What is the best way to cache and reuse immutable singleton objects in Java?

I have a class representing a set of values that will be used as a key in maps.

开发者_JAVA百科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:

• If the number of distinct objects is small and fixed, you should use an enum
  • They're not instantiatiable beyond the declared constants, and EnumMap is optimized for it
• Otherwise, you can cache immutable instances as you planned:
  • If the values are indexable by numbers in a contiguous range, then an array can be used
    • This is how e.g. Integer cache instances in a given range for valueOf
  • Otherwise you can use some sort of 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 is java.util.LinkedHashMap, which allows you to enforce LRU-like policies if you @Override the removeEldestEntry.

There is also LRUMap from Apache Commons Collections that implement this policy more directly.

See also

• Java Tutorials/enums
• Effective Java 2nd Edition, Item 1: Consider static factory methods instead of constructors
• Wikipedia/Flyweight pattern

Related questions

• Easy, simple to use LRU cache in java
• LRU LinkedHashMap that limits size based on available memory
  • Guava MapMaker, soft keys and values, etc
• How would you implement an LRU cache in Java 6?

What you're trying to make sounds like an example of the Flyweight Pattern, so looking for references to that might help clarify your thinking.

Storing them in a map of some sort is indeed a common implementation.

What do your objects look like? If your objects are fairly simple I think you should consider not caching them - object creation is usually quite fast. I think you should evaluate whether the possibly small performance boost is worth the added complexity and effort of a cache.