Best way to translate this java code into kotlin - kotlin

Question

URL url = new URL(urlSpec);
HttpURLConnection connection = (HttpURLConnection)url.openConnection();
InputStream in = connection.getInputStream();
int bytesRead = 0;
byte[] buffer = new byte[1024];
while ((bytesRead = in.read(buffer)) > 0) {
out.write(buffer, 0, bytesRead);
}
out.close();
I am especially curious about this part
while(bytesRead = in.read(buffer))
We know that asigements are treated as statements in kotlin while in java they are treated as expressions, so this construct is only possible in java.
What is best way to translate this java code into kotlin?

Answer 1

Instead of translating the code literally, make use of Kotlin's stdlib which offers a number of useful extension functions. Here's one version
val text = URL(urlSpec).openConnection().inputStream.bufferedReader().use { it.readText() }
To answer the original question: You're right, assignments are not treated as expressions. Therefore you will need to separate the assignment and the comparison. Take a look at the implementation in the stdlib for an example:
public fun Reader.copyTo(out: Writer, bufferSize: Int = DEFAULT_BUFFER_SIZE): Long {
var charsCopied: Long = 0
val buffer = CharArray(bufferSize)
var chars = read(buffer)
while (chars >= 0) {
out.write(buffer, 0, chars)
charsCopied += chars
chars = read(buffer)
}
return charsCopied
}
Source: https://github.com/JetBrains/kotlin/blob/a66fc9043437d2e75f04feadcfc63c61b04bd196/libraries/stdlib/src/kotlin/io/ReadWrite.kt#L114

Answer 2

You could use apply block to execute the assignment:
val input= connection.getInputStream();
var bytesRead = 0;
val buffer = ByteArray(1024)
while (input.read(buffer).apply { bytesRead = this } > 0) {
out.write(buffer, 0, bytesRead);
}

Answer 3

You could use something like this
This operation may be little heavy as a function is created each iteration.
val url = URL("urlSpec")
val connection = url.openConnection() as HttpURLConnection
val `in` = connection.inputStream
val buffer = ByteArray(1024)
var bytesRead: Int? = null
while ({ bytesRead = `in`.read(buffer); bytesRead }() != null) {
out.write(buffer, 0, bytesRead!!)
}
out.close()