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Data objects

When printing a plain object declaration in Kotlin, the string representation contains both its name and the hash of the object:

object MyObject

fun main() {
    println(MyObject) 
    // MyObject@hashcode
}

Output:

MyObject@3941a79c

However, by marking an object declaration with the data modifier, you can instruct the compiler to return the actual name of the object when calling toString(), the same way it works for data classes:

data object MyDataObject {
    val number: Int = 3
}

fun main() {
    println(MyDataObject) 
    // MyDataObject
}

Output:

MyDataObject

Additionally, the compiler generates several functions for your data object:

• toString() returns the name of the data object;
• equals()/hashCode() enables equality checks and hash-based collections.

Note: You can’t provide a custom equals or hashCode implementation for a data object.

The equals() function for a data object ensures that all objects that have the type of your data object are considered equal. In most cases, you will only have a single instance of your data object at runtime, since a data object declares a singleton. However, in the edge case where another object of the same type is generated at runtime (for example, by using platform reflection with java.lang.reflect or a JVM serialization library that uses this API under the hood), this ensures that the objects are treated as being equal.

The generated hashCode() function has a behavior that is consistent with the equals() function, so that all runtime instances of a data object have the same hash code.

Differences between data objects and data classes

While data object and data class declarations are often used together and have some similarities, there are some functions that are not generated for a data object:

• No copy() function. Because a data object declaration is intended to be used as singletons, no copy() function is generated. Singletons restrict the instantiation of a class to a single instance, which would be violated by allowing copies of the instance to be created;
• No componentN() function. Unlike a data class, a data object does not have any data properties. Since attempting to destructure such an object without data properties wouldn’t make sense, no componentN() functions are generated.

Use data objects with sealed hierarchies

Data object declarations are particularly useful for sealed hierarchies like sealed classes or sealed interfaces. They allow you to maintain symmetry with any data classes you may have defined alongside the object.

In this example, declaring EndOfFile as a data object instead of a plain object means that it will get the toString() function without the need to override it manually:

sealed interface ReadResult
data class Number(val number: Int) : ReadResult
data class Text(val text: String) : ReadResult
data object EndOfFile : ReadResult

fun main() {
    println(Number(7)) 
    // Number(number=7)
    println(EndOfFile) 
    // EndOfFile
}

Links

Data objects

Further reading

Data Objects in Kotlin

Next questions

What’s data classes?

What do you know about companion object?

What do you know about object keyword?

What do you know about sealed classes and interfaces?

credit goes to @swayangjit