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Lecture 4 Review

Today we started our discussion of Java generics. I motivated generics by showing Friday’s source code, which used Object... p varargs, and where p[0] had to be an IListFactory and p[1] an IList. Confusing the indices or passing the wrong value would lead to a ClassCastException.

We then looked at how to make a Box<E>, a wrapper that could contain any kind of value, but you had to specify its type. Later we worked out on the board what a generic ILambda would look like, including an Equals example that compared two Integer values. Then we made another Equals<P> that could compare any two values of the same type, so we specified one of the type variables of ILambda<R,P>, but left the other one open.

The last thing I mentioned was that SpecialBox<Integer> was a subclass of Box<Integer> when SpecialBox<E> extends Box<E>, but I’ll have to revisit that again, because the important thing is that Box<Integer> is not a subtype of Box<Object> even though Integer is a subtype of Object.

We also ran into an interesting problem with boxing when we write Equals: p[0] and p[1] are Integers, so p[0]==p[1] does an identity comparison, not an integer equality test, so we had to do p[0].equals(p[1]), but that lead well to the exercise Equals<P>.

I also wasn’t sure about whether primitive types could be used in generics; I correctly said no, but I wasn’t sure, so I’ll reiterate that. I’m pretty happy with this lecture. We’ll see how well my students understood the concepts, because on Friday we’ll talk about upper and lower bounds and wildcards.

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