Kotlin cannot access class of Java library, which Java can access

Question

I am relatively new to Kotlin and recently started to convert some Java projects to Kotlin for practice. I know the Java code is working but I am having trouble getting my Kotlin port running. It seems like Kotlin cannot access Java classes the same way Java can.

I have a class in a Java library which looks like this one:

package foo.bar.utils;

import java.util.List;
import java.util.ArrayList;

public class Foo {

    private List bars;

    public Foo() {
        bars = new ArrayList<>();
    }

    static public class Bar {

        private Qux quxs;

        public Qux getQuxs() {
            return quxs;
        }

        public Bar setQuxs(final Qux quxs) {
            this.quxs = quxs;
            return this;
        }

        static class Qux {

            @Override
            public String toString() {
                return "Qux []";
            }
        }

        static public class QuxClass extends Qux {
            private String id;

            public String getId() {
                return id;
            }

            public QuxClass setId(final String id) {
                this.id = id;
                return this;
            }
        }
    }
}

The working Java code looks like this:

package com.example;

import foo.bar.utils.Foo;

public class JavaMain {

    public static void main(String... args) {
        Foo.Bar bar = new Foo.Bar();
        Foo.Bar.QuxClass qux = new Foo.Bar.QuxClass();
        bar.setQuxs(qux);

        System.out.println(bar);
    }
}

The non-working Kotlin code looks like this:

package com.example

import foo.bar.utils.Foo

fun main() {
    val bar = Foo.Bar()
    val qux = Foo.Bar.QuxClass()
    bar.quxs = qux

    println(bar)
}

I omitted code that (I believe) is unnecessary to reproduce this problem.

The Kotlin code does in fact compile but at runtime, it throws Exception in thread "main" java.lang.IllegalAccessError: tried to access class foo.bar.utils.Foo$Bar$Qux from class com.example.MainKt. IntelliJ shows, when hovering over bar.quxs, a hint that Type Foo.Bar.Qux! is inaccessible in this context due to: public/*package*/ open class Qux defined in foo.bar.utils.Foo.Bar but I have troubles understanding that hint.

A possible fix is to change the accessibility of the inner class Qux to public.

I tried OpenJDK 12 and Amazon Corretto 8 with the same result.

I also tried to inspect the decompiled code of the Kotlin and the Java code but could not spot any noteworthy differences.

Calling setQuxs() did not help either.

Is there a way to alter my Kotlin code to get the port running? I really want to understand why Kotlin acts like this.

user_14013929 · Answer

The problem is easy to see when you check out the JVM bytecode for your kotlin snippet (With a regular bytecode viewer, not the one included with Intellij). Your line bar.quxs = qux becomes:

aload0
aload1
checkcast foo/bar/utils/Foo$Bar$Qux
invokevirtual foo/bar/utils/Foo$Bar.setQuxs(Lfoo/bar/utils/Foo$Bar$Qux;)Lfoo/bar/utils/Foo$Bar;

Or represented as java code:

bar.setQuxs((foo.bar.utils.Foo.Bar.Qux)qux);

The bytecode for your JavaMain class does not include this unnecessary checkcast:

aload1
aload2
invokevirtual foo/bar/utils/Foo$Bar.setQuxs(Lfoo/bar/utils/Foo$Bar$Qux;)Lfoo/bar/utils/Foo$Bar;

Why the kotlin compiler adds this checkcast I can not say. I would lean towards classifying this as a bug or at least unexpected behaviour in the kotlin compiler with no easy fix on your end (Besides modifying the code of the library).

user_13896305 · Answer

New keyword is not for creating object in Kotlin you add import like this

import foo.bar.utils.Foo.Bar
import foo.bar.utils.Foo.Bar.QuxClass

then create object like

    val bar = Bar()
    val qux = QuxClass()

Kotlin cannot access class of Java library, which Java can access

2 Answers