Effective Java

---

ITEM 38: EMULATE EXTENSIBLE ENUMS WITH INTERFACES
179
The resulting code is a bit less complex, and the 
test
method is a bit more flexi-
ble: it allows the caller to combine operations from multiple implementation
types. On the other hand, you forgo the ability to use 
EnumSet
(Item 36) and
EnumMap
(Item 37) on the specified operations.
Both programs shown previously will produce this output when run with com-
mand line arguments 
4
and 
2
:
4.000000 ^ 2.000000 = 16.000000
4.000000 % 2.000000 = 0.000000
A minor disadvantage of the use of interfaces to emulate extensible enums is
that implementations cannot be inherited from one enum type to another. If the
implementation code does not rely on any state, it can be placed in the interface,
using default implementations (Item 20). In the case of our 
Operation
example,
the logic to store and retrieve the symbol associated with an operation must be
duplicated in 
BasicOperation
and 
ExtendedOperation
. In this case it doesn’t
matter because very little code is duplicated. If there were a larger amount of
shared functionality, you could encapsulate it in a helper class or a static helper
method to eliminate the code duplication.
The pattern described in this item is used in the Java libraries. For example,
the 
java.nio.file.LinkOption
enum type implements the 
CopyOption
and
OpenOption
interfaces. 
In summary,