"overload resolution ambiguity" when function parameter is marked "NonNull"

Question

Here are my Kotlin (Version 1.3.61) + Java(1.8.0_201) code that failed to compile:

Maven:

<dependency> <groupId>com.google.code.findbugs</groupId> <artifactId>jsr305</artifactId> <version>2.0.1</version> </dependency>

Test.Kt:

fun main(args: Array<String>) { Point().setX(1) }

Point.java:

import javax.annotation.Nonnull; public class Point { public void setX(int x) { } public void setX(@Nonnull Integer x) { } }

Failed to compile due to the following error:

Error:(2, 13) Kotlin: Overload resolution ambiguity: public open fun setX(@Nonnull x: Int): Unit defined in Point public open fun setX(x: Int): Unit defined in Point

If I removed the @NonNull annotation in the second setX function, then this demo could compile. I thought Java annotation was only extra metadata and would not affect the code itself, so I had no idea why the ambiguity arose here.

Also, I run javap -s Point.class and found out that they had same output:

public void setX(java.lang.Integer); descriptor: (Ljava/lang/Integer;)V

Can someone help to explain what is going on here? Thanks in advance.

Maybe take a step back and explain why in the first place you even have those two methods. I wouldn't see how having both of these would make a difference for the caller — Lino
– Lino, Commented Jan 8, 2020 at 7:46
A similar code was generated from Avro in my project. Though they are not supposed to be called directly, they are public. Therefore I had this question. @LinosaysReinstateMonica — pikatao
– pikatao, Commented Jan 8, 2020 at 17:32

mightyWOZ · Accepted Answer · 2020-01-08 07:53:28Z

From kotlin in action

sometimes Java code contains information about nullability, expressed using annotations. When this information is present in the code, Kotlin uses it. Thus @Nullable String in Java is seen as String? by Kotlin, and @NotNull String is just String. The interesting question is what happens when the annotations aren’t present. In that case, the Java type becomes a platform type in Kotlin.

When you use Java declarations from Kotlin, Java primitive types become non-null types (not platform types), because they can’t hold null values.

in your case first method has a java primitve type as parameter and because primitives can't be null kotlin translates it to

setX(x: Int)

and since second method contains @Nonnull annotation, kotlin uses this information and hence it translates the method to

setX(x: Int)

so as you can see for kotlin they are two different methods with exact same signature, hence the ambiguity.

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"overload resolution ambiguity" when function parameter is marked "NonNull"

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