29

I'm trying to convert the following code to Kotlin AND still have one of the classes (Foo) used by Java. What is the proper way of making this conversion?

Original Java:

public class Foo {
   public static final String C_ID = "ID";
   public static final String C_NAME = "NAME";
   public static final String[] VALUES = {"X", "Y", "Z"};

   public static String[] getAll() {
       return new String[] {C_ID, C_NAME};
   }
}

public class Bar {
    public void doStuff() {
        String var1 = Foo.C_ID;
        String[] array1 = Foo.VALUES;
        String[] array2 = Foo.getAll();
    }
}

Auto conversion fo Foo to Kotlin

object Foo {
    val C_ID = "ID"
    val C_NAME = "NAME"
    val VALUES = arrayOf("X", "Y", "Z")

    val all: Array<String>
        get() = arrayOf(C_ID, C_NAME)
}

Problem:

Bar class can no longer access C_ID or VALUES (error: "private access")

if I put "const" in front of C_ID, it works... but I cannot do the same with VALUES ("const" can ONLY be used on primatives or String)

Is there a different way I should be doing this (so both Java code and Kotlin code can access everything in Foo)?

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1
  • 2
    although const val may have the effect you need, it's purpose is different (compiler time constants) and one should not use it for interop. Commented Feb 1, 2016 at 3:54

4 Answers 4

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39

The current semantics come from Kotlin Beta Candidate:

@JvmField and objects

We have made the strategy for generating pure fields (as opposed to get/set pairs) more predictable: from now on only properties annotated as @JvmField, lateinit or const are exposed as fields to Java clients. Older versions used heuristics and created static fields in objects unconditionally, which is against our initial design goal of having binary-compatibility-friendly APIs by default.

Also, singleton instances are now accessible by the name INSTANCE (instead of INSTANCE$).

According to this and to the reference, there are three ways of working with properties of a Kotlin object from Java:

  • Use Foo.INSTANCE.

    By default, properties of object won't be static fields for Java, but Java can access the properties through Foo object instance -- Foo.INSTANCE.

    So the expression will be Foo.INSTANCE.getC_ID().

  • Mark a property with @JvmStatic annotation:

    object Foo {
    @JvmStatic val C_ID = "ID"
    //...
}

    This will generate static getter for C_ID instead of Foo instance getter which will be accessible as Foo.getC_ID().

  • Use @JvmField annotation on property declaration:

    object Foo {
    @JvmField val C_ID = "ID"
    //...
}

    This will make Kotlin compiler generate a static field for Java instead of property. Then in Java you can access it as a static field: Foo.C_ID.

    But it won't work on properties without backing fields like all in your example.

For primitives, as you stated, one can use const which will have the same effect as @JvmField in terms of visibility in Java.

By the way, when it comes to methods, the situation is the same, and there is @JvmStatic annotation for them.

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Comments

7

In your foo class you can put those properties and the method inside a companion object:

class Foo {

  companion object {
     val C_ID:String = "ID"
     val C_NAME:String = "NAME"
     @JvmField val VALUES = arrayOf("X", "Y", "Z")

     fun getAll():Array<String> {
        return arrayOf(C_ID, C_NAME)
     }
  }
}

Then you can call Foo.getAll(), and Foo.C_ID, Foo.C_NAME and Foo.VALUES.

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Comments

1

You should be able to access the values "the kotlin way":

object Foo {
val C_ID = "ID"
val C_NAME = "NAME"
val VALUES = arrayOf("X", "Y", "Z")

val all: Array<String>
    get() = arrayOf(C_ID, C_NAME)
}

fun main(args: Array<String>) {
    Foo.all.forEach { it->println(it) }
}

With as result:

ID
NAME

Process finished with exit code 0
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3 Comments

The question is rather about accessing parts of Kotlin object in Java, what do you mean by 'Kotlin way' and why should the C_ID and C_NAME be accessible through array?
The op has a val "all" that has an arrayOf as getter. I decided to use that in my example code. I am not a kotlin expert, but this is how I would do it
Sorry, didn't notice the all val. Though the question still seems to be more about Java interop.
1

it's better if you create new kotlin file just for constants.

create Constants.kt file and paste below code.

object Constants {
val C_ID = "ID"
val C_NAME = "NAME"
val VALUES = arrayOf("X", "Y", "Z")

val all: Array<String>
    get() = arrayOf(C_ID, C_NAME)
}

in your main activity you can access the constants by the constant name the android studio will automatically import the constants. here is my mainActivity:

import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.util.Log
import com.example.architecturecompintro.Constants.C_ID
import com.example.architecturecompintro.Constants.C_NAME
import com.example.architecturecompintro.Constants.VALUES
import com.example.architecturecompintro.Constants.all

class MainActivity : AppCompatActivity() {

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
    val TAG = "info"

        Log.i(TAG, C_ID)
        Log.i(TAG,C_NAME)

        for(item in VALUES) {
            Log.i(TAG,item)
        }
        val arrayItem = all

        for(item in arrayItem) {
            Log.i(TAG,item)
        }
    }
}

I was able to get log output successfully

logcat output

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