F.8. Enumerations

In Fig. D.5, we introduced the basic enum type, which defines a set of constants represented as unique identifiers. In that program the enum constants represented the game’s status. In this section we discuss the relationship between enum types and classes. Like classes, all enum types are reference types. An enum type is declared with an enum declaration, which is a comma-separated list of enum constants—the declaration may optionally include other components of traditional classes, such as constructors, fields and methods. Each enum declaration declares an enum class with the following restrictions:

1. enum constants are implicitly final, because they declare constants that shouldn’t be modified.

2. enum constants are implicitly static.

3. Any attempt to create an object of an enum type with operator new results in a compilation error.

The enum constants can be used anywhere constants can be used, such as in the case labels of switch statements and to control enhanced for statements.

Figure F.10 illustrates how to declare instance variables, a constructor and methods in an enum type. The enum declaration (lines 5–37) contains two parts—the enum constants and the other members of the enum type. The first part (lines 8–13) declares six enum constants. Each is optionally followed by arguments which are passed to the enum constructor (lines 20–24). Like the constructors you’ve seen in classes, an enum constructor can specify any number of parameters and can be overloaded. In this example, the enum constructor requires two String parameters. To properly initialize each enum constant, we follow it with parentheses containing two String arguments, which are passed to the enum’s constructor. The second part (lines 16–36) declares the other members of the enum type—two instance variables (lines 16–17), a constructor (lines 20–24) and two methods (lines 27–30 and 33–36).

Click here to view code image

 1   // Fig. F.10: Book.java
 2   // Declaring an enum type with constructor and explicit instance fields
 3   // and accessors for these fields
 4
 5   public enum Book
 6   {
 7      // declare constants of enum type                                  
 8      JHTP( "Java How to Program", "2012" ),                          
 9      CHTP( "C How to Program", "2007" ),                             
10      IW3HTP( "Internet & World Wide Web How to Program", "2008" ),
11      CPPHTP( "C++ How to Program", "2012" ),                         
12      VBHTP( "Visual Basic 2010 How to Program", "2011" ),          
13      CSHARPHTP( "Visual C# 2010 How to Program", "2011" );          
14
15      // instance fields
16      private final String title; // book title
17      private final String copyrightYear; // copyright year
18
19      // enum constructor
20      Book( String bookTitle, String year )
21      {
22         title = bookTitle;
23         copyrightYear = year;
24      } // end enum Book constructor
25
26      // accessor for field title
27      public String getTitle()
28      {
29         return title;
30      } // end method getTitle
31
32      // accessor for field copyrightYear
33      public String getCopyrightYear()
34      {
35         return copyrightYear;
36      } // end method getCopyrightYear
37   } // end enum Book

Fig. F.10 | Declaring an enum type with constructor and explicit instance fields and accessors for these fields.

Lines 16–17 declare the instance variables title and copyrightYear. Each enum constant in Book is actually an object of type Book that has its own copy of instance variables title and copyrightYear. The constructor (lines 20–24) takes two String parameters, one that specifies the book’s title and one that specifies its copyright year. Lines 22–23 assign these parameters to the instance variables. Lines 27–36 declare two methods, which return the book title and copyright year, respectively.

Figure F.11 tests the enum type Book and illustrates how to iterate through a range of enum constants. For every enum, the compiler generates the static method values (called in line 12) that returns an array of the enum’s constants in the order they were declared. Lines 12–14 use the enhanced for statement to display all the constants declared in the enum Book. Line 14 invokes the enum Book’s getTitle and getCopyrightYear methods to get the title and copyright year associated with the constant. When an enum constant is converted to a String (e.g., book in line 13), the constant’s identifier is used as the String representation (e.g., JHTP for the first enum constant).

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 1   // Fig. F.11: EnumTest.java
 2   // Testing enum type Book.
 3   import java.util.EnumSet;
 4
 5   public class EnumTest
 6   {
 7      public static void main( String[] args )
 8      {
 9         System.out.println( "All books: " );
10
11         // print all books in enum Book
12         for ( Book book : Book.values() )
13            System.out.printf( "%-10s%-45s%s ", book,
14                book.getTitle(), book.getCopyrightYear() );
15
16         System.out.println( " Display a range of enum constants: " );
17
18         // print first four books
19         for ( Book book : EnumSet.range( Book.JHTP, Book.CPPHTP ) )
20            System.out.printf( "%-10s%-45s%s ", book,
21                book.getTitle(), book.getCopyrightYear() );
22      } // end main
23   } // end class EnumTest

Fig. F.11 | Testing enum type Book.

Lines 19–21 use the static method range of class EnumSet (declared in package java.util) to display a range of the enum Book’s constants. Method range takes two parameters—the first and the last enum constants in the range—and returns an EnumSet that contains all the constants between these two constants, inclusive. For example, the expression EnumSet.range( Book.JHTP, Book.CPPHTP ) returns an EnumSet containing Book.JHTP, Book.CHTP, Book.IW3HTP and Book.CPPHTP. The enhanced for statement can be used with an EnumSet just as it can with an array, so lines 12–14 use it to display the title and copyright year of every book in the EnumSet. Class EnumSet provides several other static methods for creating sets of enum constants from the same enum type.