16. Writing a custom template processor

The built-in STR and FMT can return only String instances and cannot throw exceptions. However, both of them are actually instances of the functional interface StringTemplate.Processor<R,E extends Throwable>, which defines the process() method:

R process(StringTemplate stringTemplate) throws E 

By implementing the Processor<R,E extends Throwable> interface, we can write custom template processors that return R (any result type), not just String. Moreover, if something goes wrong during processing (for instance, validation issues are present), we can throw checked exceptions (E extends Throwable).

For instance, let’s assume that we need to interpolate strings with expressions representing phone numbers. So we accept only the expressions that are phone numbers, matching the following regular expression:

private static final Pattern PHONE_PATTERN = Pattern.compile( 
  "\\d{10}|(?:\\d{3}-){2}\\d{4}|\\(\\d...