How to extend a class that has multiple constructors in Kotlin?

Question

I am in the processing of learning Kotlin and ran into a problem I couldn't figure out. I would like to extend the Java class RuntimeException in Kotlin and be able to use any one of three of its constructors, in different circumstances (based on what info I have at the time I want to throw an exception). In java my class would look like this:

public class PhotoLibException extends RuntimeException {

    public PhotoLibException(String message, RuntimeException ex) {
        super(message, ex);
    }

    public PhotoLibException(String message) {
        super(message);
    }

    public PhotoLibException(RuntimeException ex) {
        super(ex);
    }
}

When I try to do this in Kotlin, I used this answer as a guide: Kotlin secondary constructor however, I had a problem trying to figure out how to invoke the appropriate super constructor correctly. For example, using functions seemed to be a good approach, like this:

fun PhotoLibException(message: String): PhotoLibException {
    val ex = null
    return PhotoLibException(message, ex)
}

fun PhotoLibException(ex: Exception): PhotoLibException {
    val message = ""
    return PhotoLibException(message, ex)
}

class PhotoLibException(message: String, ex: Exception?): RuntimeException(message, ex) {
}

However, in this Kotlin example above, I am always invoking the super constructor with two args, and not invoking the constructor most appropriate to the situation. So what I have above works, but doesn't do exactly what it would do in Java where a different constructor is invoked in each situation. I also tried instantiating a new RuntimeException inside each fun above and casting it to PhotoLibException, but I wasn't allowed to do that.

Can anyone suggest how I would do this correctly in Kotlin?

Andrey Breslav · Accepted Answer · 2015-03-20 12:27:57Z

186

Update: Since M11 (0.11.*), you can use secondary constructors to solve this problem:

class PhotoLibException : RuntimeException {
    constructor(message: String, ex: Exception?): super(message, ex) {}
    constructor(message: String): super(message) {}
    constructor(ex: Exception): super(ex) {}
}

Currently, there's no way to call different super-constructors in different context from the same class. It will be supported in the upcoming months, though.

edited Mar 20, 2015 at 12:27

answered Dec 30, 2014 at 12:39

Andrey Breslav

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4 Comments

Rakesh N Over a year ago

Hi, does this solution hold good even today (as of kotlin 1.2.41)? I know it works but are there even simpler alternatives?

urSus Over a year ago

@Andrey Breslav How would I add a property to this? I.e. Say some boolean which is only PhotoLibException field passed in via constructor

Niroshan Over a year ago

Make sure your not initialising the any of the parent constructor on the same line as declaring the child class. Eg "class Child : Parent (- don't do this initialisation here-) "

user8681 Over a year ago

Note that this only works if the class doesn't have a primary constructor - all constructors have to be "secondary". That's probably what @Niroshan meant.

Brill Pappin · Accepted Answer · 2020-06-18 13:24:29Z

5

Use the @JvmOverloads annotation.

class PhotoLibException: RuntimeException {
   @JvmOverloads constructor(message: String, ex: Exception?)
}

answered Jun 18, 2020 at 13:24

Brill Pappin

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