The servlet is a simple implementation that behaves based on the value of an 'operation' parameter (op) that is passed as part of the request.

private static final String PARAM_OP = "op";

The op operation can take one of the values: categories, byCategory, byAuthor, addBookForm, and addBook.

The categories operation requests a listing of the currently registered categories.

private static final String OP_CATEGORIES = "categories";

The byCategory and byAuthor operations request a listing of the books in a given category and by a given author, respectively.

private static final String OP_BYCATEGORY = "byCategory";
private static final String OP_BYAUTHOR = "byAuthor";

Their category and author parameters are passed using:

private static final String PARAM_CATEGORY = "category";
private static final String PARAM_AUTHOR = "author";

The addBookForm operation requests the display of the form for adding a book:

private static final String OP_ADDBOOKFORM = "addBookForm";

It will pass the parameters of the book using the category and author keys, as defined previously along with the isbn, title, and rating:

private static final String PARAM_ISBN = "isbn";
private static final String PARAM_TITLE = "title";
private static final String PARAM_RATING = "rating";

The parameters for adding a book are passed to the addBook operation as follows:

private static final String OP_ADDBOOK = "addBook";

And last, but not least, the authentication form is displayed with the loginForm operation and the authentication request using the login operation, along with the user and pass parameters.

private static final String OP_LOGINFORM = "loginForm";
private static final String OP_LOGIN = "login";
private static final String PARAM_USER = "user";
private static final String PARAM_PASS = "pass";

Those constants will be used in the code and embedded in the generated HTML.