CODEWITHSUNDEEP
kotlin
Water Cooler v2
Water Cooler v2

Reputation: 33880

Convert a nullable type to its non-nullable type?

I have a bunch of beans that have nullable properties like so:

package myapp.mybeans;

data class Foo(val name : String?);

And I have a method in the global space like so:

package myapp.global;

public fun makeNewBar(name : String) : Bar
{
  ...
}

And somewhere else, I need to make a Bar from the stuff that's inside Foo. So, I do this:

package myapp.someplaceElse;

public fun getFoo() : Foo? { }
...
val foo : Foo? = getFoo();

if (foo == null) { ... return; }


// I know foo isn't null and *I know* that foo.name isn't null
// but I understand that the compiler doesn't.
// How do I convert String? to String here? if I do not want
// to change the definition of the parameters makeNewBar takes?
val bar : Bar = makeNewBar(foo.name);

Also, doing some conversion here with foo.name to cleanse it every time with every little thing, while on the one hand provides me compile-time guarantees and safety it is a big bother most of the time. Is there some short-hand to get around these scenarios?

Upvotes: 51

Views: 38620

Answers (2)

Sir Codesalot
Sir Codesalot

Reputation: 7293

You could use an extension:

fun <T> T?.default(default: T): T {
    return this ?: default
}

Then use it like this:

fun getNonNullString(): String {
    return getNullableString().default("null")
}

Upvotes: 7

miensol
miensol

Reputation: 41678

You need double exclamation mark like so:

val bar = makeNewBar(foo.name!!)

As documented in Null Safety section:

The third option is for NPE-lovers. We can write b!!, and this will return a non-null value of b (e.g., a String in our example) or throw an NPE if b is null:

val l = b!!.length

Thus, if you want an NPE, you can have it, but you have to ask for it explicitly, and it does not appear out of the blue.

Upvotes: 56

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