Accessing a Kotlin object nested in companion object from Java

Question

I have a structure like this in Kotlin

companion object Constants {
    /**
     * Collection of fields and values relative to phone books.
     */
    object PhoneBooks {
        /**
         * Field indicating the ID of a phone book.
         * Each phone book must have an unique ID.
         */
        const val PB_ID_KEY = "PB_ID"

        /**
         * Field indicating the status of phone book.
         */
        const val PB_STATUS_KEY = "PB_Status"

        /**
         * One of the possible [PB_STATUS_KEY] values, when the phone book is in indexing state
         * (usually at startup or in update phase).
         */
        const val PB_INDEXING = "Indexing"
        [...]

The problem is that I must have the possibility to access the constants' values in sub-objects from Java, but it seems not possible. How can I solve that without changing the structure?

Works fine here using `import static com.yourcompany.yourproject.YourClass.Constants.PhoneBooks;`, and then `PhoneBooks.PB_ID_KEY`. — JB Nizet, Nov 22 '19 at 09:56
I don't care about rep. And I would be happy to see a more elaborate answer, from someone with more knowledge than me. But at least my comment unblocks you. — JB Nizet, Nov 22 '19 at 10:06
Anyways, thank you so much. I think that also a not-so-much elaborated answer can unblock many other people ;) — Lore, Nov 22 '19 at 10:10
By "scratch file" do you mean https://www.jetbrains.com/help/idea/scratches.html? If it doesn't work there, it may be an IDE bug... — Alexey Romanov, Dec 02 '19 at 17:19

score 4 · Accepted Answer · edited Jun 20 '20 at 09:12

4

JB Nizet commented:

Works fine here using

import static com.yourcompany.yourproject.YourClass.Constants.PhoneBooks;

and then

PhoneBooks.PB_ID_KEY

It works very good for me! So I think it's useful to make it visible as an answer.

edited Jun 20 '20 at 09:12

Community

1
1

answered Nov 25 '19 at 15:25

Lore

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score 2 · Answer 2 · answered Nov 22 '19 at 13:48

In the comments above JB Nizet shows how to solve the issue with a static import

However Looking at the provided code I would use an Enum

// kotlin
enum class PhoneBooks(val param:String) {
    PB_ID_KEY("PB_ID"),
    PB_STATUS_KEY("PB_Status"),
    PB_INDEXING("Indexing")

}

// java
System.out.println(PhoneBooks.PB_ID_KEY.getParam());

A big advantage here is code readability PhoneBooks.PB_ID_KEY flags PB_ID_KEY as a phone book constant in a clean way

like Kotlin sealed classes the kolin compiler add some nice checks for enums (exhaustive) and they are designed to provide clean readable code for pattern matching

see @Rolands answer here Kotlin: Using enums with when

sure, my answer only relates to the code posted in the question, there are many possible solutions based on a given context — Nigel Savage, Nov 22 '19 at 14:32

score 1 · Answer 3 · answered Dec 04 '19 at 14:24

try using interface :)

  companion object Constants {
/**
 * Collection of fields and values relative to phone books.
 */
interface PhoneBooks {
    /**
     * Field indicating the ID of a phone book.
     * Each phone book must have an unique ID.
     */
    const val PB_ID_KEY = "PB_ID"

    /**
     * Field indicating the status of phone book.
     */
    const val PB_STATUS_KEY = "PB_Status"

    /**
     * One of the possible [PB_STATUS_KEY] values, when the phone book is in indexing state
     * (usually at startup or in update phase).
     */
    const val PB_INDEXING = "Indexing"
    [...]

Accessing a Kotlin object nested in companion object from Java

3 Answers3