How to extend a data class with toString

Question

I have created a data class

data class Something (
    val a : String,
    val b : Object,
    val c : String
)

as later in my program, I need the string representation of this data class I tried to extend the toString method.

override fun Something.toString() : String = a + b.result() + c

The problem here is, it does not allow extending (overriding) the toString function, as it is not applicable to top-level functions.

How to properly override/extend the toString method of a custom dataclass?

Answer 1

In Kotlin, extension functions cannot override member functions, moreover, they are resolved statically. It implies that if you write an extension function fun Something.toString() = ..., s.toString() won't be resolved to it, because member always wins.

But in your case, nothing stops you from overriding toString inside Something class body, because data classes can have bodies just like regular classes:

data class Something(
    val a: String,
    val b: Any,
    val c: String
) {
    override fun toString(): String = a + b + c
}