Assignment not allowed in expression?

Question

In Java, we can usually perform assignments as part of the while condition. However, Kotlin complains about this. Therefore, the following code does not compile:

 val br = BufferedReader(InputStreamReader( conn.inputStream)) var output: String println("Output from Server .... \n") while ((output = br.readLine()) != null) { // <--- error here: Assignments are not expressions, and only expressions are allowed in this context println(output) } 

According to this other thread, this seems like a better solution:

 val reader = BufferedReader(reader) var line: String? = null; while ({ line = reader.readLine(); line }() != null) { // <--- The IDE asks me to replace this line for while(true), what the...? System.out.println(line); } 

But this?

Answer 1

No, the best way, IMO, would be

 val reader = BufferedReader(reader) reader.lineSequence().forEach { println(it) } 

And if you want to make sure the reader is properly closed (as it would with the try-with-resources operator in Java), you can use

 BufferedReader(reader).use { r -> r.lineSequence().forEach { println(it) } } 

Answer 2

Here is the shortest stdlib- based solution that also safely closes the reader:

 reader.forEachLine { println(it) } 

Answer 3

And here is a brief general solution in the style of Kotlin, Roman Elizarov:

 while (true) { val line = reader.readLine() ?: break println(line); } 

Here break is of type Nothing , which does not affect type inference for line .

Answer 4

In cases where you just want to replace while ((x = y.someFunction()) != null) you can use the following:

 generateSequence { y.someFunction() } .forEach { x -> /* what you did in your while */ } 

generateSequence will retrieve all values ​​one by one until the first null is reached. You can replace .forEach with reduce or fold (or anything else that seems appropriate ;-)) if you want to keep the last value or sum the values ​​elsewhere.

However, for your specific use case, you can simply use what JB Nizet showed in your answer , or use useLines :

 reader.useLines { it.forEach(::println) } 

.forEachLine is probably the next best shorthand solution for this particular readLine -problem (already answered here ), if you know that you just want to read all the lines and then stop.

Answer 5

(This example is for a while loop) Hope this example helps you ..

Vary from

while ((c = is .read (buffer))> 0) {sb.append (String (buffer, 0, c, Charset.forName (UTF8))))}

in

while ({c = is .read (buffer); c} ()> 0) {sb.append (String (buffer, 0, c, Charset.forName (UTF8))))}