how scala use class or something else defined in java file?

Using Java from Scala is fairly seamless if you are using SBT.

I minimal example project showing free mixing of Scala and Java could look like:

• project root
  • build.sbt
  • src/
    • Hey.java
    • There.scala

~~ build.sbt ~~

javaSource in Compile <<= baseDirectory(_ / "src")

scalaSource in Compile <<= baseDirectory(_ / "src")

~~ src/Hey.java ~~

public class Hey {
    public static String yay = "Yay!!";
}

~ src/There.scala~~

object There extends App {
    println(Hey.yay)
}

Then run There either by your IDE or by

$ sbt 'run-main There'

Similarly, if you have a Java library you want to use from your Scala project, you can use it as a Maven dependency without regard for whether it is Scala or Java:

~~ build.sbt ~~

libraryDependencies ++= Seq(
    "com.neuronrobotics" % "nrjavaserial" % "3.7.5.1", // Java library
    "org.scala-lang.modules" % "scala-xml_2.11" % "1.0.3" // Scala library
)

If you are using Maven as your build system, using libraries is as easy as in SBT. Mixing Scala and Java sources is a little trickier using Maven, and requires tapping onto a Maven lifecycle stage before compile.