how to write a test for this class ```class Task var title S

Question

how to write a test for this class ```class Task var title String bodyText String = var body String = bodyText get return if this body isEmpty NO BODY FOUND else this body fun printTask println Task $title n println Body println body ``` The current TaskTest kt file ```import org junit jupiter api Test import org junit jupiter api Assertions class TaskTest val testSample1 Task = Task Learn Encapsulation val testSample2 Task = Task Learn Encapsulation Encapsulation is good for reducing change Test fun printTask ```

Sam · Answer

It s hard to test this because `println` is an example of a function that operates via side effects i e it does something instead of returning something

mkrussel · Answer

You could replace `System out` with your own object that builds a string

Dron Bhattacharya · Answer

< mkrussel> I cannot understand your suggestion Can you elaborate a little bit

mkrussel · Answer

I ve not done this in Kotlin but Java has <https docs oracle com javase 8 docs api java lang System html setOut java io PrintStream > You can use that to redirect `System out` which the `println` function uses You then create a `PrintStream` that takes an `ByteArrayOutputStream` you can then convert those bytes to a String to validate the printing

mkrussel · Answer

Then revert `System out` back to the original print stream

Sam · Answer

Possibly a simpler version of the same thing might be something like this ```class Task var title String bodyText String = var body String = bodyText get return if this body isEmpty NO BODY FOUND else this body fun printTask printer Task $title n printer Body printer body companion object var printer String gt Unit = println class TaskTest val testSample1 Task = Task Learn Encapsulation val testSample2 Task = Task Learn Encapsulation Encapsulation is good for reducing change Test fun printTask val output = mutableListOf lt String gt Task printer = output add it testSample1 printTask assertTrue output contains Learn Encapsulation ```

Sam · Answer

Easier to replace a ` String gt Unit` function than the whole system output stream

Sam · Answer

Not the sort of thing I would want to do in real code but good enough for learning

mkrussel · Answer

I would probably go with making the printer a constructor argument versus changing global state

Sam · Answer

Yep would definitely do that if I was going to use something like this for real

Adam S · Answer

generally what applications do is use a Logger rather than printing directly to console There are libraries that provide loggers but you don t need a library as < Sam> amp < mkrussel> demoed the most basic logger I can think of is just a type alias ```typealias Logger = String gt Unit``` So anywhere you want to print pass in a Logger ```typealias Logger = String gt Unit class Task private val logger Logger fun printTask logger Task logger Body ```

Adam S · Answer

and it s perfectly fine to print to stdout so you can set that up as the default behaviour ```class Task private val logger Logger = println ``` that uses a function reference to `println ` so for every message it will just print to stdout

Adam S · Answer

and then in tests you override the default logger to capture the log messages by for example collecting them into a list ```fun main collect the log messages into this list val logMessages = mutableListOf lt String gt override the default logger val task = Task logger = logMessages add task printTask now we can access all captured messages println captured messages $logMessages ``` So that ^ will collect all your log messages into a list Now each test can be run independently and the logs won t be muddled up

Adam S · Answer

Here s a runnable example that I updated to use a more object oriented approach which I think is more readable <https pl kotl in JkGeW1VJz|run in Kotlin Playground>

Klitos Kyriacou · Answer

By the way if you are trying to access the backing field you should use `field` Otherwise this will cause an endless recursion and eventually a stack overflow ``` var body String = bodyText get return if this body isEmpty NO BODY FOUND else this body ``` More details <https kotlinlang org docs properties html backing fields>

Ties · Answer

I just offered to explain things like side effects and how it impacts testability in a call stuck out tongue might be more effective than slack messages

Dron Bhattacharya · Answer

I have ended up writing this Is it OK Task kt ```class Task var title String private val bodyText String = var body String = bodyText get = field ifEmpty NO BODY FOUND fun printTask String val text String = Task $title Body $body trimIndent also println it return text ``` TaskTest kt ```class TaskTest val titles = listOf lt String gt Learn Encapsulation Learn interface val bodys = listOf lt String gt NO BODY FOUND Interface is good for reducing change val testSample1 Task = Task titles 0 val testSample2 Task = Task titles 1 bodys 1 Test fun printTask NoBody val expected = Task $ titles 0 Body $ bodys 0 trimIndent assertEquals expected testSample1 printTask Test fun printTask val expected = Task $ titles 1 Body $ bodys 1 trimIndent assertEquals expected testSample2 printTask ```

Dron Bhattacharya · Answer

Is there anything more to test for this class like `instance creation` of the class I am new to this

Dron Bhattacharya · Answer

I am using a String template in the `printTask ` function But when I change the template I have to change it manually in the test class too I have to change the template in 3 places I know it can be better but what to apply here is not clear to me abstract class interface or something else Can I do anything here

Dron Bhattacharya · Answer

I have tried to solve the manually modifying String template issue in this way Now if I have to modify the String that gets printed I can change it in 1 place and it reflects in other places Task kt ```import java util UUID class Task var title String bodyText String = val id = UUID randomUUID hashCode var body String = bodyText get = field ifEmpty NO BODY FOUND init println Task created with ID=$id fun printTask = TaskPrintString title body formattedString also println it data class TaskPrintString private val title String private val body String val formattedString String = Task $title Body $body trimIndent ``` TaskTest kt ```class TaskTest val titles = listOf lt String gt Learn Encapsulation Learn interface val bodys = listOf lt String gt NO BODY FOUND Interface is good for reducing change Test fun printTask NoBody val testSample1 Task = Task titles 0 val expected = TaskPrintString titles 0 bodys 0 formattedString assertEquals expected testSample1 printTask Test fun printTask withBody val testSample2 Task = Task titles 1 bodys 1 val expected = TaskPrintString titles 1 bodys 1 formattedString assertEquals expected testSample2 printTask ```