Java means two different things:
- the Java language - this includes the language, with its syntax, rules, features and overall semantics
The Java language and Scala both run on the same platform. AFAIK Scala can call down to Java code quite easily, so if there is any real interest in learning Java to understand/leverage Scala, then it is to understand how this works and how you can best use Java code from within a Scala app. As a language, Java is unlikely to teach you anything C# wouldn't have taught you by now (I assumed from your profile you have some C# experience).
This question's .NET-analogy would be:
Do you need to know C# / VB.NET before trying F#?
In either case, the answer is a pretty firm "No."