Sample

MainActivity

class MainActivity : AppCompatActivity() {

    companion object {
        const val TAG = "MainActivity"

        private fun loadNativeDependencies() {
            System.loadLibrary("swiftkotlin")
        }
    }

    private external fun viewLoaded()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        loadNativeDependencies()


        var myExampleClass = MyExample()

        var sumDoubles = myExampleClass.sumDoubles(12.5, 12.5)

        myExampleClass.name = "Marco"

        Log.v(TAG, ".::Java::.")
        Log.v(TAG,"Sum is "+sumDoubles.toString())
        Log.v(TAG, MyExample.concat("Hello", "World"))
        Log.v(TAG, MyExample.MY_INT.toString())
        Log.v(TAG, "name "+myExampleClass.name)

        viewLoaded()
    }
}

Kotlin Class

class MyExample {

    companion object {
        @JvmStatic val MY_BYTE: Byte = 127
        @JvmStatic val MY_SHORT: Short = 2700
        @JvmStatic val MY_INT: Int = 1000
        @JvmStatic val MY_LONG: Long = 12700000
        @JvmStatic val MY_FLOAT: Float = 1000.4534F
        @JvmStatic val MY_DOUBLE: Double = 12034343.3434454
        @JvmStatic val MY_BOOLEAN: Boolean = true
        @JvmStatic val MY_CHAR: Char = 'S'

        @JvmStatic fun returnInt() : Int = 20
        @JvmStatic fun sumInts(a: Int, b:Int) : Int = a + b
        @JvmStatic fun concat(str1: String, str2: String) : String = "$str1 $str2"
    }

    var name: String? = null

    init {
        println("Init Block")
    }

    constructor()

    constructor(name: String?){
        this.name = name
    }

    fun sumDoubles(d1: Double, d2: Double) : Double = d1 + d2

    fun sayHello(){
        name = "Hi $name"
    }
}

Swift Class

	import Foundation
import java_swift
import java_lang
import java_util

open class MyExample: JavaObject {
    
    public convenience init?( casting object: JavaObject, _ file: StaticString = #file, _ line: Int = #line ) {
        self.init( javaObject: nil )
        object.withJavaObject {
            self.javaObject = $0
        }
    }
    
    //Global reference of the Java MyExample object
    private static var MyExampleJNIClass: jclass?
    
    //**********************************/
    //         CONSTRUCTORS             /
    //**********************************/
    
    /** Constructor without params
     */
    private static var constructor1: jmethodID?
    public convenience init() {
        
        var __locals = [jobject]()
        
        //creating arrays of params for the constructor
        var params = [jvalue]( repeating: jvalue(), count: 1 )

        //creating newMyExampleObject local ref
        let newMyExampleObject = JNIMethod
            .NewObject( className: "com/jmarkstar/sample/MyExample",
                        classCache: &MyExample.MyExampleJNIClass,
                        methodSig: "()V",
                        methodCache: &MyExample.constructor1, args: &params, locals: &__locals )
        
        //creating newMyExampleObject global ref.
        self.init( javaObject: newMyExampleObject )
        
        //deleting newMyExampleObject
        JNI.DeleteLocalRef( newMyExampleObject )
    }
    
    /** Constructor with name param
     */
    private static var constructor2: jmethodID?
    public convenience init( name: String? ) {
        
        var __locals = [jobject]()
        
        //creating arrays of params for the constructor
        var params = [jvalue]( repeating: jvalue(), count: 1 )
        params[0] = JNIType.toJava( value: name, locals: &__locals )
        
        //creating newMyExampleObject local ref
        let newMyExampleObject = JNIMethod
            .NewObject( className: "com/jmarkstar/sample/MyExample",
            classCache: &MyExample.MyExampleJNIClass,
            methodSig: "(Ljava/lang/String;)V",
            methodCache: &MyExample.constructor2, args: &params, locals: &__locals )
        
        //creating newMyExampleObject global ref.
        self.init( javaObject: newMyExampleObject )
        
        //deleting newMyExampleObject
        JNI.DeleteLocalRef( newMyExampleObject )
    }
    
    //**********************************/
    //        STATIC FIELDS             /
    //**********************************/
    
    private static var MY_BYTE_FieldID: jfieldID?
    open static var MY_BYTE: Int8 {
        get{
            let __value = JNIField
                .GetStaticByteField(
                    fieldName: "MY_BYTE",
                    fieldType: "B",
                    fieldCache: &MY_BYTE_FieldID,
                    className: "com/jmarkstar/sample/MyExample", classCache: &MyExample.MyExampleJNIClass )
            return __value
        }
    }
    
    private static var MY_SHORT_FieldID: jfieldID?
    open static var MY_SHORT: Int16 {
        get{
            let __value = JNIField
                .GetStaticShortField(
                    fieldName: "MY_SHORT",
                    fieldType: "S",
                    fieldCache: &MY_SHORT_FieldID,
                    className: "com/jmarkstar/sample/MyExample", classCache: &MyExample.MyExampleJNIClass )
            return __value
        }
    }
    
    private static var MY_INT_FieldID: jfieldID?
    open static var MY_INT: Int {
        get{
            let __value = JNIField
                .GetStaticIntField(
                    fieldName: "MY_INT",
                    fieldType: "I",
                    fieldCache: &MY_INT_FieldID,
                    className: "com/jmarkstar/sample/MyExample", classCache: &MyExample.MyExampleJNIClass )
            return Int(__value)
        }
    }
    
    private static var MY_LONG_FieldID: jfieldID?
    open static var MY_LONG: Int64 {
        get{
            let __value = JNIField
                .GetStaticLongField(
                    fieldName: "MY_LONG",
                    fieldType: "J",
                    fieldCache: &MY_LONG_FieldID,
                    className: "com/jmarkstar/sample/MyExample", classCache: &MyExample.MyExampleJNIClass )
            return __value
        }
    }
    
    
    //**********************************/
    //        INSTANCE FIELDS           /
    //**********************************/
    
    private static var name_FieldID: jfieldID?
    open var name: String? {
        get{
            let __value = JNIField.GetObjectField(
                    fieldName: "name",
                    fieldType: "Ljava/lang/String;",
                    fieldCache: &MyExample.name_FieldID,
                    object: javaObject )
            defer { JNI.DeleteLocalRef( __value ) }
            return __value != nil ? String( javaObject: __value ) : nil
        }
        set(newName){
            var __locals = [jobject]()
            let __value = JNIType.toJava( value: newName, mapClass: "java/lang/String", locals: &__locals )
            
            JNIField.SetObjectField(
                fieldName: "name",
                fieldType: "Ljava/lang/String;",
                fieldCache: &MyExample.name_FieldID,
                object: javaObject,
                value: __value.l,
                locals: &__locals )
        }
    }
    
    //**********************************/
    //        STATIC METHODS            /
    //**********************************/
    
    private static var returnInt_Method_Ref: jmethodID?
    open class func returnInt() -> Int {
        var __locals = [jobject]()
        var params = [jvalue]( repeating: jvalue(), count: 1 )
        
        let returnedInt = JNIMethod.CallStaticIntMethod(
            className: "com/jmarkstar/sample/MyExample",
            classCache: &MyExample.MyExampleJNIClass,
            methodName: "returnInt",
            methodSig: "()I",
            methodCache: &returnInt_Method_Ref,
            args: &params,
            locals: &__locals )
        
        return Int(returnedInt)
    }
    
    private static var sumInts_Method_Ref: jmethodID?
    open class func sumInts(a: Int, b: Int) -> Int {
        var __locals = [jobject]()
        
        var params = [jvalue]( repeating: jvalue(), count: 2 )
        params[0] = JNIType.toJava( value: a, locals: &__locals )
        params[1] = JNIType.toJava( value: b, locals: &__locals )
        
        let returnedInt = JNIMethod.CallStaticIntMethod(
            className: "com/jmarkstar/sample/MyExample",
            classCache: &MyExample.MyExampleJNIClass,
            methodName: "sumInts",
            methodSig: "(II)I",
            methodCache: &sumInts_Method_Ref,
            args: &params,
            locals: &__locals )
        
        return Int(returnedInt)
    }
    
    //**********************************/
    //        INSTANCE METHODS            /
    //**********************************/
    
    private static var sumDoubles_MethodID: jmethodID?
    open func sumDoubles(d1: Double, d2: Double) -> Double {
        
        var __locals = [jobject]()
        
        var params = [jvalue]( repeating: jvalue(), count: 2 )
        params[0] = JNIType.toJava( value: d1, locals: &__locals )
        params[1] = JNIType.toJava( value: d2, locals: &__locals )
        
        let __return = JNIMethod.CallDoubleMethod(
            object: javaObject,
            methodName: "sumDoubles",
            methodSig: "(DD)D",
            methodCache: &MyExample.sumDoubles_MethodID,
            args: &params,
            locals: &__locals )
        
        return __return
    }
    
    private static var sayHello_MethodID: jmethodID?
    open func sayHello(){
        var __locals = [jobject]()
        var params = [jvalue]( repeating: jvalue(), count: 1 )
        JNIMethod.CallVoidMethod(
            object: javaObject,
            methodName: "sayHello",
            methodSig: "()V",
            methodCache: &MyExample.sayHello_MethodID,
            args: &params,
            locals: &__locals )
    }
}

Using Swift class to call Kotlin class

import Foundation
import java_lang
import java_util
import Foundation

open class main {
    
    public init() {
        NSLog("\(type(of: self)): \(#function)")
    }
    

    // one-off call to bind the Java and Swift sections of app
    @_silgen_name("Java_com_jmarkstar_sample_MainActivity_viewLoaded")
    public static func viewLoaded( __env: UnsafeMutablePointer<JNIEnv?>, __this: jobject?){
        
        NSLog("\(type(of: self)): \(#function)")
        
        NSLog(".::SWIFT::.")
        
        let myExample = MyExample(name: "Marco")
        
        NSLog("NAME instance field: \(myExample.name!)")
        myExample.name = "Shelly"
        NSLog("NAME instance field: \(myExample.name!)")
        
        let result: Double = myExample.sumDoubles(d1: 25.342323, d2: 34.232356)
        NSLog("result sumDoubles: \(result)")
        
        myExample.sayHello()
        
        NSLog("NAME instance field: \(myExample.name!)")
        
        NSLog("returned int: \(MyExample.returnInt())")
        
        NSLog("retured sumInts: \(MyExample.sumInts(a: 30, b: 30))")
        
        NSLog("MY BYTE static field: \(MyExample.MY_BYTE)")
        NSLog("MY SHORT static field: \(MyExample.MY_SHORT)")
        NSLog("MY INT static field: \(MyExample.MY_INT)")
        NSLog("MY LONG static field: \(MyExample.MY_LONG)")
        
        
    }
}