Overview of Packages and Directory Structure
One file can contain several related classes, but only one of them can be public. If the public class is called wheat.java, then the file must be called wheat.
A set of classes in different files can be grouped together in a package. Each of the files must be in the same directory and contain the command
The name of the directory must the same as the package.
File: wheat.java: stone.java:
package mill package mill
public class wheat . . . public class stone . . .