Can java run a compiled scala code?

You can run byte code generated by Scala if you include all necessary runtime libs for Scala (scala-library.jar, scala-swing.jar ...) in the classpath. The scala command does this automatically, and supports Scala specific command line arguments.

Scala is designed to integrate easily with applications that run on modern virtual machines, primarily the Java virtual machine (JVM). The main Scala compiler, scalac, generates Java class files that can be run on the JVM. -> http://www.artima.com/scalazine/articles/steps.html

As long as you have installed the scala runtime you should be fine: compile classes with scalac and run them with java.

Yes, it can. Scala is compiled down to Java bytecode. But remember that it depends on the Scala runtime classes, so you need to still have Scala's jar files on the classpath.

If so, why do we have an exclusive command scala?

Convenience wrapper.