I thought only data classes have 

equals/hashcode

 method implemented by default. But it turns out,

class SomeClass(val a: String)

fun main(){
    val someClass1 = SomeClass("abcd")
    val someClass2 = SomeClass("abcd")

    println(someClass1 == someClass2) // this code is also valid
}

==

is also supported in regular classes. How does it behave? Is

==

is same as

===

in case of non-data classes?

all objects in Kotlin implement

equals()

and

hashcode()

, they're just almost always useless (because they're referential equality, rather than structural equality). Your example probably only appears to work because your JVM is using the same interned string instance for "abcd", but there's no guarantee it will do that

I see. But in the snippet,

println(someClass1 == someClass2)

this code prints

false

, as expected. Because

someClass1

and

someClass2

don't reference the same object. So, effectively

==

would be same as

===

in normal classes unless

equals

is overridden with some custom logic.

@nilTheDev Exactly:

==

(or

equals

) let's you define a custom way to represent equality. It is defined in

Any

, so you can use it for whatever class. The default implementation is to use

===

.