need help...success in compile with .jar, but fail in running

I have a jar file that contains a package. I enter this command and it compiles:

javac -cp "C:\Program Files\Java\jdk1.5.0_05\bin\myPac.jar" app.java

but when I try to run the program by typing this command:

java -cp "C:\Program Files\Java\jdk1.5.0_05\bin\myPac.jar" app

It doesn't run and i get this exception error:

Exception in thread "main" java.lang.NoClassDefFoundError: app

Any ideas?

Does the jar file contain a manifest file?
Not sure, but I believe you might have to specify the class file to run in a manifest file.

Does the java command need a -jar switch to run a jar file?

Yep, that's another thing you are missing.
Your running should look like this:
java -jar jar-file


But you also need the Main-Class header in the manifest file to indicate the entry point to the app.