These steps are discussed in more detail in the following sections.

The enumeration type BookTypeEL is encoded as a Java enum, and we probide enumeration literals as well as assign label (human readable) to each enumeration literal:

public enum BookTypeEL {
  TEXTBOOK( "TextBook"), BIOGRAPHY( "Biography");

  // associated label for an enumeration literal
  private final String label;

  private BookTypeEL( String label) {
    this.label = label;
  }

  public String getLabel() {
    return this.label;
  }
}

We encode the model class Book in the form of a Java class and we use the appropriate JPA annotations to be used with the Book class hierarchy:

@Entity
@Inheritance( strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn( name = "category", discriminatorType = DiscriminatorType.STRING, length = 16)
@DiscriminatorValue( value = "BOOK")
@Table( name = "books")
...
public class Book {
  @Id
  @Column( length = 10)
  @NotNull( message = "An ISBN is required!")
  @Pattern( regexp = "\\b\\d{9}(\\d|X)\\b", message = "The ISBN must be a 10-digit string or a 9-digit string followed by 'X'!")
  private String isbn;
  @Column( nullable = false)
  @NotNull( message = "A title is required!")
  private String title;
  @Column( nullable = false)
  @NotNull( message = "A year is required!")
  @UpToNextYear
  @Min( value = 1459, message = "The year must not be before 1459!")
  private Integer year;

  // constructors, set, get and other methods
}

When using the JPA single table storage for a class hierarchy, the @Inheritance annotations must be set for the superclass (e.g, Book in our example). It provide the strategy parameter which for single table storage must have the value InheritanceType.SINGLE_TABLE. The @DiscriminatorColumn specifies the table column name, i.e., name = "category", which stores the value that makes the difference between different types of subclasses (e.g., Book, TextBook and Biography in our example). We expect to use BookTypeEL for the values of name parameter, but this is not possible because the name parameter requires a constant expression, while BookCategoryEL.TEXTBOOK.name() is not considered to be a constant (being a function cally), so a compiler exception is thrown. Last, the @DiscriminatorValue specifies an unique value to be stored in the discriminator table column used to identify the class type (e.g., for the Book we want to have BOOK as discriminator value, so we use value = "BOOK").

Further, we define the TextBook and Biography subtypes:

@Entity
@DiscriminatorValue( value = "TEXTBOOK")
...
public class TextBook extends Book {
  @NotNull( message = "A subject area value is required")
  String subjectArea;
  
  // constructors, set, get and other methods
}

The TextBook and Biography are subclasses of Book, so we define them by using extends Book. For each of the subclass, we add the @DiscriminatorValue JPA annotation with an unique value to identify the type.

We have to take care of handling the category discriminator and the subjectArea and about segment properties both in the "List all books" use case as well as in the "Create book" and "Update book" use cases by

We add a "Special type" column to the display table of the "List all books" use case in books.html:

<h:dataTable value="#{bookController.books}" var="b">
  ...
  <h:column>
    <f:facet name="header">Special type</f:facet>
    #{b.getClass().getSimpleName() != 'Book' ? b.getClass().getSimpleName() : ''}
  </h:column>
</h:dataTable>

A conditional expression (using the Java ? ternary operator) is used to check if the JavaBean class name is Book, on which case we don't show it, or it is something else (e.g. TextBook or Biography), and then it is shown. This expression also shows how you can call/use various methods from the JavaBeans, not only the methods custom defined by the programmer of the application.

In both use cases, we need to allow selecting a special category of book ('textbook' or 'biography') with the help of a select control, as shown in the following HTML fragment:

<h:panelGrid id="bookPanel" columns="3">
  ...
  <h:outputText value="Special type: " />
  <h:selectOneMenu id="bookType" value="#{viewScope.bookTypeVal}">
    <f:selectItem itemLabel="---"/>
    <f:selectItems value="#{book.typeItems}" />
    <f:ajax event="change" execute="@this" 
            render="textBookPanel biographyBookPanel standardBookPanel" />
  </h:selectOneMenu>
  <h:message for="bookType" errorClass="error" />
</h:panelGrid>

<h:panelGroup id="standardBookPanel">  
  <h:commandButton value="Create" rendered="#{viewScope.bookTypeVal == null}"
                   action="#{bookController.add( book.isbn, book.title, book.year)}" />
</h:panelGroup>
    
<h:panelGroup id="textBookPanel">  
  <h:panelGrid rendered="#{viewScope.bookTypeVal == 'TEXTBOOK'}" columns="3">
    <h:outputText value="Subject area: " />
    <h:inputText id="subjectArea" value="#{textBook.subjectArea}"/>
    <h:message for="subjectArea" errorClass="error" />
  </h:panelGrid>
  <h:commandButton value="Create" rendered="#{viewScope.bookTypeVal == 'TEXTBOOK'}"
                   action="#{textBookController.add( book.isbn, book.title, book.year, textBook.subjectArea)}" />
</h:panelGroup>

<h:panelGroup id="biographyBookPanel">  
  <h:panelGrid rendered="#{viewScope.bookTypeVal == 'BIOGRAPHY'}" columns="3">
    <h:outputText value="About: " />
    <h:inputText id="about" value="#{biographyBook.about}"/>
    <h:message for="about" errorClass="error" />
  </h:panelGrid>
  <h:commandButton value="Create" rendered="#{viewScope.bookTypeVal == 'BIOGRAPHY'}"
                   action="#{biographyBookController.add( book.isbn, book.title, book.year, biographyBook.about)}" />
</h:panelGroup>

There are a few important remarks on the above view code:

For a class in the class hierarchy, one corresponding controller class is defined. It contains the specific add, update and destroy methods (see . The shared methods, such as getAllObjects are defined by the controller of the top level class (e.g., for our example, this is BookController). See minimal app for more details on how to implement the controller class and the corresponding CRUD methods.

The Update Book test case is very similar with the Create Book test case. The Delete Book test case remains unchanged, see See minimal app for more details.