Understanding and Implementing Information Management Concepts and Techniques

Java EE Web App Tutorial Part 3: Dealing with Enumerations

Learn how to build a back-end web application with enumeration attributes, using Java EE with Java Server Faces (JSF) as the user interface technology, the Java Persistence API (JPA) for object-to-storage mapping, and a MySQL database

Gerd Wagner

Mircea Diaconescu

Warning: This tutorial may still contain errors and may still be incomplete in certain respects. Please report any issue to Gerd Wagner at G.Wagner@b-tu.de or Mircea Diaconescu at M.Diaconescu@b-tu.de.

This tutorial is also available in the following formats: PDF. See also the project page, or run the example app from our server, or download it as a ZIP archive file.

This tutorial article, along with any associated source code, is licensed under The Code Project Open License (CPOL), implying that the associated code is provided "as-is", can be modified to create derivative works, can be redistributed, and can be used in commercial applications, but the article must not be distributed or republished without the authors' consent.

2017-06-14

Revision History
Revision 0.320150724gw
improve entity class model, use both simple enums and code list enums
Revision 0.220150630gw
various revisions
Revision 0.120150529md
create first version

Table of Contents

Foreword
1. Enumeration Attributes
1. Enumerations
1.1. Simple enumerations
1.2. Code lists
1.3. Record enumerations
1.4. Extensible enumerations
2. Enumeration Attributes
3. Enumerations in Computational Languages
3.1. Enumerations in SQL
3.2. Enumerations in XML Schema
3.3. Enumerations in JavaScript
4. Dealing with Enumeration Attributes in a Design Model
2. Enumeration Attributes in Java EE
1. New Issues
2. Make a JPA Entity Class Model
3. Write the Model Code
3.1. Code the enumerations
3.2. Code the JPA entity class
3.3. Define a converter for serializing enumeration attribute values
3.4. Code the enumeration attribute setters
3.5. Write a serialization function
3.6. Database schema
3.7. Creating test data
4. Write the View Code
4.1. Selection lists
4.2. Radio button groups and checkbox groups
5. Displaying Value Sets for Multi-Valued Enumeration Attributes
6. Run the App and Get the Code
7. Practice Project
7.1. Adding ratings and genres as enumeration attributes

List of Figures

1.1. An example of an extensible enumeration
1.2. A single and a multiple select element with no selected option
1.3. A radio button group
1.4. A checkbox group
1.5. An information design model for the object type Book
2.1. A JPA entity class model for the object type Book
2.2. The user interface for creating a new book record

List of Tables

1.1. Representing an enumeration of records as a table
1.2. A class defining a record enumeration
1.3. Sample data for Book

Foreword

This tutorial is Part 3 of our series of six tutorials about model-based development of back-end web applications with Java, using JPA and JSF. It shows how to build a simple web app where model classes have enumeration attributes that need to be rendered in the user interface in the form of selection lists and other choice widgets.

A distributed web app is composed of at least two parts: a front-end part, which, at least, renders the user interface (UI) pages, and a back-end part, which, at least, takes care of persistent data storage. A back-end web app is a distributed web app where essentially all work is performed by the back-end component, including data validation and UI page creation, while the front-end only consists of a web browser's rendering of HTML-forms-based UI pages. Normally, a distributed web app can be accessed by multiple users, possibly at the same time, over HTTP connections.

In the case of a Java/JPA/JSF back-end app, the back-end part of the app can be executed by a server machine that runs a web server supporting the Java EE specifications Java Servlets, Java Expression Language (EL), JPA and JSF, such as the open source server Tomcat/TomEE.

The app supports the four standard data management operations (Create/Read/Update/Delete). The other parts of the tutorial are:

  • Part 1: Building a minimal app.

  • Part 2: Handling constraint validation.

  • Part 4: Managing unidirectional associations, such as the associations between books and publishers, assigning a publisher to a book, and between books and authors, assigning authors to a book.

  • Part 5: Managing bidirectional associations, such as the associations between books and publishers and between books and authors, also assigning books to authors and to publishers.

  • Part 6: Handling subtype (inheritance) relationships between object types.

Chapter 1. Enumerations and Enumeration Attributes

1. Enumerations

In all application domains, there are string-valued attributes with a fixed list of possible string values. These attributes are called enumeration attributes, and the fixed value lists defining their possible string values are called enumerations. For instance, when we have to manage data about people, we often need to include information about their gender. The possible values of a gender attribute may be restricted to one of the enumeration labels "male","female" and "undetermined", or to one of the enumeration codes "M", "F" and "U". Whenever we deal with codes, we also need to have their corresponding labels, at least in a legend explaining the meaning of each code.

Instead of using the enumeration string values as the internal values of an enumeration attribute, it is preferable to use a simplified internal representation for them, such as the positive integers 1, 2, 3, etc., which enumerate the possible values. However, since these integers do not reveal their meaning (which is indicated by the enumeration label) in program code, for readability we rather use special constants, called enumeration literals, such as MALE or M, prefixed by the name of the enumeration like in this.gender = GenderEL.MALE. Notice that we follow the convention that the names of enumeration literals are written all upper case, and that we also use the convention to suffix the name of an enumeration datatype with "EL" standing for "enumeration literal" (such that we can recognize from the name GenderEL that each instance of this datatype is a "gender enumeration literal").

There are also enumerations having records as their instances, such that one of the record fields provides the name of the enumeration literals. An example of such an enumeration is the following list of units of measurement:

Table 1.1. Representing an enumeration of records as a table

Units of Measurement
Unit Symbol Unit Name Dimension
m meter length
kg kilogram mass
g gram mass
s second time
ms milisecond time


Notice that since both the "Unit Symbol" and the "Unit Name" fields are unique, either of them could be used for the name of the enumeration literals.

In summary, we can distinguish between the following three forms of enumerations:

  1. simple enumerations define a list of self-explanatory enumeration labels;

  2. code lists define a list of code/label pairs.

  3. record enumerations consist of a list of records, so they are defined like classes with simple attributes defining the record fields.

These three forms of enumerations are discussed in more detail below.

Notice that, since enumerations are used as the range of enumeration attributes, they are considered to be datatypes.

Enumerations may have further features. For instance, we may want to be able to define a new enumeration by extending an existing enumeration. In programming languages and in other computational languages, enumerations are implemented with different features in different ways. See also the Wikipedia article on enumerations.

1.1. Simple enumerations

A simple enumeration defines a fixed list of self-explanatory enumeration labels, like in the example of a GenderEL enumeration shown in the following UML class diagram:

Since the labels of a simple enumeration are being used, in capitalized form, as the names of the corresponding enumeration literals (GenderEL.MALE, GenderEL.FEMALE, etc.), we may also list the (all upper case) enumeration literals in the UML enumeration datatype, instead of the corresponding (lower or mixed case) enumeration labels.

1.2. Code lists

A code list is an enumeration that defines a fixed list of code/label pairs. Unfortunately, the UML concept of an enumeration datatype does not support the distinction between codes as enumeration literals and their labels. For defining both codes and labels in a UML class diagram in the form of an enumeration datatype, we may use the attribute compartment of the data type rectangle and use the codes as attribute names defining the enumeration literals, and set their initial values to the corresponding label. This approach results in a visual representation as in the following diagram:

In the case of a code list, we can use both the codes or the labels as the names of enumeration literals, but using the codes seems preferable for brevity (GenderEL.M, GenderEL.F, etc.). For displaying the value of an enumeration attribute, it's an option to show not only the label, but also the code, like "male (M)", provided that there is sufficient space. If space is an issue, only the code can be shown.

1.3. Record enumerations

A record enumeration defines a record type with a unique field designated to provide the enumeration literals, and a fixed list of records of that type. In general, a record type is defined by a set of field definitions (in the form of primitive datatype attributes), such that one of the unique fields is defined to be the enumeration literal field, and a set of operation definitions.

Unfortunately, record enumerations, as the most general form of an enumeration datatype, are not supported by the current version of UML (2.5) where the general form of an enumeration is defined as a special kind of datatype (with optional field and operation definitions) having an additional list of unique strings as enumeration literals (shown in a fourth compartment). The UML definition does neither allow designating one of the unique fields as the enumeration literal field, nor does it allow populating an enumeration with records.

Consequently, for showing a record enumeration in a UML class diagram, we need to find a workaround. For instance, if our modeling tools allows adding a drawing, we could draw a rectangle with four compartments, such that the first three of them correspond to the name, properties and operations compartments of a datatype rectangle, and the fourth one is a table with the names of properties/fields defined in the second compartment as column headers, as shown in the table below.

Table 1.2. A class defining a record enumeration

UnitEL

«el» unitSymbol: String

unitName: String

dimension: String

Unit Symbol Unit Name Dimension
m meter length
kg kilogram mass
g gram mass
s second time
ms millisecond time

1.4. Extensible enumerations

Figure 1.1. An example of an extensible enumeration

An example of an extensible enumeration

There may be cases of enumerations that need to be extensible, that is, it must be possible to extend their list of enumeration values (labels or code/label pairs) by adding a new one. This can be expressed in a class diagram by appending an ellipsis to the list of enumeration values, as shown in Figure 1.1.

Since enumeration values are internally represented by enumeration literals, which are normally stored as plain positive integers in a database, a new enumeration value can only be added at the end of the value list such that it can be assigned a new index integer without re-assigning the indexes of other enumeration values. Otherwise, the mapping of enumeration indexes to corresponding enumeration values would not be preserved.

Alternatively, if new enumeration values have to be inserted in-between other enumeration values, and their indexes re-assigned, this implies that

  1. enumeration indexes are plain sequence numbers and do no longer identify an enumeration value;

  2. the value of an enumeration literal can no longer be an enumeration index, but rather has to be an identifying string: preferably the enumeration code in the case of a code list, or the enumeration label, otherwise.

.

2. Enumeration Attributes

An enumeration attribute is an attribute that has an enumeration as its range.

In the user interface, an output field for an enumeration attribute would display the enumeration label, rather than its internal value, the corresponding enumeration index.

Figure 1.2. A single and a multiple select element with no selected option

A single and a multiple select element with no selected option

For allowing user input to an enumeration attribute, we can use the UI concept of a (drop-down) selection list, which may be implemented with an HTML select element, such that the enumeration labels would be used as the text content of its option elements, while the enumeration indexes would be used as their values. We have to distinguish between single-valued and multi-valued enumeration attributes. In the case of a single-valued enumeration attribute, we use a standard select element. In the case of a multi-valued enumeration attribute, we use a select element with the HTML attribute setting multiple="multiple".

In the case of using a single select element for an optional enumeration attribute, we need to include in its options an element like "---" for indicating that nothing has been selected. Then, the UI page for the CRUD use case "Create" shows "---" as the initially selected option.

For both cases, an example is shown in Figure 1.2. While the single select element for "Original language" shows the initially selected option "---" denoting "nothing selected", the multiple select element "Other available languages" shows a small window displaying four of the options that can be selected.

For usability, the multiple selection list can only be implemented with an HTML select element, if the number of enumeration literals does not exceed a certain threshold (like 20), which depends on the number of options the user can see on the screen without scrolling.

Figure 1.3. A radio button group

A radio button group

For user input for a single-valued enumeration attribute, a radio button group can be used instead of a single selection list, if the number of enumeration literals is sufficiently small (say, not larger than 7). A radio button group is implemented with an HTML fieldset element acting as a container of labeled input elements of type "radio", all having the same name, which is normally equal to the name of the represented enumeration attribute.

Figure 1.4. A checkbox group

A checkbox group

For user input for a multi-valued enumeration attribute, a checkbox group can be used instead of a multiple selection list, if the number of enumeration literals is sufficiently small (say, not larger than 7). A checkbox group is implemented with an HTML fieldset element acting as a container of labeled input elements of type "checkbox", all having the same name, which is normally equal to the name of the represented enumeration attribute.

3. Enumerations in Computational Languages

Defining enumerations is directly supported in information modeling languages (such as in UML Class Diagrams), in data schema languages (such as in XML Schema, but not in SQL), and in many programming languages (such as in C++ and Java, but not in JavaScript).

3.1. Enumerations in SQL

Unfortunately, standard SQL does not support enumerations. Some DBMS, such as MySQL and Postgres, provide their own extensions of SQL column definitions in the CREATE TABLE statement allowing to define enumeration-valued columns.

A MySQL enumeration is specified as a list of enumeration labels with the keyword ENUM within a column definition, like so:

CREATE TABLE people (
    name VARCHAR(40),
    gender ENUM('MALE', 'FEMALE', 'UNDETERMINED')
);

A Postgres enumeration is specified as a special user-defined type that can be used in columns definitions:

CREATE TYPE GenderEL AS ENUM ('MALE', 'FEMALE', 'UNDETERMINED');
CREATE TABLE people (
   name text,
   gender GenderEL
)

3.2. Enumerations in XML Schema

In XML Schema, an enumeration datatype can be defined as a simple type restricting the primitive type xs:string in the following way:

<xs:simpleType name="BookCategoryEL">
  <xs:restriction base="xs:string">
    <xs:enumeration value="NOVEL"/>
    <xs:enumeration value="BIOGRAPHY"/>
    <xs:enumeration value="TEXTBOOK"/>
    <xs:enumeration value="OTHER"/>
  </xs:restriction>
</xs:simpleType>

3.3. Enumerations in JavaScript

In JavaScript, we can define an enumeration as a special JS object having a property for each enumeration literal such that the property's name is the enumeration literal's name (the enumeration label or code in upper case) and its value is the corresponding enumeration index. One approach for implementing this is using the Object.defineProperties method:

var BookCategoryEL = null;
Object.defineProperties( BookCategoryEL, {
  NOVEL: {value: 1, writable: false},
  BIOGRAPHY: {value: 2, writable: false},
  TEXTBOOK: {value: 3, writable: false},
  OTHER: {value: 4, writable: false},
  MAX: {value: 4, writable: false},
  labels: {value:["novel","biography","textbook","other"], 
           writable: false}
});

This definition allows using the enumeration literals BookCategoryEL.NOVEL, BookCategoryEL.BIOGRAPHY etc., standing for the enumeration indexes 1, 2 , 3 and 4, in program statements. Notice how this definition takes care of the requirement that enumeration literals like BookCategoryEL.NOVEL are constants, the value of which cannot be changed during program execution. This is achieved with the help of the property descriptor writable: false in the Object.defineProperties statement.

We can also use a more generic approach and define a meta-class Enumeration for creating enumerations in the form of special JS objects:

function Enumeration( enumLabels) {
  var i=0, LBL="";
  this.MAX = enumLabels.length;
  this.labels = enumLabels;
  // generate the enum literals as capitalized keys/properties
  for (i=1; i <= enumLabels.length; i++) {
    LBL = enumLabels[i-1].toUpperCase();
    this[LBL] = i;
  }
  // prevent any runtime change to the enumeration
  Object.freeze( this);
};

Using this Enumeration class allows to define a new enumeration in the following way:

var BookCategoryEL = new Enumeration(["novel","biography","textbook","other"])

Having an enumeration like BookCategoryEL, we can then check if an enumeration attribute like category has an admissible value by testing if its value is not smaller than 1 and not greater than BookCategoryEL.MAX. Also, the label can be retrieved in the following way:

formEl.category.value = BookCategoryEL.labels[this.category - 1];

As an example, we consider the following model class Book with the enumeration attribute category:

function Book( slots) {
  this.isbn = "";     // string
  this.title = "";    // string
  this.category = 0;  // number (BookCategoryEL)
  if (arguments.length > 0) {
    this.setIsbn( slots.isbn); 
    this.setTitle( slots.title); 
    this.setCategory( slots.category);
  }
};

For validating input values for the enumeration attribute category, we can use the following check function:

Book.checkCategory = function (c) {
  if (!c) {
    return new MandatoryValueConstraintViolation(
        "A category must be provided!");
  } else if (!Number.isInteger(c) || c < 1 || 
             c > BookCategoryEL.MAX) {
    return new RangeConstraintViolation(
        "The category must be a positive integer " +
        "not greater than "+ BookCategoryEL.MAX +" !");
  } else {
    return new NoConstraintViolation();
  }
};

Notice how the range constraint defined by the enumeration BookCategoryEL is checked: it is tested if the input value c is a positive integer and if it is not greater than BookCategoryEL.MAX.

4. Dealing with Enumeration Attributes in a Design Model

We again consider the simple data management problem that we have considered before. So, again, the purpose of our app is to manage information about books. But now we have four additional enumeration attributes, as shown in the UML class diagram in Figure 1.5 below:

  1. the single-valued mandatory attribute originalLanguage with the enumeration datatype LanguageEL as its range,

  2. the multi-valued optional attribute otherAvailableLanguages with range LanguageEL,

  3. the single-valued mandatory attribute category with range BookCategoryEL

  4. the multi-valued mandatory attribute publicationForms with range PublicationFormEL

Figure 1.5. An information design model for the object type Book

An information design model for the object type Book

Notice that the attributes otherAvailableLanguages and publicationForms are multivalued, as indicated by their multiplicity expressions [*] and [1..*]. This means that the possible values of these attributes are sets of enumeration literals, such as the set {ePub, PDF}, which can be represented in JavaScript as a corresponding array list of enumeration literals, [PublicationFormEL.EPUB, PublicationFormEL.PDF].

The meaning of the design model and its enumeration attributes can be illustrated by a sample data population:

Table 1.3. Sample data for Book

ISBN Title Original language Other languages Category Publication forms
0553345842 The Mind's I English (en) de, es, fr novel paperback, ePub, PDF
1463794762 The Critique of Pure Reason German (de) de, es, fr, pt, ru other paperback, PDF
1928565379 The Critique of Practical Reason German (de) de, es, fr, pt, ru other paperback
0465030793 I Am A Strange Loop English (en) es textbook hardcover, ePub

Chapter 2. Implementing Enumeration Attributes in a Java EE Web App

In this chapter, we show how to build a back-end web application with enumeration attributes, using Java with JPA and JSF. In addition to the topic of enumeration attributes, we also show how to deal with multi-valued attributes.

1. New Issues

Compared to the Validation App discussed in Part 2 we have to deal with the following new issues:

  1. Enumeration datatypes have to be defined in a suitable way as part of the model code.

  2. Enumeration attributes have to be defined in model classes and handled in the user interface with the help of suitable choice widgets.

In terms of coding, the new issues are:

  1. In the model code we have to take care of

    1. enumerations to be defined in the form of Java enum classes;

    2. single-valued enumeration attributes, like Book::originalLanguage, requiring specific JPA annotations;

    3. multi-valued enumeration attributes, like Book::publicationForms, requiring specific JPA annotations and JPA attribute converters for serializing their collection values in the form of suitable strings;

    4. extending the methods Book.create, and Book.update such that they take care of the enumeration attributes.

  2. In the user interface code we have to take care of

    1. adding new table columns in retrieveAndListAll.xhtml;

    2. adding suitable choice widgets in create.xhtml and upate.xhtml;

    3. rendering multi-valued enumeration attributes in the table view of retrieveAndListAll.xhtml and in the choice widgets of create.xhtml and upate.xhtml (with the help of an array of JSF SelectItems, each consisting of an enumeration value and its label).

2. Make a JPA Entity Class Model

Using the information design model shown in Figure 1.5 above as the starting point, we make a JPA entity class model, essentially by adding JPA annotations, setters, and getters as explained before:

Figure 2.1. A JPA entity class model for the object type Book

A JPA entity class model for the object type Book

3. Write the Model Code

3.1. Code the enumerations

Simple enumerations, like BookCategoryEL and PublicationFormEL, are coded in the following way with the help of Java's enum construct:

public enum BookCategoryEL {
  NOVEL, 
  BIOGRAPHY, 
  TEXTBOOK, 
  OTHER;
}

Notice how the enumeration literals are defined with capitalized names in a comma-separated list.

For a code list, like LanguageEL, where each enumeration instance consists of a code and a label, we use the codes as enumeration literals and define an attribute label for storing the labels, as well as a private constructor that allows creating enumeration instances consisting of a code and a label. This is possible because in Java, an enum is a special kind of class. Each entry in the list of enumeration literals, auch as EN( "English")), represents an invocation of the enum's constructor:

public enum LanguageEL {
  EN( "English"),
  DE( "German"),
  FR( "French"),
  ES( "Spanish");

  private final String label;
  private LanguageEL( String label) {this.label = label;}
  public String getLabel() {return this.label;}
}

3.2. Code the JPA entity class

The entity class Book is coded with the help of JPA annotations for class attributes, as well as the setter and getter methods of every class attribute.

@Entity @Table( name="books")
@ViewScoped @ManagedBean( name="book")
public class Book {
  @Id @Column( length=10)
  @NotNull( message="An ISBN value is required!")
  @Pattern( regexp="\\b\\d{9}(\\d|X)\\b", message = "...")
  private String isbn;
  
  @Column( nullable=false)
  @NotNull( message="A title is required!")
  private String title;

  @Column( nullable=false)
  @NotNull( message="An original language is required!")
  @Enumerated( EnumType.STRING)
  private LanguageEL originalLanguage;

  @Column( nullable=false)
  @NotNull( message="A category is required!")
  @Enumerated( EnumType.STRING)
  private BookCategoryEL category;

  @Convert( converter = 
      pl.m.converter.OtherAvailableLanguagesConverter.class)
  private Set<LanguageEL> otherAvailableLanguages;
 
  @Column( nullable=false)
  @Size( min=1, message="At least one publication form is required!")
  @Convert( converter=pl.m.converter.PublicationFormsConverter.class)
  private Set<PublicationFormEL> publicationForms;

  ...
}

For the case of single-valued enumeration attributes (like originalLanguage and category), the JPA annotation @Enumerated is used for specifying the storage serialization. It takes one of the following two parameter values:

  • EnumType.STRING means that enumeration literals are converted to strings when they are serialized ( e.g., in the case of the originalLanguage attribute of the Book class, the values saved in the database are one of "EN", "DE", "FR" or "ES") ;

  • EnumType.ORDINAL means that enumeration literals are converted to their index integer when they are serialized (i.e.,the values saved in the database are 1, 2 ,3 etc.).

We store the collection values of a multi-valued enumeration attribute (like otherAvailableLanguages and publicationForms) in a database table column as serialized arrays in the form of ["value1", "value2", ...]. In the case of the originalLanguage attribute, an example of a saved value would be ["EN", "FR", "DE"]. Achieving this behavior is possible with JPA by using custom converters to map database table column values to Java types and vice versa. The @Convert annotation allows specifying a Java converter that is responsible for the mappings.

Also, in the case of a multi-valued enumeration attribute, we use the @Size annotation to specify the minimum, and if required also the maximum, number of elements stored by this attribute. For example, in the case of publicationForms, we use @Size( min=1, message = "At least one publication form is required!"), which enforces to have at least one value for this attribute, otherwise the error message is displayed.

@Entity @Table( name="books")
@ViewScoped @ManagedBean( name="book")
public class Book {
  ...
  public Book( String isbn, String title, Integer year, 
      LanguageEL originalLanguage, 
      Set<LanguageEL> otherAvailableLanguages, 
      BookCategoryEL category, 
      Set<PublicationFormEL> publicationForms) { ...}
  public LanguageEL getOriginalLanguage() {...}
  public void setOriginalLanguage( 
      LanguageEL originalLanguage) {...}  
  public Set<LanguageEL> getOtherAvailableLanguages() {...}
  public void setOtherAvailableLanguages( 
      Set<LanguageEL> otherAvailableLanguages) {...}
  public BookCategoryEL getCategory() {...}
  public void setCategory( BookCategoryEL category) {...}  
  public Set<PublicationFormEL> getPublicationForms() {...}
  public void setPublicationForms( 
      Set<PublicationFormEL> publicationForms) {...}

  public static Book retrieve(...) {...}
  public static List<Book> retrieveAll(...) {...}
  public static void create(...) throws Exception {...}
  public static void update(...) throws Exception {...}
  public static void delete(...) throws Exception {...}

  public String getOtherAvailableLanguagesValues() {...}
  public SelectItem[] getLanguageItems() {...}
  public SelectItem[] getCategoryItems() {...}
  public String getPublicationFormsValues() {...}
  public SelectItem[] getPublicationFormsItems() {...}
}

The Book constructor is extended with the new parameters corresponding to the new enumeration attributes. Notice the new methods in the last code block, like getPublicationFormsValues and getPublicationFormsItems, which are used for handling the enumeration attributes in the UI. We discuss each of them in the following subsections.

3.3. Define a converter for serializing enumeration attribute values

A JPA attribute converter is a special Java class that implements the AttributeConverter interface with the methods convertToDatabaseColumn and convertToEntityAttribute. For the case of otherAvailableLanguages attribute, we define the following converter class:

@Converter
public class OtherAvailableLanguagesConverter implements 
    AttributeConverter< Set<LanguageEL>, String> {
  @Override
  public String convertToDatabaseColumn( Set<LanguageEL> attrValue) {
    JSONArray jsonArr = new JSONArray( attrValue);
    return jsonArr.toString();
  }
  @Override
  public Set<LanguageEL> convertToEntityAttribute( String colValue) {
    Set<LanguageEL> result = new HashSet<LanguageEL>();
    try {
      JSONArray jsonArr = new JSONArray( colValue);
        for ( int i = 0, length = jsonArr.length(); i < length; i++) {
          result.add( LanguageEL.valueOf( jsonArr.getString( i)));
        }
    } catch ( JSONException e) {
      e.printStackTrace();
    }
    return result;
  }
}

An attribute converter class needs to be annotated with @Converter. In our example, the convertToDatabaseColumn method is responsible to convert the entity attribute value (e.g., otherAvailableLanguages) to a JSON array which is stored in the database as a String.

The convertToEntityAttribute method is responsible for de-serializing a table column's value to the corresponding the entity attribute value. In our example, this means to map the JSON array string to a Java Set of enumeration literals of type LanguageEL.

The code above shows the custom attribute converter class for the otherAvailableLanguages attribute. The attribute converter class for the publicationForms attribute is defined in the same way.

3.4. Code the enumeration attribute setters

Both for single-valued and for multi-valued enumeration attributes an ordinary setter is defined. In the case of a multi-valued enumeration attribute, this setter assigns an entire set of values (in the form of a Java Set) to the attribute.

3.5. Write a serialization function

The object serialization function now needs to include the values of enumeration attributes:

public String toString() {
  int i = 0, n = this.publicationForms.size();
  String result = "{ isbn: '" + this.isbn + "', title:'" + this.title
    + "', year: " + this.year + ", originalLanguage: '" 
    + this.originalLanguage.getLabel();
  result += "', otherAvailableLanguages: [";
  for (LanguageEL oal : this.otherAvailableLanguages) {
    result += "'" + oal.getLabel() + "'";
    if (i < n-1) result += ", ";
    i++;
  }
  result += "]";
  result += ", category: '" + this.category.name().toLowerCase();
  result += "', publicationForms: [";
  i = 0;
  for (PublicationFormEL pf : this.publicationForms) {
    result += "'" + pf.name().toLowerCase() + "'";
    if (i < n-1) result += ", ";
    i++;
  }
  result += "]}";
  return result;
}

3.6. Database schema

As we discussed in Part 1, the database schema can be automatically generated by a JPA-enabled server like TomEE. The generated schema for our Book entity class is like so:

CREATE TABLE IF NOT EXISTS `books` (
  `ISBN` varchar(10) NOT NULL,
  `CATEGORY` varchar(255) NOT NULL,
  `ORIGINALLANGUAGE` varchar(255) NOT NULL,
  `OTHERAVAILABLELANGUAGES` varchar(255) DEFAULT NULL,
  `PUBLICATIONFORMS` varchar(255) NOT NULL,
  `TITLE` varchar(255) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

For every attribute (like category), a column with the same name but using upper case is created, (e.g., CATEGORY). This is the default naming, which is fine for our example application, but it can be changed, if required, by using the @Column( name="...") annotation, as in @Column( name="book_category").

The single-valued and multi-valued enumeration attributes are by default created as varchar columns with the default maximum length of 255 characters. However, if a longer (or shorter) length is desirable, this can be enforced by using the @Column( length=...) annotation.

3.7. Creating test data

In the test data records that are created by Book.createTestData, we now have to provide values for single- and multi-valued enumeration attributes:

public static void createTestData( EntityManager em, UserTransaction ut)
    throws NotSupportedException, SystemException, IllegalStateException,
    SecurityException, HeuristicMixedException, RollbackException
    HeuristicRollbackException {
  Book book = null;
  // first clear existing books
  Book.clearData( em, ut);
  ut.begin();
  Book book = new Book( "006251587X", "Weaving the Web", 2000, 
      LanguageEL.EN,
      new HashSet<LanguageEL>() {
        { add( LanguageEL.DE); add( LanguageEL.FR);}
      }, 
      BookCategoryEL.TEXTBOOK, 
      new HashSet<PublicationFormEL>() {
        { add( PublicationFormEL.HARDCOVER);
          add( PublicationFormEL.PDF);}
      }
  );
  em.persist( book);
}

4. Write the View Code

The example app's user interface for creating a new book record with ISBN, title and four enumeration attributes looks as in Figure 2.2 below.

Figure 2.2. The user interface for creating a new book record

The user interface for creating a new book record

4.1. Selection lists

We use JSF selection lists for rendering the enumeration attributes originalLanguage and otherAvailableLanguages in the code of the facelet files WebContent/views/books/create.xhtml and .../update.xhtml:

<ui:composition template="/WEB-INF/templates/page.xhtml"> 
 ...
 <ui:define name="main">
  <h:form id="createBookForm">
   ...
   <div>
    <h:outputLabel for="originalLanguage" value="Original language">
     <h:selectOneMenu id="originalLanguage"
        value="#{book.originalLanguage}">
      <f:selectItem itemValue="" itemLabel="---" />
      <f:selectItems value="#{book.languageItems}" />
     </h:selectOneMenu>
    </h:outputLabel>
    <h:message for="originalLanguage" errorClass="error" />
   </div>
   <div class="multi-sel">
    <h:outputLabel for="otherAvailableLanguages" 
       value="Also available in">
     <h:selectManyListbox id="otherAvailableLanguages"
        value="#{book.otherAvailableLanguages}">
      <f:selectItems value="#{book.languageItems}" />
     </h:selectManyListbox>
    </h:outputLabel>
    <h:message for="otherAvailableLanguages" errorClass="error" />
   </div>
   ...
   <div>
    <h:commandButton value="Save"
       action="#{bookCtrl.create( book.isbn, book.title, book.year, 
           book.originalLanguage, book.otherAvailableLanguages, 
           book.category, book.publicationForms)}" />
   </div>
  </h:form>
 </ui:define>
</ui:composition>

The JSF element h:selectOneMenu allows creating single selection lists with the HTML select element. The list is populated with language options due to its child element <f:selectItems value="#{book.languageItems}"/>. Using the expression #{book.languageItems} results in calling the method getLanguageItems() on the book object. This method returns a set of SelectItem objects, which are used to populate the selection list. The corresponding method code is as follows:

public SelectItem[] getLanguageItems() {
  SelectItem[] items = new SelectItem[LanguageEL.values().length];
  int i = 0;
  for (LanguageEL lang : LanguageEL.values()) {
    items[i++] = new SelectItem( lang.name(), lang.getLabel());
  }
  return items;
} 

A multiple selection list, corresponding to an HTML element <select multiple="multiple" .../>, is created with the JSF element h:selectManyListbox using the same getLanguageItems method for obtaining the selection list items.

4.2. Radio button groups and checkbox groups

Since the enumeration attributes category and publicationForms have not more than seven possible values, we can use a radio button group and a checkbox group for rendering them:

<ui:composition template="/WEB-INF/templates/page.xhtml">
  ...
  <ui:define name="main">
    <h:form id="createBookForm">
      <div>
        <h:outputLabel for="category" value="Category">
          <h:selectOneRadio id="category" value="#{book.category}">
            <f:selectItems value="#{book.categoryItems}" />
          </h:selectOneRadio>
        </h:outputLabel>
        <h:message for="category" errorClass="error" />
      </div>
      <div>
        <h:outputLabel for="publicationForms" 
          value="Publication forms ">
          <h:selectManyCheckbox id="publicationForms"
            value="#{book.publicationForms}">
            <f:selectItems value="#{book.publicationFormsItems}" />
          </h:selectManyCheckbox>
        </h:outputLabel>
        <h:message for="publicationForms" errorClass="error" />
      </div>
      <div>
        <h:commandButton value="Save"
          action="#{bookCtrl.create( book.isbn, book.title, book.year, 
            book.originalLanguage, book.otherAvailableLanguages, 
            book.category, book.publicationForms)}" />
      </div>
    </h:form>
  </ui:define>
</ui:composition>

The radio button group is obtained by using the JSF element h:selectOneRadio. It renders a set of <input type="radio" ... /> elements. Using the same technique as for selection lists, the radio button group is populated with a set of SelectItem objects. The corresponding getCategoryItems method from the Book class is similar to getLanguageItems.

The checkbox group, consisting of <input type="checkbox" ... /> elements, is created with the JSF element h:selectManyCheckbox and populated in the same way as a radio button group or a selection list.

5. Displaying Value Sets for Multi-Valued Enumeration Attributes

In the case of a multi-valued enumeration attribute like otherAvailableLanguages or publicationForms, the Retrieve/List All view must show a value in the form of a comma-separated list, like "English, German, Spanish", as shown in the following table:

ISBN Title Year Orig. lang. Other avail. lang. Category Publication forms
006251587X Weaving the Web 2000 English German, French textbook pdf, hardcover
0465026567 Gödel, Escher, Bach 1999 French other paperback, epub
0465030793 I Am A Strange Loop 2008 Spanish English, German textbook pdf, epub

For this purpose, we define a method that creates the desired serialization of a multi-valued attribute and use it in the code of the facelet file retrieveAndListAll.xhtml from WebContent/views/books/. For the publicationForms attribute, the method code is as follows:

public String getPublicationFormsValues() {
  String result = "";
  if (this.publicationForms != null) {
    int i=0, n = this.publicationForms.size();
    for (PublicationFormEL pf : this.publicationForms) {
      result += pf.name().toLowerCase();
      if (i < n-1) result += ", ";
      i++;
    }
  }
  return result;
}

Notice that in the case of the publicationForms attribute, the underlying enumeration PublicationFormEL does not have a label property. Instead, we use the enumeration literal name in lowercase as the label.

6. Run the App and Get the Code

You can run the enumeration app on our server or download the code as a ZIP archive file.

Follow our instructions to get your environment prepared for running Java EE web applications.

7. Practice Project

If you have any questions about how to carry out the following projects, you can ask them on our discussion forum.

7.1. Adding ratings and genres as enumeration attributes

The purpose of the app to be built is managing information about movies. The app deals with just one object type, Movie, and with two enumerations, as depicted in the following class diagram.

First make a list of all the constraints that have been expressed in this model. Then code the app by following the guidance of this tutorial and the Validation Tutorial.

Compared to the validation app practice project, two attributes have been added: the optional single-valued enumeration attribute rating, and the multi-valued enumeration attribute genres.