Kotlin - What is the use of class without body?

Question

In Kotlin both the header and the body are optional; if the class has no body, curly braces can be omitted.

So we can define class like,

class Empty

What is the use of this type of class?

Answer 1

You can use it for some custom exceptions:

class Empty : Exception()

or as a marker interface:

interface Empty

or as a data class:

data class Empty(val s: String)

or as a marker annotation:

annotation class Empty

~ That's a good post to read.

Answer 2

Kotlin allows to declare any type without body, for example:

interface Interface;

class Class;

annotation class Annotation;

sealed class SealedClass;

data class DataClass(var value: String);

object ObjectClass;

enum class EnumClass;

class CompanionClass {
    companion object
}

the usage of each definition can be described as below:

• interface - as a marker interface.
• annotation - describe the annotated type has some ability. e.g: junit4 @Before and @After annotations.
• object - it often present as a token or a lock or a placeholder and .etc. e.g: synchronized(lock){ /*thread safe working*/ }
• data class - quickly define a java POJO class with getters, setters? , equals, hashCode, toString and componentN operators for destructuring in kotlin.
• others - they are meaningless, just are the language syntax.