What are the differences between interfaces in Kotlin and Java?

Main differences:

• Default method implementation: In Java 8 and above, interfaces can implement methods with bodies (default methods). In Kotlin, interface methods have default bodies. The default keyword is not required.

• Abstract and open methods: In Java, all interface methods are implicitly abstract. To make a method with a body, default must be used. In Kotlin, interface methods with bodies are implicitly open and can be overridden in implementing classes. Methods without bodies are implicitly abstract.

• Properties: Java interfaces cannot contain fields, only constants (final static). Kotlin interfaces can declare abstract properties, which must be implemented in classes, or properties with default getters/setters (backstage fields are not stored).

• Constructors: Java interfaces cannot contain constructors. Kotlin interfaces cannot contain constructors.

• Visibility modifiers: In Java, interface methods are implicitly public. In Kotlin, interface methods are implicitly public open.

Feature	Kotlin	Java
Method implementation	Allowed (by default)	Allowed (with default)
Method abstractness	Without body - abstract, with body - open	Without body - abstract, with body - default
Properties	Allowed (abstract or with accessors)	Not allowed (only constants)
Constructors	Not allowed	Not allowed
Visibility modifier	public open by default	public by default

Kotlin example:

interface MyInterface {
    val myProperty: String // Abstract property

    fun abstractMethod() // Abstract method

    fun defaultMethod() {
        // Default implementation
        println("Default implementation in Kotlin interface")
    }
}

Java example:

interface MyInterfaceJava {
    // No properties, only constants allowed
    // public static final String MY_CONSTANT = "value";

    void abstractMethod(); // Abstract method (implicitly public and abstract)

    default void defaultMethod() { // Default method
        // Default implementation
        System.out.println("Default implementation in Java interface");
    }
}