Classes and Objects: A Deeper Look - Java: How to Program Early Objects - page 335

8.9 enum Types
In Fig. 6.8, 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 (as you'll see
momentarily). Each enum declaration declares an enum class with the following restrictions:
1. enum constants are implicitly final .
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.

Declaring Instance Variables, a Constructor and Methods in an enum Type
Figure 8.10 demonstrates 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 constants. Each is op-
tionally followed by arguments that 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 pa-
rameters 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 con-
taining two String arguments. 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).
Lines 16-17 declare the instance variables title and copyrightYear . Each enum con-
stant in enum type Book is actually an object of enum 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 topic's title and one that specifies its copyright
year. Lines 22-23 assign these parameters to the instance variables. Lines 27-36 declare
methods, which return the topic title and copyright year, respectively.

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// Fig. 8.10: Book.java

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// Declaring an enum type with a constructor and explicit instance fields

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// and accessors for these fields

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public enum Book

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{

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// declare constants of enum type
JHTP( "Java How to Program" , "2015" ),
CHTP( "C How to Program" , "2013" ),
IW3HTP( "Internet & World Wide Web How to Program" , "2012" ),

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9

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Fig. 8.10 | Declaring an enum type with a constructor and explicit instance fields and accessors
for these fields. (Part 1 of 2.)