You can use the Go To Class feature for quick navigation through mixed Scala/Java code base.
Press Ctrl+N or open the Go To menu and select Class, then start typing a class name. Your input will be looked up against both Scala and Java class, interface, trait etc. names.
You can use CamelHumps and wildcard (/*) masks to flexibly filter the list.
To find a method, field or type alias, use the Go To Symbol feature.
Press Ctrl+Alt+Shift+N or open the Go To menu and select View, then start typing a symbol name.
The completion rules described above also apply here.