I thought only data classes have `equals/hashcode`...

# getting-started

[nilTheDev]

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?

[Paul Griffith]

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

[Paul Griffith]

== vs === always means 'structural equality operator' vs 'referential equality operator', it's just that without manually defined equals there's nothing to use on a given object besides the referential equality

[nilTheDev]

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.

[CLOVIS]

@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 ===.

[CLOVIS]

data class have a better default implementation, which uses properties; but it is defined for any Kotlin objects.