Just like Java, Scala is also compiled into bytecode which can easily be executed by the JVM. This means that the runtime platforms of Scala and Java are the same because both generate bytecodes as the compilation output. So, you can easily switch from Java to Scala, you can and also easily integrate both, or even use Scala in your Android application to add a functional flavor.
Note that, while using Java code in a Scala program is quite easy, the opposite is very difficult, mostly because of Scala's syntactic sugar.
Also, just like the javac command, which compiles Java code into bytecode, Scala has the scalas command, which compiles the Scala code into bytecode.