how to make kotlin interface property with default value to be used in java code

Question

Using kotlin 1.6.21, and java 11. Having a kotlin interface which has property with default value. How to make use it in java code?

interface ISomeKInterface {
   val flag: Int
       get() = return 1

   fun onProc(data: String) {
       if (flag == 1) {
          // do something_1
       } else if (flag == 2) {
          // do something_2
       } else {
          // do the other
       }
   }
}

in kotlin it could do

object : ISomeKInterface {
    override val flag: Int = 2
    override fun onProc(data: String) {
       if (flag == 1) {
          // do something_1
       } else if (flag == 2) {
          // do different implementation for flag 2
       } else {
          // do the other
       }
   }
}

Edit: in kotlin it works with either override the property or override the getter

fun test_() {
        val theInterfaceImpl: ISomeKInterface = object : ISomeKInterface {

            //override val flag: Int = 2

            // the override the getter does the same as above override flag:Int = 2
            override val flag: Int
                get() {
                    val superFlag: Int = super.flag
                    println("+++ test override get(), flag:$superFlag, ret flag: 2")
                    return 2
                }
        // if no change to the default implementation this override is not required.
        override fun onProc(data: String) {
                println("+++ test override onProc(), flag: $flag default onProc(:$data)")
                super.onProc(data)
            }
        }
        val flag = theInterfaceImpl.flag
        println("+++ test current flag:$flag")
        theInterfaceImpl.onProc("eee888")
    }

But it does not compile if trying to use it in java. In java it requires the @Override public void onProc(String data), but it cannt call the super's default implementation - for the case that the default implementation of the ISomeKInterface is goog enough and does not need different implementation override.

    @Test
    public void test_() {

        ISomeKInterface theInterfaceImpl = new ISomeKInterface() {
            @Override
            public int getFlag() {
//                int superFlag = ISomeKInterface.super.getFlag(); //<== got error on ISomeKInterface.super.
//                System.out.println("+++ flag:"+superFlag+", ret flag: "+2);
                return 2;
            }

            @Override
            public void onProc(@NonNull String data) {

                // it is require the implement the onPro(data: String although it has the default implementation already)
                // but what if the super's default onProc() works fine, and does not need provide different implementation??? it seems cannot call the super.onProc here

                System.out.println("+++ test override onProc(:"+data);
                ISomeKInterface.super.onProc(data);  //<===
            }

        };
        int flag = theInterfaceImpl.getFlag();
        System.out.println("+++ current flag:"+flag);
        theInterfaceImpl.onProc("eee888");
    }

error: abstract method onProc(String) in ISomeKInterface cannot be accessed directly
                ISomeKInterface.super.onProc(data);  //<===
                                     ^

Answer 1

I use kotlin interface in java , so easy

   public class Test {
    public static void main(String[] args) {
        ISomeKInterface iSomeKInterface = new ISomeKInterface() {
            @Override
            public void onProc() {
            }

            @Override
            public int getFlag() {
                return 2;
            }
        };
        System.out.println(iSomeKInterface.getFlag());
    }
}

Answer 2

Actually, the code override val flag: Int = 2 not change the flag value in ISomeKInterface ,and you can use super.flag get the the value which is 1.

further on，kotlin generate get,set function automatic， so if you want use in java, u need change the getFlag() function:

  ISomeKInterface anInterface = new ISomeKInterface() {
    @Override
    public void onProc(@NonNull Object data) {

    }

    @Override
    public int getFlag() {
        return 2;
    }
};

Answer 3

Default Method in Java 8:

public interface Alarm {

    default String turnAlarmOn() {
        return "Turning the alarm on.";
    }
    
    default String turnAlarmOff() {
        return "Turning the alarm off.";
    }
}

Reference: https://www.baeldung.com/java-static-default-methods