Question

I’m converting someone else’s java code to scala (for the curious, it’s the example here) and I hit a compiler error on the following (simplified somewhat):

var out = new Formatter(new StringBuilder(), Locale.US)
out.format("%s-%d ", someObject, someInteger);

And here’s the error message I get:

[error]   (java.util.Locale,java.lang.String,<repeated...>
[java.lang.Object])java.util.Formatter <and>
[error]   (java.lang.String,<repeated...>[java.lang.Object])java.util.Formatter
[error]  cannot be applied to (java.lang.String, java.lang.Object, Int)
...
[error] one error found

This works if I change the second line to:

out.format("%s-%d ", someObject, someInteger.asInstanceOf[Object]);

Can someone explain why this is?

Does this mean that it’s ok in java to pass integers where object arguments are expected but not in scala?

Answer 1

The other answers all add something, but I’m not sure they explain what the problem is.

It all comes down to Int being a class in Scala while int is a primitive in Java. So, in Java, when you write that a method expects an Object, and you pass an int, Java will auto-box it in a java.lang.Integer.

Now, Java’s java.lang.Object equivalent in Scala is AnyRef, but Int is not a subclass of AnyRef. There’s a proper type for that: Any. An Any can contain both things which are subclasses of java.lang.Object as well as the stuff that are primitives in Java. If you say a method expects Any and pass an Int, then it will be auto-boxed, but not for AnyRef.

So, whenever you interface with Java and a method expects you to pass boxed primitives, you’ll have to force the boxing yourself. You can create a method expecting Any in Scala, and then cast it to AnyRef and call the Java equivalent, to make things easier if you are going to call that method a lot.