At the moment, I am using PayPal to implement my payment process. However, I'm not really happy with the so outdated API that PayPal provides for Java. I have googled for a while and I saw a number of alternatives in several articles such as:
I'd be very grateful if you could give me some opinions on the pros and cons of any of these systems. Besides, based on your experience, please tell me which ones you think has good API for Java developers.