Hello at all I ve got some java objects that I d need to con

Question

Hello at all I ve got some java objects that I d need to convert to kotlin Is there an idiomatic way of specifying constraints on data classes Like the following Java ```public class TotallyMadeUpClass private String username public String getUsername return this username public void setUsername String username if username length gt 10 throw new IllegalArgumentException this username = username ```

Wesley Acheson · Answer

it doesn t need to be strings it doesn t need the illegal argument exception I just want a way to specify the constraint in the language

Ruckus · Answer

```class TotallyMadeUpClass userName String var userName = userName set value require value length lt = 10 field = value ```

Wesley Acheson · Answer

Okay so vars not vals I assume also that this cannot be used as a constructor parameter I m also crossposting to stack overflow

Ruckus · Answer

Edited to show using constructor param For future note if you re asking on SO it s common practice to post the question there and then just post a link here with some context so people can go check it out if they want That way you don t split the discussion

Wesley Acheson · Answer

Thank you

Wesley Acheson · Answer

I ve put it on StackOverflow anway As maybe will help someone else find the answer Feel free to answer or I can copy your text into an answer <https stackoverflow com questions 62283491 kotlin data class enforce value constraints>

Wesley Acheson · Answer

I ll remember to do that in the future

Ruckus · Answer

Not a big deal and it s not a hard requirement or anything just something that has become a common practice of sorts

Philip Puffinburger · Answer

If you do want to use a data class and val s you should be able to do ```data class TotallyMadeUpClass val username String init require username length gt 10 ``` That should validate whether you set it from the constructor or the copy method

Wesley Acheson · Answer

< Philip Puffinburger> Perfect thats what I m looking for It would be even cooler if it had TypeScript like constraints but this is what I needed I was already trying to see if I could force < Ruckus> example to something like this

Wesley Acheson · Answer

Thank you both I didn t know the require keyword either ```fun main println TotallyMadeUpClass OK println TotallyMadeUpClass This username should be rejected as its too long data class TotallyMadeUpClass val username String init require username length lt 10 Testing to see if I can get a constraint message ``` `TotallyMadeUpClass username=OK ` `Exception in thread main java lang IllegalArgumentException Testing to see if I can get a constraint message` `at com blah TotallyMadeUpClass lt init gt ConstraintTest kt 10 ` `at com blah merchantapi ConstraintTestKt main ConstraintTest kt 5 ` `at com blah merchantapi ConstraintTestKt main ConstraintTest kt `

Wesley Acheson · Answer

sorry about formatting

Matteo Mirk · Answer

Here is a little variation to show how it could be done in case of a standard class having a constructor parameter that doesn t correspond to a property ```class Username username String val user = $ username toLowerCase init require username length lt 10 user name must be shorter than 10 characters usage val u = Username Joe u user joe ``` notice the absence of `val var` in the constructor argument The read only property `user` is initialized only after the init check passes