How to create jar file with package structure?

How to create jar file with package structure?

I have a folder structures
/com/cdy/ws/a.class files
/com/cdy/ws/b.class files
/com/cdy/ws/c.class files
When I run the following command "jar cvf asd.jar *.class" it gives jar
with all the class files. But the folder structure is not getting
generated. All the class files have to be under "com.cdy/ws" but all the
classes are in same level of META-INF. Can anyone tell me what is the
command to generate the package structure?
Thanks Anderson