I have a class representing a set of values that will be used as a key in maps.
This class is immutable and I want to make it a singleton for every distinct set of values, using the static factory pattern. The goal is to prevent identical objects from being created many (100+) times and to optimize the equals method.
I am looking for the best way to cache and reuse previous instances of this class. The first thing that pops to mind is a simple hashmap, but are there alternatives?
There are two situations:
enum
EnumMap
is optimized for itInteger
cache instances in a given range forvalueOf
Map
Depending on the usage pattern, you may choose to only cache, say, the last N instances, instead of all instances created so far. This is the approach used in e.g.
re.compile
in Python's regular expression module. If N is small enough (e.g. 5), then a simple array with a linear search may also work just fine.For
Map
based solution, perhaps a useful implementation isjava.util.LinkedHashMap
, which allows you to enforce LRU-like policies if you@Override
theremoveEldestEntry
.There is also
LRUMap
from Apache Commons Collections that implement this policy more directly.See also
enums
Related questions
MapMaker
, soft keys and values, etc