Calling Java from Kotlin – Interoperability

Things to consider when calling Java from Kotlin.

fun main() {
    val car = Car("silver", "Audi", "A3", "3.2 V6", "Leather", 2004)
    car.colour = "Akoya silver" // Can only refer to this like a property if getter and setters are set in Java. Will work without setter if property is public, but not usually desirable
    // Wildcard assignments in Java are converted to use-site variance in Kotlin. ? extends someClass -> out someClass, ? super someClass -> in someClass
    // If you call a Java method that throws an exception and you don't handle the exception in the code, you don't have to declare that the Kotlin function throws the exception - Kotlin doesn't care
    // varargs: If you want to call a Java method that wants multiple arguments:
    car.variableMethod(5, "hello", "goodbye") // This works
    val strings = arrayOf("hello", "goodbye")
//    car.variableMethod(10, strings) // But it doesn't allow us to pass in an array
    car.variableMethod(10, *strings) // So we need to use the spread operator to unpack the array
    // Kotlin doesn't have void, so it will return Unit
    // As previously discussed, when Java method expects primitive types we cannot use arrayOf. We need to use e.g. intArrayOf or call .toIntArray on the array:
//    car.wantsIntArray(arrayOf(1, 2, 3)) // Nope
    car.wantsIntArray(intArrayOf(1, 2, 3)) // Fine
    car.wantsIntArray(arrayOf(1, 2, 3).toIntArray()) // Fine
    // Any? is top of Kotlin hierarchy, Object is top of Java. What happens when Java calls Object explicitly? Will be treated like Any in Kotlin.
    // However, does not have methods such as wait(), notify(), clone() or finalize(). If wanting to call these cast the variable to java.lang.Object in your Kotlin code:
//    car.anObject.equals("bh") // Has equals() method and some others but missing the methods listed above
//    (car.anObject as java.lang.Object).notify() // All methods now available on casting
    // Static methods and fields are translated to companion objects. You can't pass the companion object around as a value but you can use its members
    println("x = ${Car.x}") // Accessing static variable
    println(Car.xString()) // The same as calling a companion object
    // Single Abstract Method (SAM) interface as of Java 8, can use lambda. E.g. Runnable interface is a SAM (see Car.java demoMethod and demoMethod2)
    // If a Java method wants an object that implements a SAM interface e.g. Runnable then you can pass it a Kotlin function:
    car.demoMethod3 ({ println("I'm in a Thread") }) // Function signature has to match parameter, in this case Runnable
}

package academy.learnprogamming.javainteroperability;


import org.jetbrains.annotations.NotNull; // N.B. Had to prompt for download from Maven
import org.jetbrains.annotations.Nullable;

public class Car {

    public static int x = 5;

    private String colour;
    private String manufacturer;
    private String model;
    private String variant;
    private String interior;
    private int year;
    private Object anObject;

    public Car(String colour, String manufacturer, String model, String variant, String interior, int year) {
        this.colour = colour;
        this.manufacturer = manufacturer;
        this.model = model;
        this.variant = variant;
        this.interior = interior;
        this.year = year;
    }

    public void demoMethod() {
        new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("I'm in a Thread");
            }
        }).start();
    }

    public void demoMethod2() {
        new Thread(() -> System.out.println("I'm in a Thread")).start();
    }

    public void demoMethod3(Runnable r) {
        new Thread(r).start();
    }

    public static String xString() {
        return "This is x: " + x++;
    }

    public String getColour() {
        return colour;
    }

    public void setColour(@NotNull String colour) { // @NotNull comes from JetBrains, not Java. Not enforced by Java.
        this.colour = colour;
    }

    public String getManufacturer() {
        return manufacturer;
    }

    public void setManufacturer(@Nullable String manufacturer) {
        this.manufacturer = manufacturer;
    }

    public String getModel() {
        return model;
    }

    public void setModel(String model) {
        this.model = model;
    }

    public String getVariant() {
        return variant;
    }

    public void setVariant(String variant) {
        this.variant = variant;
    }

    public @Nullable String getInterior() {
        return interior;
    }

    public void setInterior(String interior) {
        this.interior = interior;
    }

    public int getYear() {
        return year;
    }

    public void setYear(int year) {
        this.year = year;
    }

    public Object getAnObject() {
        return anObject;
    }

    public void setAnObject(Object anObject) {
        this.anObject = anObject;
    }

    public void variableMethod(int num, String... strings) {
        for (String string: strings) {
            System.out.println(string);
        }
    }

    public void wantsIntArray(int[] theArray) {
        for (int i: theArray) {
            System.out.println(i);
        }
    }

    @Override
    public String toString() {
        return "Car{" +
                "colour='" + colour + '\'' +
                ", manufacturer='" + manufacturer + '\'' +
                ", model='" + model + '\'' +
                ", variant='" + variant + '\'' +
                ", interior='" + interior + '\'' +
                ", year=" + year +
                '}';
    }
}