How do we add interface or classes to a package?

Well, the process differs whether you are using either an IDE or the command line binaries.

If you are using an IDE, then it is most likely that your IDE comes with extensive functionality for both managing and creating packages. Look under the "Packages" menu, or, when you are using Eclipse, just right click on the package name in the Package Explorer window and a flood of possibilities will come right up.

If you are using the command line binaries, then there are 2 steps to putting your classes in a package. First, you have to put your class or interface in the appropriate folder structure. For example, if you are using the package com.foo.bar.beebop, then you have to put your class in a folder named beebop, which is in a folder named bar, which is in a folder named foo, which is in a folder named com, which is in your class directory. Then, you have to add a package statement, which is only allowed to be at the top of your class file. For example, if you're using the package com.foo.bar.beebop, then put the following statement at the top of your class:

package com.foo.bar.beebop;

Then skip a line, then come your import statements (if you have any), then stick in another line (if you have inport statements), then your class.

Enjoy!