Interfaces

Kotlin interfaces can declare abstract members and provide default implementations—similar to Java 8+ interfaces and C# default interface methods. A class may implement multiple interfaces.

Implementing interfaces

interface Greeter {
    fun greet(name: String): String
}

class LoudGreeter : Greeter {
    override fun greet(name: String) = "HELLO, $name"
}

Interface properties can be abstract or have getters; no backing field unless you are in a class implementation.

Compared to Java

After Java interfaces, Kotlin feels familiar—no separate implements keyword; use a colon and comma-separated supertypes: class Foo : Bar, Baz.

Important interview questions and answers

1. Q: Interface vs abstract class?
   A: Interfaces support multiple inheritance of behavior contracts; abstract classes carry state and single inheritance—pick interfaces for capabilities, abstract classes for shared base state.
2. Q: Can interfaces have code?
   A: Yes—default method bodies in interfaces are common for small shared helpers.

Self-check

1. How many interfaces can one class implement?
2. What keyword marks interface implementation?

Tip: Keep interfaces small—default implementations help evolve APIs without breaking implementers.