We were taught very early in Java that comparing objects using == won't produce the expected results, since it tests for reference equality, and we need to use equals() for that.

JVM does string interning to prevent that in some basic cases, so for the sake of the example we'll use new String() to avoid that:

String s1 = "ABC";
String s2 = new String(s1);

System.out.println(s1 == s2); // false

Kotlin translates == to equals():

val s1 = "ABC"
val s2 = String(s1.toCharArray())

println(s1 == s2) // true

If you do want to check for reference equality, use ===:

println(s1 === s2) // false