Answer.

Data object is a new type of declaration in Kotlin (since version 1.9) that combines the properties of a singleton object and a data class. It is designed for cases when you want to have only one instance of an object and automatically generated methods equals, hashCode, toString (like in a data class).

Background

Previously, to store unique entities with auto-generated toString() and equals() methods, developers had to write object with manual implementations or compromise by using data class with a single object (rather than instances).

Problem

Singleton objects (object) do not have auto-generated equals, hashCode, toString, while data class implies multiple instances, which is not suitable for exhaustive enum-like models.

Solution

With the emergence of data object, both tasks are solved succinctly:

The class can only be a single instance (singleton).
The compiler generates the necessary multimethods.

Code example:

data object LoadingState

data object NoData

println(LoadingState == LoadingState) // true
println(LoadingState.toString()) // LoadingState

Key features:

Always a singleton — a second instance cannot be created.
The compiler adds methods equals, hashCode, toString like for a data class.
Especially convenient to use together with sealed interface for modeling states.

Trick questions.

How does data object differ from a regular object?

Only data object receives auto-generation of equals, hashCode, toString methods at the compiler level; regular object uses implementations from Any (referential equality, limited toString).

Can a data object have constructor parameters?

No, a data object cannot have a constructor with parameters. It is always parameterless, as there exists only one instance.

Can a data object be inherited?

Yes, a data object can implement interfaces, including sealed interface, and be part of state hierarchies.

Example:

sealed interface Result

data object Success : Result

data object Failure : Result

Typical mistakes and anti-patterns

Trying to add constructor fields to a data object (this is prohibited).
Using data class when a single instance is implied.

Real-life example

Negative case

A developer models "no data" as object NoData, then compares it with equals() to an external object and gets an unexpected result (referential comparison, not substantive).

Pros:

Simplicity of declaration.

Cons:

Inadequate behavior when using equals/toString.

Positive case

A data object is used for loading statuses, equality and printing happen according to expected data class rules. The model layer reliably uses pattern matching on states.