Java Reference
In-Depth Information
•
The constructor cannot explicitly invoke a superclass constructor.
The chaining to the super constructor is handled automatically by
the compiler.
•
An enum constructor cannot use a non-constant static field of the
enum.
This last restriction requires a little explanation. Because each enum
constant is a static field of the enum type, the constructors are executed
during static initialization of the enum class. The enum constant declar-
ations must be the first declarations in the type, so the constructors for
these values will always be the first code executed during static initial-
ization. Any other static fields will be initialized afterward. So if a con-
structor were to refer to a static (non-constant) field of the enum, it
would see the default uninitialized value. This would nearly always be
an error, and so it is simply disallowed.
The work-around is quite straight-forward: Declare a static initialization
block that does what the constructor would like to have done. This block
can refer to any enum constant or can iterate through all of them be-
cause all will have been constructed before the block can execute.
Exercise 6.4
: Expand your traffic light color enum from Exercise 6.1 on
can be retrieved with
getColor
.
6.3.2. Constant Specific Behavior
Many enums define simple, passive types whose sole purpose is to
provide the named enum constantsan enum like
Suit
is a good example.
Occasionally, enum constants will have state associated with them that
will be set by a constructor and accessed with a method on the enum.
For example, consider an enum that defined the nine planets of our solar
system, and that allowed the mass of each planet to be set and queried.
In some circumstances, the enum may represent an entity with inherent
behavior that varies between the different enum constantsthis is known
as
constant-specific behavior.
