Java streams .orElseThrow

Question

I want convert a piece of code from a Connection Pool project i have been working on to use streams

the original code is

for (Map.Entry<JdbConnection,Instant> entry : borrowed.entrySet()) {
  Instant leaseTime = entry.getValue();
  JdbConnection jdbConnection = entry.getKey();
  Duration timeElapsed = Duration.between(leaseTime, Instant.now());
  if (timeElapsed.toMillis() > leaseTimeInMillis) {
    //expired, let's close it and remove it from the map
    jdbConnection.close();
    borrowed.remove(jdbConnection);

    //create a new one, mark it as borrowed and give it to the client
    JdbConnection newJdbConnection = factory.create();
    borrowed.put(newJdbConnection,Instant.now());
    return newJdbConnection;
  }
}

throw new ConnectionPoolException("No connections available");

I have got to the point of this

borrowed.entrySet().stream()
                   .filter(entry -> Duration.between(entry.getValue(), Instant.now()).toMillis() > leaseTimeInMillis)
                   .findFirst()
                   .ifPresent(entry -> {
                     entry.getKey().close();
                     borrowed.remove(entry.getKey());
                   });


JdbConnection newJdbConnection = factory.create();
borrowed.put(newJdbConnection,Instant.now());
return newJdbConnection;

The above can compile but the moment i add orElseThrow after IfPresent i am getting the following

/home/prakashs/connection_pool/src/main/java/com/spakai/ConnectionPool.java:83: error: void cannot be dereferenced
                       .orElseThrow(ConnectionPoolException::new);

Answer 1

That's because ifPresent returns void. It can't be chained. You could do something like:

Entry<JdbConnection, Instant> entry =
    borrowed.entrySet().stream()
        .filter(entry -> Duration.between(entry.getValue(), Instant.now())
                            .toMillis() > leaseTimeInMillis)
        .findFirst()
        .orElseThrow(ConnectionPoolException::new));
entry.getKey().close();
borrowed.remove(entry.getKey());

What you were looking for would read well:

.findFirst().ifPresent(value -> use(value)).orElseThrow(Exception::new);

But for it to work, ifPresent would have to return the Optional, which would be a little odd. It would mean you could chain one ifPresent after another, doing multiple operations on the value. That might have been a good design, but it isn't the one the creators of Optional went with.

Answer 2

Use map instead of isPresent, and return with an Optional instead of an exception.

borrowed.entrySet().stream()
               .filter(entry -> Duration.between(entry.getValue(), Instant.now()).toMillis() > leaseTimeInMillis)
               .findFirst()
               .map(entry -> {
                 entry.getKey().close();
                 borrowed.remove(entry.getKey());
                 JdbConnection newJdbConnection = factory.create();
                 borrowed.put(newJdbConnection,Instant.now());
                 return newJdbConnection;
               })