Is `x and 0xFF toByte ` a noop Aka `x and 0xFF toByte == x` kotlinlang #getting-started

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Title

r

reactormonk

07/04/2022, 1:47 PM

Is

x and 0xFF.toByte()

a noop? Aka

x and 0xFF.toByte() == x

is always true?

j

Joffrey

07/04/2022, 1:49 PM

What type is

?

r

reactormonk

07/04/2022, 1:50 PM

Also

Byte

j

Joffrey

07/04/2022, 1:52 PM

Then I indeed don't quite get the point of this 🤔 Should be a noop IMO. But I'm not well versed in Kotlin's bitwise operators

If this code is old, it might come from the days when Kotlin didn't have bitwise operators on Bytes and you had to convert to

Int

to do this 🤷

r

reactormonk

07/04/2022, 1:53 PM

I pasta'd it from Java code, where it was on `Int`s

j

Joffrey

07/04/2022, 1:54 PM

Ah, then forget about my last point

s

Sam

07/04/2022, 2:13 PM

Can you share the Java code? Typically

& 0xFF

would be used to truncate the sign bits or other unwanted extras. Translating it to Kotlin probably requires some understanding of the intended behaviour.

And Kotlin has unsigned types which might or might not make your life easier

r

reactormonk

07/04/2022, 2:15 PM

https://github.com/skjolber/external-nfc-api/blob/master/core/src/main/java/com/github/skjolber/nfc/command/ACR1255BluetoothCommands.java#L157=

j

Joffrey

07/04/2022, 2:16 PM

response.getData()[0]

is already a byte, though. I'm not sure what the Java code wanted to do initially with this

s

Sam

07/04/2022, 2:21 PM

I imagine the intention is to print the byte as if it is an unsigned value.

j

Joffrey

07/04/2022, 2:22 PM

but I fail to see what difference it makes to

& 0xFF

(even in terms of sign bits) if the original value is a byte

s

Sam

07/04/2022, 2:22 PM

0xFF

in Java is an int, because it is too big to be a signed byte

:thank-you: 1

Consider this Java code:

byte signed = -1;
System.out.println("Signed value is " + signed);
// Signed value is -1
int unsigned = 0xFF & signed;
System.out.println("Unsigned value is " + unsigned);
// Unsigned value is 255

j

Joffrey

07/04/2022, 2:24 PM

Got it, I missed that

& 0xFF

also converts the original value to

int

before truncating the bits

s

Sam

07/04/2022, 2:28 PM

As for the Kotlin equivalent:

val data: Byte = -1
println("Signed value is " + data)
println("Unsigned value is " + data.toUByte())

(or

data.toUByte().toInt()

if you want to match the Java version more exactly)

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