It works in Kotlin

this_fails_with_java_lang_NoClassDefFoundError.kt

package com.cprieto.samples

class ShortToStringConverterListener: ArrayInitBaseListener() {
    private val builder = StringBuilder()

    override fun enterInit(ctx: ArrayInitParser.InitContext?) {
        builder.append("{")
    }

    override fun toString(): String {
        return builder.toString()
    }
}

this_works.kt

package com.cprieto.samples

class ShortToStringConverterListener: ArrayInitBaseListener() {
    private val builder = StringBuilder()

    override fun enterInit(ctx: ArrayInitParser.InitContext?) {
        builder.append("{")
    }

    override fun toString() = builder.toString()
}