JAR - Package Directory - Can't Find Or Load Main Method

I am trying to understand how package(directory structure) and JAR files work interchangeably. From my understanding JAR is a package file format typically used to aggregate many Java classes. Since many classes are usually contained inside a package, I should be able to create a JAR file from a package, and use them interchangeably. A single JAR file is easier to handle then multiple classes and folders.

To test my understanding of the concept, I am working with the following project structure,

/development_directory
____/BookPackage
________Book.java
____BookDemo.java

Book.java contains the following code,

package BookPackage;
public class Book{
private String bookname;
public Book(String str){
bookname = str;
}
public String getBookName(){
return bookname;
}
}

and BookDemo.java contains the following code,

import BookPackage.Book;
class BookDemo{
public static void main(String ... args){
Book b = new Book("Lord of the Rings");
System.out.println(b.getBookName());
}
}

From the development_directory, I used the following commands to compile and run the project,

development_directory/ javac BookDemo.java
development_directory/ java BookDemo

...and it compiles and works as expected.Now, I will try to convert the BookPackage folder(package) into single jar file for portability, and I am using the following command to do that,development_directory/ jar -cf bookpkg.jar BookPackage ...this command generates the bookpkg.jar file.

I am going to remove the BookPackage folder to see if bookpkg.jar file really works as a replacement for the BookPackage directory structure.

After removal of BookPackage folder, the development_directory structure looks like this,

/development_directory
____bookpkg.jar
____BookDemo.java

Now, I tried to use the following commands to see if the program works as before,

// to compile
development_directory/ javac -cp ./bookpkg.jar BookDemo.java

...it compiles without any error - this means the replacement of bookpkg.jar for BookPackage(package) folder structure is working - Am I correct?

// to run
development_directory/ java -cp ./bookpkg.jar BookDemo

...And I hit an error,

Error: Could not find or load main class BookDemo

But the BookDemo class is at default namespace - why it JRE won't be able to find the main method?

And, the way I am trying to replace package folder structure with a single JAR file - am I getting the concept correctly?