Add java-class-prefix option to djinni

java-class-prefix will prefix all Java class names with passed prefix
when generating output.

src/source/Main.scala

@@ -42,6 +42,7 @@ object Main {
     var javaNullableAnnotation: Option[String] = None
     var javaNonnullAnnotation: Option[String] = None
     var javaUseFinalForRecord: Boolean = true
+    var javaClassPrefix: Option[String] = None
     var jniOutFolder: Option[File] = None
     var jniHeaderOutFolderOptional: Option[File] = None
     var jniNamespace: String = "djinni_generated"
@@ -98,6 +99,8 @@ object Main {
         .text("The output for the Java files (Generator disabled if unspecified).")
       opt[String]("java-package").valueName("...").foreach(x => javaPackage = Some(x))
         .text("The package name to use for generated Java classes.")
+      opt[String]("java-class-prefix").valueName("<prefix>").foreach(x => javaClassPrefix = Some(x))
+          .text("The prefix to be prepended to generated Java class names.")
       opt[String]("java-cpp-exception").valueName("<exception-class>").foreach(x => javaCppException = Some(x))
         .text("The type for translated C++ exceptions in Java (default: java.lang.RuntimeException that is not checked)")
       opt[String]("java-annotation").valueName("<annotation-class>").foreach(x => javaAnnotation = Some(x))
@@ -224,6 +227,11 @@ object Main {
     var objcFileIdentStyle = objcFileIdentStyleOptional.getOrElse(objcIdentStyle.ty)
     val objcppIncludeObjcPrefix = objcppIncludeObjcPrefixOptional.getOrElse(objcppIncludePrefix)
 
+    // Add Java prefix to identstyle
+    if (javaClassPrefix.isDefined) {
+      javaIdentStyle = javaIdentStyle.copy(ty = IdentStyle.prefix(javaClassPrefix.get, javaIdentStyle.ty))
+    }
+
     // Add ObjC prefix to identstyle
     objcIdentStyle = objcIdentStyle.copy(ty = IdentStyle.prefix(objcTypePrefix,objcIdentStyle.ty))
     objcFileIdentStyle = IdentStyle.prefix(objcTypePrefix, objcFileIdentStyle)