Java code that cannot be used from Kotlin (for in-house study sessions)

• Overview ――We will announce the problems that you are addicted to in your work. ――Actually, I'm not sure, so please comment if you can understand it. --Premise --Use Java Library from Kotlin on Android Kotlin / JVM

The code I was really into

Desk.java

package library_package;

abstract class BaseDesk {
    public enum Color { BLUE, ORANGE }
}

public class Desk extends BaseDesk {
    private Color color;

    public Desk(Color color) {
        this.color = color;
    }
}

Of course, using such a Java implementation from Java works well.

java

new Desk(Desk.Color.BLUE);

When used from Kotlin, I get a Compile Error.

kotlin

Desk(Desk.Color.BLUE) // Unresolved reference: Color

kotlin

import library_package.BaseDesk // Cannot access 'BaseDesk': it is package-private in 'library_package'

Desk(BaseDesk.Color.BLUE) // Cannot access 'Color': it is public in 'BaseDesk'

How to avoid

To solve this problem (maybe?), A static method called getColorEnum () was prepared.

Desk.java

package library_package;

abstract class BaseDesk {
    public enum Color { BLUE, ORANGE }

    public static Color getColorEnum(String name) {
        return Color.valueOf(name);
    }
}

kotlin

@Suppress("INACCESSIBLE_TYPE") //suppress warning is necessary
Desk(Desk.getColorEnum("BLUE")) //I was able to put a Color instance in the Desk constructor

Why do I get an error / not?

Looking at what kind of case causes an error ...

Foo.java

class BaseFoo {
    public static int i = 0;

    public static int inc() {
        return ++i;
    }

    public static class FooFoo {
        public static int j = 0;
    }
}

public class Foo extends BaseFoo {}

java

int foo_i = Foo.i;
int foo_inc = Foo.inc();
int foo_foo_j = Foo.FooFoo.j;

kotlin

val foo_i = Foo.i
val foo_inc = Foo.inc()
val foo_foo_j: Int = Foo.FooFoo.j // Unresolved reference: FooFoo

Kotlin doesn't seem to be able to access the static inner class of the super class. I took a quick look at the underlying language specifications, but didn't understand.

Why could it be used as a method return value?

Java seems to be able to define an originally invisible class as a return value.

public class Bar {
    public static InternalBar getBar() {
        return new InternalBar();
    }

    private static class InternalBar extends Bar {}
}

Bar bar = Bar.getBar(); // OK

You can't do this with Kotlin.

open class Baz {
    companion object {
        fun getInternalBaz(): InternalBaz { // 'public' function exposes its 'private' return type InternalBaz
            return InternalBaz()
        }

        fun getBaz(): Baz { // OK
            return InternalBaz()
        }
    }
    private class InternalBaz : Baz()
}

What is INACCESSIBLE_TYPE warning?

There is a problem that the behavior of Kotlin Compiler, which causes an error when it cannot be referenced, does not match the Java specifications as described above and cannot be compiled. Currently, it is set to Warning instead of Compile Error, but it will be properly accessed in the future. It seems that it will be changed again so that it will be a compile error only when it is not possible.

See https://youtrack.jetbrains.com/issue/KT-11398.

What if getColorEnum () is not provided on the Library side?

You can create your own Utility Method in Java.

DeskColorResolver.java

import library_package.Desk;

public class DeskColorResolver {
    public static Desk.Color getColor(String name) {
        return Desk.Color.valueOf(name);
    }
}

kotlin

val desk = Desk(DeskColorResolver.getColorEnum("BLUE"))

Can be written. Is this really the correct answer ...?

thanks!