Helper for implementing a *pure* partial function: it will possibly be invoked multiple times for a single “application”, because its only abstract method is used for both isDefinedAt() and apply(); the former is mapped to isCheck == true and the latter to isCheck == false for those cases where this is important to know.

Failure to match is signaled by throwing noMatch(), i.e. not returning normally (the exception used in this case is pre-allocated, hence not that expensive).

new JavaPartialFunction<Object, String>() {
  public String apply(Object in, boolean isCheck) {
    if (in instanceof TheThing) {
      if (isCheck) return null; // to spare the expensive or side-effecting code
      return doSomethingWithTheThing((TheThing) in);
    } else {
      throw noMatch();
    }
  }
}

The typical use of partial functions from Apache Pekko looks like the following:

if (pf.isDefinedAt(x)) {
  pf.apply(x);
}

i.e. it will first call JavaPartialFunction.apply(x, true) and if that does not throw noMatch() it will continue with calling JavaPartialFunction.apply(x, false).