Call Java method from Scala named "clone"

Question

I want to call a Java method with the following API from Scala:

// Java API
public interface JavaAPI {
  JustSomeAPISpecificType clone();
}

// Scala code in class a.b.c.D
val api: JavaAPI = factory.getAPI
val result: JustSomeAPISpecificType = api.clone

However, this does not work, as the Scala compiler (2.10) thinks I want to call Object.clone:

Error: method clone in class Object cannot be accessed in Option[JavaAPI] Access to protected method clone not permitted because prefix type Option[JavaAPI] does not conform to class D in package a.b.c where the access take place

Any ideas? Thanks.

And it isn't because the interface is package protected? (No access modifier in Scala means public but package protected in Java) — johanandren, Sep 08 '14 at 18:00
JavaAPI is public. And putting () at the end of clone didn't make any difference. — Michael Rueegg, Sep 08 '14 at 18:02
Edited the question to make it more clear (added public modifier to Java interface and matched class names in error message). — Michael Rueegg, Sep 09 '14 at 03:57

Andrzej Jozwik · Accepted Answer · 2014-09-09T08:27:28.433

2

First of all see Object#clone() signature

protected Object clone()
                throws CloneNotSupportedException

You want to override this method.

In your example now you have two methods:

protected Object clone()
                    throws CloneNotSupportedException

public JustSomeAPISpecificType clone()

See:

JustSomeAPISpecificType.java

public enum JustSomeAPISpecificType{
INSTANCE
}

Factory.java

public class Factory {

        public static class JavaApiImpl implements JavaAPI{

                public JustSomeAPISpecificType clone(){
                        return JustSomeAPISpecificType.INSTANCE;
                }
        }

        public static JavaAPI getAPI(){
                return new JavaApiImpl();
        }
}

Now add your files to src/main/java and run sbt console from root:

scala> val api = Factory.getAPI
api: JavaAPI = Factory$JavaApiImpl@4243eb68

scala> api.getClass.getMethods.filter(m => m.getName.startsWith("clone")).mkString("\n")
res0: String = 
public JustSomeAPISpecificType Factory$JavaApiImpl.clone()
public java.lang.Object Factory$JavaApiImpl.clone() throws java.lang.CloneNotSupportedException

Update If you cast - you will see the method clone (if the factory is public).

scala> api.asInstanceOf[Factory.JavaApiImpl].clone
res4: JustSomeAPISpecificType = INSTANCE

edited Sep 09 '14 at 08:27

answered Sep 09 '14 at 07:09

Andrzej Jozwik

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Thanks for your comment, but it's the API of a given Java library that provides this clone method. How can I convince the Scala compiler to use JavaAPI.clone() now? – Michael Rueegg Sep 09 '14 at 07:15
Try the reflection api. But you want to break the rules. See [clone contract](http://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#clone--) `x.clone().getClass() == x.getClass()`. – Andrzej Jozwik Sep 09 '14 at 07:22
Okay, thank you, by using reflection I'm able to call clone() of the Java API, although I was hoping for a more elegant solution... By the way, I'm aware of the clone contract and don't want to break it. It's just the Java library I'm using. – Michael Rueegg Sep 09 '14 at 08:17
This is bug is scala. See https://groups.google.com/forum/#!topic/scala-user/WaMDZqm8CC0 or http://stackoverflow.com/a/15861169/651140 – Andrzej Jozwik Sep 09 '14 at 09:09
Thanks, but unfortunately the Java factory is not public. I can live with the reflection approach for now. But note that the bug you mentioned was fixed in Scala 2.7.2 according to the bug tracker and I'm using 2.10.3. – Michael Rueegg Sep 09 '14 at 09:38
It was solved - and again occurred. It is regression. Your code is a good test. – Andrzej Jozwik Sep 09 '14 at 09:55
I use reflection for now until this bug is fixed and mark this as resolved. – Michael Rueegg Sep 09 '14 at 10:00

Call Java method from Scala named "clone"

1 Answers1