From the JDOM site:

Jason (Hunter) founded the JDOM project in early 2000 along with Brett McLaughlin

That is, around the time Java 1.3 was being introduced. There was nothing in the way of maturity when it came to Java development--the most experienced developers only had 4 years using Java at best. There were no broadly used IoC containers, no Hibernate. Apache Struts was just getting its start.

Which is all to say that Jason and Brett were just wrong. A lot of people didn't "get it" yet. If they had C++ backgrounds, well, you didn't need interfaces in C++, why the heck use them in Java? Well, there's a lot of good reasons, which others can direct you to.

They were definitely wrong about

anything that can be done with interfaces can be done with subclassing

Java does not allow multiple inheritance of classes, but it does allow the implementation of multiple interfaces. That can make a real difference.

From the JDOM site:

Jason (Hunter) founded the JDOM project in early 2000 along with Brett McLaughlin

That is, around the time Java 1.3 was being introduced. There was nothing in the way of maturity when it came to Java development--the most experienced developers only had 4 years using Java at best. There were no broadly used IoC containers, no Hibernate. Apache Struts was just getting its start.

Which is all to say that Jason and Brett were just wrong. A lot of people didn't "get it" yet. If they had C++ backgrounds, well, you didn't need interfaces in C++ (well, they were there in C++, but you didn't really need them, right?), why the heck use them in Java? Well, there's a lot of good reasons, which others can direct you to.

They were definitely wrong about

anything that can be done with interfaces can be done with subclassing

Java does not allow multiple inheritance of classes, but it does allow the implementation of multiple interfaces. That can make a real difference.