Question

In Effective Java, Joshua Bloch favors interfaces over abstract classes. However, he notes that a skeletal implementation should come with every interface.

I feel that a skeletal implementation is almost the same as an abstract class. How do these two concepts differ?

Answer 1

^{Edited after re-reading the aforementioned section in Effective Java}

According to this section of the book a skeletal implementation is an abstract class. He recommends this approach because, after a skeletal implementation is in place, it becomes trivial to implement the interface and selectively override methods, even with an anonymous class (as he does in his book).

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^{Previous answer, edited slightly for continuity}

A skeletal implementation could theoretically be a complete implementation and therefore concrete. Then it could be used with composition, because it can be instantiated. Whereas abstract classes require inheritance.