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But I don't think a "fully abstract class" is the same thing as an "interface", at least in Java? As far as I know, you can implement multiple "interfaces" but you can still only "extend" one abstract class, even if it is "fully abstract" in that it has no concrete member variables and all methods are abstract.

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In Java it isn't but this is specific to Java (and clones of Java like C#).

This is because Java has single-inheritance enforcement for classes.

C++ for example has multiple inheritance. So the way you do an interface is you just write an abstract class, then extend it to implement it.

In my experience, whether you use an abstract class or an interface class as your reference depends very much on the problem you are trying to solve - specifically, is there common code shared across all implementations?

A C++ pure abstract class is equivalent to a Java interface.

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