Effective Java

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ITEM 37: USE ENUMMAP INSTEAD OF ORDINAL INDEXING
175
the transitions by source phase, and the second creates an 
EnumMap
with mappings
from destination phase to transition. The merge function in the second collector
(
(x,
y)
->
y)
) is unused; it is required only because we need to specify a map fac-
tory in order to get an 
EnumMap
, and 
Collectors
provides telescoping factories.
The previous edition of this book used explicit iteration to initialize the phase
transition map. The code was more verbose but arguably easier to understand.
Now suppose you want to add a new phase to the system: 
plasma
, or ionized
gas. There are only two transitions associated with this phase: 
ionization
, which
takes a gas to a plasma; and 
deionization
, which takes a plasma to a gas. To update
the array-based program, you would have to add one new constant to 
Phase
and
two to 
Phase.Transition
, and replace the original nine-element array of arrays
with a new sixteen-element version. If you add too many or too few elements to
the array or place an element out of order, you are out of luck: the program will
compile, but it will fail at runtime. To update the 
EnumMap
-based version, all you
have to do is add 
PLASMA
to the list of phases, and 
IONIZE(GAS,
PLASMA)
and
DEIONIZE(PLASMA,
GAS)
to the list of phase transitions: