Effective Java

by Joshua Bloch

To improve the performance, explicitly compile the regular expression into a Pattern instance (which is immutable) as part of class initialization, cache it, and reuse the same instance for every invocation of the isRomanNumeral method:

Beware the other edge of this sword: thread-safety (a likely factor when we're talking about "performance critical situations" that call for caching instances). A frequent example of this in practice is DateFormat, where (similar to this example) you want might instantiate an instance with a format pattern once, and re-use it (call `.format()`) many times. However, DateFormat uses internal state (that is, private instance fields whose values change) while formatting. This means that concurrent calls to `.format()` can mess each other up. In this case, you have a number of options: just take the cost of instantiating new objects each use; keep a thread-local copy; synchronize usage of the object, introduce a pool of instances that can be "checked out" for use... See another related note just below on "When an object is immutable, it is obvious it can be reused safely."