commented Nov 22, 2018 at 12:11

DodgyCodeException · Accepted Answer · 2018-11-22 13:19:39Z

2

I think the comments have answered both of my questions, so let me compile them into a single answer.

Yes, it's a known bug, reported at youtrack.jetbrains.com/issue/KT-6294. You can vote for it (if you have a Jetbrains account) to show the developers that you would like it fixed. Thank you Alexey Belkov for bringing this to my attention.

A common way to directly translate it would be:

val br = Files.newBufferedReader(Paths.get("filename"))
var line: String
while (br.readLine().also { line = it} != null)
    println(line)

However, a better way in this particular case is:

val br = Files.newBufferedReader(Paths.get("filename"))
br.forEachLine {
    println(it)
}

Or just

File("filename").forEachLine {
    println(it)
}

(which uses a convenience method, equivalent to Java's convenience method Files.lines(Path)).

edited Nov 22, 2018 at 13:19

answered Nov 22, 2018 at 13:12

DodgyCodeException

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