``` if x = null y = x ``` is there a shorter way to do this

Question

``` if x = null y = x ``` is there a shorter way to do this I recall there being some way by using the ` ` operator but im not sure if that was in kotlin or another language

Marko Mitic · Answer

`if x =null y=x` smile

LastExceed · Answer

pls

Marko Mitic · Answer

`y= x y`

Marc Knaup · Answer

`x let y = it `

LastExceed · Answer

thanks

Shawn · Answer

I personally wouldn t recommend that as a good use of the safe navigation operator or the elvis operator either

LastExceed · Answer

should i just keep it as is

Shawn · Answer

there s nothing wrong with a simple null check every now and again especially when it comes to like assignments or side effecty code

Marc Knaup · Answer

Best approach depends on the context

Marko Mitic · Answer

Try to cut down `var`s and assignments as much as possible for clean functional and thread safe code

Shawn · Answer

that s also true

Marko Mitic · Answer

`val z = x y` might be better option

Marc Knaup · Answer

The approach with `if` won t even work reliably in cases where a smart cast is not possible

LastExceed · Answer

i have 2 objects of a data class that contains lots of nullables character update packet in a game server that doesn t always contain every information i keep the first packet i receive to store the current character data position ect and want to update it everytime i get a new packet so i gotta do this operation for every information the packet might contain every time i receive a new packet looks somewhat like this ```oldCharacterData Update newCharacterData fun CharacterData Update newData CharacterData if newData position = null this position = newData position the same code for all 47 fields ```

LastExceed · Answer

i figured since i need to do it 47 times in a row it d be nice to be able to do it in a 1liner

Marc Knaup · Answer

Why do you update a data class instance rather than creating a new one containing the data merged from the precious one and the update

LastExceed · Answer

i could do that but i don t see the benefit since i have to nullcheck the newer data either way

Marc Knaup · Answer

Then you could use ` copy` and the approach of Marko on line per property

Marc Knaup · Answer

It s not just about the null checks it s about immutability

LastExceed · Answer

ah yes that makes sense

LastExceed · Answer

guess im gonna do that then ty

Marc Knaup · Answer

``` data copy position = newData position data position ```

Marc Knaup · Answer

Or wrapped in an extension function even shorter

LastExceed · Answer

what exactly does the ` copy` do here

Marc Knaup · Answer

Marc Knaup · Answer

You could also use the constructor instead of copy if you change all properties

Marc Knaup · Answer

If you know at the point where you receive the data which properties you need to change you could copy the data instance with only these properties specified

LastExceed · Answer

ok i just read the docs you linked it was very helpful kotlin really is an amazing language

Marc Knaup · Answer

It is

LastExceed · Answer

is there any way to execute a secondary constructor BEFORE the primary constructor i use a secondary constructor to read the data from stream but that only works when all the fields are mutable which is otherwise unnecessary

karelpeeters · Answer

Write a factory function instead

karelpeeters · Answer

In general constructors really shouldn t do anything

Sinan Kozak · Answer

write and invoke method in companion object

Sinan Kozak · Answer

TBH i dont suggest that but in kotlin that does the trick

LastExceed · Answer

factory means a function that creates and returns an object

LastExceed · Answer

it just occurred to me isn t immutability in this case implying a performance overhead as i have to create a whole new object compared to changing a value inside that object when using mutability everytime i change something or is ` copy ` optimized enough to not bother about that

Marc Knaup · Answer

It will always cause an allocation In most cases however thinking about allocations is just over thinking of micro optimizations Unless the code is called at a high frequency and super performance critical safe code which is easy to reason about should be of much higher priority

LastExceed · Answer

position updates are sent about 20 times per second per player does that qualify as high frequency or is it negligible

LastExceed · Answer

i guess if performance was that important i d be writing C++

Marc Knaup · Answer

In that case I d recommend to finish the implementation first with a safe amp idiomatic approach and have automated testing set up Then if you see that performance is really a problem start measuring timings and then start optimizing

LastExceed · Answer

gt Why do you update a data class instance rather than creating a new one containing the data merged from the precious one and the update gt Then you could use ` copy` and the approach of Marko on line per property isnt this in the end be the same as just creating a new object manually since i have to pass a value for every property to copy either the new value or the old if the new is null

LastExceed · Answer

i get that the idea of ` copy` is precisely not having to pass the old value again but wouldnt marko s approach be doing just that

Marc Knaup · Answer

Yeah that s why I said you could also just use the constructor ` copy ` is most helpful if you only need to change some of the properties I don t know how your relevant code works so I don t know what case makes more sense for you slightly smiling face

Marc Knaup · Answer

pseudocode example ``` data class Data val a A = null val b B = null received = Data for each message when message is A gt received = received copy a=message is B gt received = received copy b=message ```