kotlin: If value is null then exit, else use that value as not nullable

Issue

Essentially this is in the title. I have a value that could be null. If it is, I just want to exit with a message. If it's not null, then there's a whole slew of work I need to do with this value.

I've found similar, but not quite this exact situation. And it's the subtle difference that's driving me nuts. Here is my code in java:

    if (value == null) {
        print("error!");
        return;
    }

    print(value);
    doFunStuff(value);
    etc(value);

All those methods using value require it to be non-null.

But I'm having a difficult time figuring this out in kotlin. With everything I try, the compiler still insists that value is still nullable and refuses to use it in the functions.

What is the kotlin way of doing this very common code flow?

Solution

If your value is a local variable or a function parameter, you won't have this problem, because the compiler will smart-cast it to not-null.

So, I'm assuming value in this case is a member property.

Option 1 is to copy it to a local variable to use in the function:

val value = value
if (value == null) {
    print("error!")
    return
}

print(value)
doFunStuff(value)
etc(value)

Option 2 is to use the let or also scope functions to do the same thing, but this might not be a good option here because so much code would become nested. This is more useful when you're only calling one or two functions with the object, in which case, you wouldn't even have to name it (just call it it).

value.let { value ->
    if (value == null) {
        print("error!")
        return
    }

    print(value)
    doFunStuff(value)
    etc(value)
}

If your entire function works with this one property, you can avoid the nesting problem like this, if you don't mind it returning something besides Unit:

fun foo() = value.also { value ->
    if (value == null) {
        print("error!")
        return
    }

    print(value)
    doFunStuff(value)
    etc(value)
}

Option 3 is to assert non-null every time you use it, but this is very ugly. This is only safe if you know the property is only ever accessed from the same thread this function is ever called on.

if (value == null) {
    print("error!")
    return
}

print(value!!)
doFunStuff(value!!)
etc(value!!)

Answered By - Tenfour04