That's not true. Scala sits on the JVM, and has some accommodations to facilitate interoperation with code written in Java, but it does not "sit on top of Java" as a language. You don't need to know Java to write Scala.
Is this actually true in practice? Are there tutorials or books that don't assume knowledge of Java? In any case, if you want to use anything in the Java standard library (I assume they haven't converted the entire thing to Scala), then you'll still need to understand Java documentation which requires some knowledge of Java.
> Are there tutorials or books that don't assume knowledge of Java?
Tutorials, books, and a Coursera course taught by Martin Odersky. Among other things.
> In any case, if you want to use anything in the Java standard library (I assume they haven't converted the entire thing to Scala), then you'll still need to understand Java documentation which requires some knowledge of Java
Sure, if you want to use something in a Java library (standard library or not) for which no one has written Scala-focussed documentation, its likely you'll need to understand Javadocs; that's not a "need to know to use Scala", that's a "need to know to effectively interface with existing Java library code", which applies just as much when leveraging Java libraries from any other language.
OTOH, the structure of Scala makes this fairly straightforward, and its probably a lower barrier if you know Scala but not Java than almost any other non-Java language from which you might call Java.
If you want to use Scala without ever calling or being called from Java, you probably picked the wrong FP language. If you want to use Scala to call and be called by Java, you will need to know Java to debug anything.
That's not true. Scala sits on the JVM, and has some accommodations to facilitate interoperation with code written in Java, but it does not "sit on top of Java" as a language. You don't need to know Java to write Scala.