How to convert Java assignment expression to Kotlin

Question

Something in java like

int a = 1, b = 2, c = 1;
if ((a = b) !=c){
    System.out.print(true);
}

now it should be converted to kotlin like

var a:Int? = 1
var b:Int? = 2
var c:Int? = 1
if ( (a = b) != c)
    print(true)

but it's not correct.

Here is the error I get:

in " (a=b)" Error:(99, 9) Kotlin: Assignments are not expressions, and only expressions are allowed in this context

Actually the code above is just an example to clarify the problem. Here is my original code:

fun readFile(path: String): Unit { 
    var input: InputStream = FileInputStream(path) 
    var string: String = "" 
    var tmp: Int = -1 
    var bytes: ByteArray = ByteArray(1024) 

    while((tmp=input.read(bytes))!=-1) { } 
}

in " (a=b)" Error:(99, 9) Kotlin: Assignments are not expressions, and only expressions are allowed in this context — K.K Song
– K.K Song, Commented Apr 27, 2016 at 3:01

hotkey · Accepted Answer · 2016-04-27 15:26:06Z

34

As @AndroidEx correctly stated, assignments are not expressions in Kotlin, unlike Java. The reason is that expressions with side effects are generally discouraged. See this discussion on a similar topic.

One solution is just to split the expression and move the assignment out of condition block:

a = b
if (a != c) { ... }

Another one is to use functions from stdlib like let, which executes the lambda with the receiver as parameter and returns the lambda result. apply and run have similar semantics.

if (b.let { a = it; it != c }) { ... }

if (run { a = b; b != c }) { ... }

Thanks to inlining, this will be as efficient as plain code taken from the lambda.

Your example with InputStream would look like

while (input.read(bytes).let { tmp = it; it != -1 }) { ... }

Also, consider readBytes function for reading a ByteArray from an InputStream.

edited Apr 27, 2016 at 15:26

answered Apr 27, 2016 at 10:50

hotkey

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Comments

Mir-Ismaili · Accepted Answer · 2021-05-24 20:53:20Z

19

Java: `(a = b) != c`

Kotlin: `b.also { a = it } != c`

Unlike the accepted answer, I recommend using Kotlin's also function, instead of let:

while (input.read(bytes).also { tmp = it } != -1) { ...

Because T.also returns T (it) itself and then you can compare it with -1. This is more similar to Java's assignment as an expression.

See "Return this vs. other type" section on this useful blog for details.

edited May 24, 2021 at 20:53

answered Nov 15, 2018 at 22:46

Mir-Ismaili

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1 Comment

ssynhtn Over a year ago

Idea's automatic java to kotlin conversion does this too

Community · Accepted Answer · 2017-05-23 10:30:04Z

12

Assignments are not expressions in Kotlin, thus you'll need to do it outside:

var a: Int? = 1
var b: Int? = 2
var c: Int? = 1

a = b
if (a != c)
    print(true)

For your other example with InputStream you could do:

fun readFile(path: String) {
    val input: InputStream = FileInputStream(path)
    input.reader().forEachLine {
        print(it)
    }
}

edited May 23, 2017 at 10:30

CommunityBot

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answered Apr 27, 2016 at 3:01

AndroidEx

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3 Comments

K.K Song Over a year ago

im try to use i/o stream, so i have to put a=b in if() or while(), how should i do

K.K Song Over a year ago

sorry, here is my original code

fun readFile(path: String): Unit {     var input: InputStream = FileInputStream(path)     var string: String = ""     var tmp: Int = -1     var bytes: ByteArray = ByteArray(1024)     while((tmp=input.read(bytes))!=-1){              } }

hotkey Over a year ago

Note: reader and forEachLine are not quite suitable for all the cases of an arbitrary byte stream, at least not with default (thus undefined) encoding which might not be single byte one and can fail at decoding the bytes. InputStream::readBytes is better for that.

w.brian · Accepted Answer · 2016-07-11 05:15:01Z

12

As pretty much everyone here has pointed out, assignments are not expressions in Kotlin. However, we can coerce the assignment into an expression using a function literal:

val reader = Files.newBufferedReader(path)
var line: String? = null
while ({ line = reader.readLine(); line }() != null) {
    println(line);
}

answered Jul 11, 2016 at 5:15

w.brian

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Comments

user2944157 · Accepted Answer · 2017-07-14 10:52:02Z

0

I think this may help you:

 input.buffered(1024).reader().forEachLine {
            fos.bufferedWriter().write(it)
        }

answered Jul 14, 2017 at 10:52

user2944157

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Ara Hakobyan · Accepted Answer · 2018-03-21 13:39:08Z

0

the simple way in kotlin is

if (kotlin.run{ a=b; a != c}){ ... }

answered Mar 21, 2018 at 13:39

Ara Hakobyan

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