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Your guess is probably right. The ^= operator and the if statement at the end are both to prevent compiler/runtime optimisations.

Initially b is false, b ^= true assigns true to b, then b ^= true assigns false to b, and the cycle continues.

By making b cycle through true and false, it makes it harder for the compiler to optimise this because it doesn't see a constant value.

Another property of ^ is that both operands must be evaluated in order to evaluate the result, unlike || or &&. The runtime can't take shortcuts.

The if statement at the end is telling the compiler and the runtime: "don't ignore b! It is of important use later!".

The last two lines force the compiler to run the whole loop, to find the value of x. Otherwise it might detect that x is not being used at all and ignore the loop, given that inside of it no "real" work is being made. Even though sum() is called repeatedly, the result of accumulating its returned value would be discarded in the end if we do nothing with x.

Of course, this assumes that the println() statement inside the loop can be safely ignored, I'm unsure if the compiler can make such a decision. That would be one aggressive compiler!

For me there are two reasons:

First Kotlin takes many ideas from the functional programming world and uses immutability as often as it can to avoid all the known problems with mutation.

So declaring every class "final" by default is (at least for me) similar.

The class cannot be changed or altered (using something like reflection) during the runtime which would make the safetychecks of the Kotlin compiler useless.

So if you want to "mutate" the default implementation of a class you have to mark it as open explicitly.

The second thought which comes to my mind is that inheritance is often missused. Some examples for common traps are explained here

There is the principle "Favor composition over inheritance" as a guideline for better designs. So declaring every class as final by default forces the developer to at least stop for a moment and think about alternative ways to solve the problem instead of using inheritance for the wrong reasons.

But as longe as there are no official statements by the kotlin developers I can only give an opinionated answer.

class BookSuggestionService {