CODEWITHSUNDEEP
javakotlin
videokojot
videokojot

Reputation: 551

Cannot implement java interface in kotlin due to nullabilty

I cannot implement this java interface in kotlin.

import org.springframework.core.convert.converter.Converter
import org.springframework.stereotype.Component

@Component
class FeatureFlagConverter : Converter<String?, FeatureFlags> {
    override fun convert(source: String?): FeatureFlags = FeatureFlags(source ?: "")
}

Compiler fails with following:

Class 'FeatureFlagConverter' is not abstract and does not implement abstract member public abstract fun convert(source: String): FeatureFlags? defined in org.springframework.core.convert.converter.Converter

Why? The interface is pure java interface (https://github.com/spring-projects/spring-framework/blob/main/spring-core/src/main/java/org/springframework/core/convert/converter/Converter.java) so the source parameter in convert method might be null, and there is no anotation to hint that it is not null.

It helps to remove the nullabilty around String. But I do not want to do that since it is Java interface and the source parameter in convert method might be null. And some java caller might pass null here.

Upvotes: 1

Views: 97

Answers (1)

Tenfour04
Tenfour04

Reputation: 93639

The documentation for the method you linked says:

source the source object to convert, which must be an instance of S (never null)

So S is required to be non-nullable.

The reason Kotlin seems to "know" that the parameter must be non-nullable is likely from nullability annotations. Kotlin supports several nullability annotations for Java. Spring's nullability annotations are meta-annotated using JSR-305, so they are supported.

Although this method doesn't mark source as @NonNull, I think the whole package is marked with non-nullability as the default here.

Upvotes: 2

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