CODEWITHSUNDEEP
kotlinnullablewith-statement
memoizr
memoizr

Reputation: 2099

How to use Kotlin's `with` expression for nullable types

The code below will not compile because the variable myType can be null. Is there a way of executing a with block for nullable types in Kotlin?

    val myType: MyType? = null
    with(myType) {
        aMethodThatBelongsToMyType()
        anotherMemberMethod()            
    }

Upvotes: 15

Views: 4078

Answers (2)

Jacob Zimmerman
Jacob Zimmerman

Reputation: 1573

You could define your own with function that accepts nullables, then determines whether to actually run based on whether the object is null.

Like this:

fun <T, R> with(receiver: T?, block: T.() -> R): R? {
    return if(receiver == null) null else receiver.block()
}

Then you can call the code the way you wanted to in the example with no issues, and the result will equal null if what you pass in is null.

Or, if the code block should (and could) be run either way, even if myType is null, then you'd define it like this instead:

fun <T, R> with(receiver: T?, block: T?.() -> R): R {
    return receiver.block()
}

Upvotes: 5

Ingo Kegel
Ingo Kegel

Reputation: 47985

You can convert a nullable type to a non-nullable type with the suffix !!:

with(myType!!) {
    aMethodThatBelongsToMyType()
    anotherMemberMethod()            
}

If the value is indeed null, it will throw a NullPointerException, so this should generally be avoided.

A better way to do this is to make the execution of the code block dependent on the value being non-null by making a null-safe call and using the apply extension function instead of with:

myType?.apply {
    aMethodThatBelongsToMyType()
    anotherMemberMethod()            
}

Yet another option is to check if the value is non-null with an if statement. The compiler will insert a smart cast to a non-nullable type inside the if-block:

if (myType != null) {
    with(myType) {
        aMethodThatBelongsToMyType()
        anotherMemberMethod()            
    }
}

Upvotes: 25

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