public static boolean containsClass(@NotNull File file,String className){
  String entryPath=className.replace('.','/') + ".class";
  return containsEntry(file,entryPath);
}

public static boolean containsClass ( @ NotNull File file , String className ) { String entryPath = className . replace ( '.' , '/' ) + ".class" ; return containsEntry ( file , entryPath ) ; }
returns true if the given . jar file exists and contains the given class .
