Java to Kotlin conversion extending Abstract class

Question

I'm converting a Java class that extends an abstract class as per the code below

public class BadRequestAlertException extends AbstractThrowableProblem {

    private static final long serialVersionUID = 1L;

    private final String entityName;

    private final String errorKey;

    public BadRequestAlertException(String defaultMessage, String entityName, String errorKey) {
        this(ErrorConstants.DEFAULT_TYPE, defaultMessage, entityName, errorKey);
    }

    public BadRequestAlertException(URI type, String defaultMessage, String entityName, String errorKey) {
        super(type, defaultMessage, Status.BAD_REQUEST, null, null, null, getAlertParameters(entityName, errorKey));
        this.entityName = entityName;
        this.errorKey = errorKey;
    }

    public String getEntityName() {
        return entityName;
    }

    public String getErrorKey() {
        return errorKey;
    }

    private static Map<String, Object> getAlertParameters(String entityName, String errorKey) {
        Map<String, Object> parameters = new HashMap<>();
        parameters.put("message", "error." + errorKey);
        parameters.put("params", entityName);
        return parameters;
    }
}

Into Kotlin as per the implementation below

class BadRequestAlertException(type: URI = ErrorConstants.DEFAULT_TYPE, defaultMessage: String, val entityName: String, val errorKey: String) : AbstractThrowableProblem(type, defaultMessage, Status.BAD_REQUEST, null, null, null, getAlertParameters(entityName, errorKey)) {

    companion object {
        private const val serialVersionUID = 1L

        private fun getAlertParameters(entityName: String, errorKey: String): Map<String, Any> {
            val parameters = HashMap<String, Any>()
            parameters["message"] = "error.$errorKey"
            parameters["params"] = entityName
            return parameters
        }
    }
}

The Kotlin implementation raises the following error Class 'BadRequestAlertException' is not abstract and does not implement abstract base class member public abstract fun getCause(): Exceptional! defined in org.zalando.problem.AbstractThrowableProblem.

Why doesn't the Java version implement the getClause() method, but Kotlin requires the method to be overridden? Is there a problem with the conversion, or just the way Kotlin extends abstract classes?

Answer 1

FWIW it appears as though the legitimate bug that originally caused this issue is fixed in Kotlin 1.5.0. See KT-45853 ("JVM / IR: "Accidental override" caused by inheriting Throwable.getCause from Java interface") for details... the following Kotlin class compiles & runs as far as I can tell:

import com.fasterxml.jackson.annotation.JsonIgnore
import java.net.URI
import org.zalando.problem.AbstractThrowableProblem
import org.zalando.problem.Exceptional
import org.zalando.problem.Status

class BadRequestAlertExceptionKt(
    type: URI = URI.create("about:blank"),
    defaultMessage: String,
    val entityName: String,
    val errorKey: String
) : AbstractThrowableProblem(
    type,
    defaultMessage,
    Status.BAD_REQUEST,
    null,
    null,
    null,
    getAlertParameters(entityName, errorKey)
) {
    companion object {
        private const val serialVersionUID = 1L

        private fun getAlertParameters(entityName: String, errorKey: String): Map<String, Any> {
            val parameters = HashMap<String, Any>()
            parameters["message"] = "error.$errorKey"
            parameters["params"] = entityName
            return parameters
        }
    }

    @JsonIgnore
    override fun getCause(): Exceptional? = super.cause
}