Hi! Implementing a type safe unix timestamp. I want to make the constructor private (should only be ...

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[iex]

Hi! Implementing a type safe unix timestamp. I want to make the constructor private (should only be initializable via factory methods) but the value used in it should be publicly readable:

@Parcelize
data class UnixTime private constructor(value: Long) : Parcelable {

    companion object {
        fun fromValue(value: Long): UnixTime =
            UnixTime(value)

        fun minTimestamp(): UnixTime =
            UnixTime(0)

        fun now(): UnixTime =
            UnixTime(Date().time / 1000)
    }
}

fun UnixTime.debugString() =
    "$value, ${toDate()}"

fun UnixTime.toDate() =
    Date(value * 1000)

This doesn't compile, and has also a warning in private. How do I implement it correctly?

[diesieben07]

A data class' constructor is implicitly always public, because it can be accessed via the copy method. Why do you not want a public constructor, if all fromValue does is call it directly anyways?

[Kroppeb]

I think the best would be to drop the data and just implement equals and hashcode

[iex]

@diesieben07 because I want to leave open the possibility to return an optional (e.g. if value is a negative number)

[diesieben07]

I don't see why the constructor needs to be private for that. The constructor can still check that the value is > 0 (or whatever other requirement you have).

[diesieben07]

You would need to do that anyways, if you have a data class, because of the generated copy method.

[iex]

but the constructor can't return an optional

[iex]

it would have to crash

[diesieben07]

Correct

[diesieben07]

Like I said, you have to do either that, or not make it a data class.

[iex]

where do I find the default implementation of equals / hashcode? To just copy it

[iex]

(for data classes I mean)

[diesieben07]

You can have IntelliJ generate it for you using Code > Generate

[Kroppeb]

equals should just check if value is equal

[Kroppeb]

oh yeah that works too

[iex]

right, perfect

[Kroppeb]

hashcode = value.hashcode. Should work

[iex]

though oh my 😅

override fun equals(other: Any?): Boolean {
    if (this === other) return true
    if (javaClass != other?.javaClass) return false
    other as UnixTime
    if (value != other.value) return false
    return true
}

[Kroppeb]

huh, I thought kotlin had better equals

[diesieben07]

Its autogenerated code. It can be simplified, if you wnat to

[iex]

probably like this?

value == (other as? UnixTime)?.value

[diesieben07]

I'd do return other is UnixTime && this.value == other.value

[iex]

why, readability?

[diesieben07]

Yes, and also your code is not correct in case value is nullable.

[iex]

oh because it would return true if value is null and other is null, right?

[diesieben07]

Yes

[iex]

(not sure if null == null is true though)

[iex]

ah ok

[diesieben07]

Well, or if other is not an instance of UnixTime

[iex]

right!

[iex]

ok, great, thanks very much you both