Add non-null return type to assertNotNull method

[adil-hussain-84]

It would be sweet if the assertNotNull(actual:) method returned the object that is passed into it back out as a non-null type.

This would help with writing JUnit 5 tests in Kotlin big time and avoid the need to force unwrap properties like this:

assertNotNull(myObject?.myProperty)
val myProperty = myObject!!.myProperty
assertEquals("Foo", myProperty.someOtherProperty)

Instead, tests like the above code snippet could be simplified to the following:

val myProperty = assertNotNull(myObject?.myProperty)
assertEquals("Foo", myProperty.someOtherProperty)

Note that the kotlin-test library already offers this mechanism in its assertNotNull method, and Apple offers a similar XCTUnwrap method in their XCTest framework.

[marcphilipp]

I think that's a good idea but we'd have to investigate whether the return type can be changed without breaking backward compatibility.

[marcphilipp]

Tentatively slated for 5.8 M1 solely for the purpose of team discussion.

[sormuras]

If such a refactoring breaks the existing API, we could introduce new methods that a) delegate to the existing ones for the null-check and b) return the given values. Similar to Objects.requireNonNull from the java.util package.

[sbrannen]

The proposal is analogous to the signature for org.junit.platform.commons.util.Preconditions.notNull(T, String).

[stale]

This issue has been automatically marked as stale because it has not had recent activity. Given the limited bandwidth of the team, it will be automatically closed if no further activity occurs. Thank you for your contribution.

[ephemient]

Please reconsider.

[awelless]

It's possible with the help of kotlin.contracts.ContractBuilder.
I can prepare a pull request containing assertNonNull and assertInstanceOf methods.

In the meantime, you can write these methods in your repo by yourself:

@OptIn(ExperimentalContracts::class)
fun assertNotNull(actual: Any?) {
    contract {
        returns() implies (actual != null)
    }

    Assertions.assertNotNull(actual)
}

@OptIn(ExperimentalContracts::class)
inline fun <reified T> assertInstanceOf(actual: Any): T {
    contract {
        returns() implies (actual is T)
    }

    return assertInstanceOf(T::class.java, actual)
}

[marcphilipp]

@awelless see #1866

[awelless]

@marcphilipp
I noticed this issue first. I think, it can be closed as a duplicate of #1866

[marcphilipp]

Since the original report is also specific to Kotlin I think you're right. 👍

[marcphilipp]

Duplicate of #1866