Suppose we have this code:
class QuickExample {
fun function(argument: SomeOtherClass) {
if (argument.mutableProperty != null ) {
doSome
i usually write it like this:
takeIf{somecondition}?.also{put somecondition is met code}?:run{put your else code here}
note the question mark after takeIf is a MUST. you can use also or apply keyword.
You could also do something like this:
class If<T>(val any: T?, private val i: (T) -> Unit) {
infix fun Else(e: () -> Unit) {
if (any == null) e()
else i(any)
}
}
You can then use it like this:
If(nullableString) {
//Use string
} Else {
}
I don't believe there is a really "short" way to achieve it, however you can simply use a conditional within with
or let
:
with(mutableVar) { if (this != null) doSomething(this) else doOtherThing() }
mutableVar.let { if (it != null) doSomething(it) else doOtherThing() }
In fact, "capturing" a mutable value is one of the main use cases of let
.
This is equivalent to your when
statement.
There is always the option you described, assigning it to a variable:
val immutable = mutableVar
if (immutable != null) {
doSomething(immutable)
} else {
doOtherThing()
}
which is always a nice fallback in case e.g. things get too verbose.
There probably isn't really a very nice way to achieve this because only the last lambda argument is allowed to be put outside the ()
, so specifying two wouldn't really fit the syntax of all of the other standard functions.
You could write one if you don't mind that (or if you'll be passing method references instead):
inline fun <T : Any, R> T?.ifNotNullOrElse(ifNotNullPath: (T) -> R, elsePath: () -> R)
= let { if(it == null) elsePath() else ifNotNullPath(it) }
...
val a: Int? = null
a.ifNotNullOrElse({ println("not null") }, { println("null") })
Note that I would personally not do this, because none of these custom constructs are very pleasant to read. IMO: stick with let
/run
and fall back to if
-else
when necessary.
add custom inline function as below:
inline fun <T> T?.whenNull(block: T?.() -> Unit): T? {
if (this == null) block()
return this@whenNull
}
inline fun <T> T?.whenNonNull(block: T.() -> Unit): T? {
this?.block()
return this@whenNonNull
}
then you can write code like this:
var nullableVariable :Any? = null
nullableVariable.whenNonNull {
doSomething(nullableVariable)
}.whenNull {
doOtherThing()
}
Update:
As mentioned by franta on the comments, if the method doSomething()
returns null, then the code on the right side of the elvis operator will be executed, which might not be the desired case for most. But at the same time, in this case, it is very likely that the doSomething()
method will only do something and not return anything.
And an alternative: as protossor has mentioned on the comments, also
can be used rather than let
, because also
returns this
object instead of the result of the function block.
mutableProperty?.also { doSomething(it) } ?: doOtherThing()
Original answer:
I would use let
with Elvis operator.
mutableProperty?.let { doSomething(it) } ?: doOtherThing()
From the doc:
If the expression to the left of ?: is not null, the elvis operator returns it, otherwise it returns the expression to the right. Note that the right-hand side expression is evaluated only if the left-hand side is null.
For a block of code after the right-hand side expression:
mutableProperty?.let {
doSomething(it)
} ?: run {
doOtherThing()
doOtherThing()
}