#Conclusion

#Notes

#Bonus Comparator is also interesting because it works around the issue of implementing equals() correctly. Better yet, it follows a pattern for fixing this type of inheritance issue: the Strategy design pattern. The Typeclasses that Haskell and Scala people get excited about are also the Strategy pattern. Inheritance isn't bad or wrong, it's just tricky. For further reading, check out Philip Wadler's paper How to make ad-hoc polymorphism less ad hoc

Conclusion

Notes

Bonus

Comparator is also interesting because it works around the issue of implementing equals() correctly. Better yet, it follows a pattern for fixing this type of inheritance issue: the Strategy design pattern. The Typeclasses that Haskell and Scala people get excited about are also the Strategy pattern. Inheritance isn't bad or wrong, it's just tricky. For further reading, check out Philip Wadler's paper How to make ad-hoc polymorphism less ad hoc

Now let's sub-class it to be a 3D point:

#Notes Josh Bloch's "Effective Java" Item 8 was the source of this example, but Bloch uses a ColorPoint which adds a color instead of a third axis and uses doubles instead of ints. Bloch's Java example is basically duplicated by Odersky/Spoon/Venners who made their example available online.

Now let's sub-class it to be a 3D point.

#Notes

1. Josh Bloch's "Effective Java" Item 8 was the source of this example, but Bloch uses a ColorPoint which adds a color instead of a third axis and uses doubles instead of ints. Bloch's Java example is basically duplicated by Odersky/Spoon/Venners who made their example available online.

2. Several people have objected to this example because if you let the parent class know about the sub-class, you can fix this issue. That's true if there are a small enough number of sub-classes and if the parent knows about them all. But the original question was about making an API which someone else will write sub-classes for. In that case, you generally can't update the parent implementation to be compatible with the sub-classes.

There are two morals of this story.

A great example of a well-spelled out contract is Comparator. Just ignore what it says about .equals() for the reasons described above.

#Conclusion

A great example of a well-spelled out contract is Comparator. Just ignore what it says about .equals() for the reasons described above. Here's an example of how Comparator can do things .equals() can't.