private static final class PrintingEventListener extends EventListener {
    private static final Factory FACTORY = new Factory() {
      final AtomicLong nextCallId = new AtomicLong(1L);

      @Override public EventListener create(Call call) {
        long callId = nextCallId.getAndIncrement();
        System.out.printf("%04d %s%n", callId, call.request().url());
        return new PrintingEventListener(callId, System.nanoTime());
      }
    };