How to use generic type declarations in java in explained case?

Question

I am having hard time understanding the best way to use generic type parameters.

Consider a priority queue implementation that implements an interface and requires all elements to be comparable of its type. The class has a constructor of with input type collection which provides initial elements. Here are a few alternatives. Can you explain differences? And provide the most general way of such declaration?

public class HeapMinPQ<E extends Comparable<E>> implements MinPQ<E> {
     ...
     public HeapMinPQ( Collection<? extends Comparable<E>> source) {
     ...
     }
}


public class HeapMinPQ<E extends Comparable<E>> implements MinPQ<E> {
     ...
     public HeapMinPQ( Collection<? extends E> source) {
     ...
     }
}


public class HeapMinPQ<E extends Comparable<? super E>> implements MinPQ<E> {
     ...
     public HeapMinPQ( Collection<? extends Comparable<E>> source) {
     ...
     }
}

I am not offended. It is not a homework. I am studying for an interview. My attempt to implement heap based priority queue. — user2259824
– user2259824, Commented Jun 28, 2016 at 0:08
relevant: stackoverflow.com/questions/2723397/… & the reason why the most generic type is E extends Comparable<? super E>. — zapl
– zapl, Commented Jun 28, 2016 at 0:45

Lew Bloch · Accepted Answer · 2016-06-28 00:47:46Z

1

Your constructors should not use Collection<? extends E> but Collection<E> as the type of the parameter. Also, your examples are incomplete; they don't show how the arguments are intended to be used.

In the first case, source contains elements guaranteed to be of type capture ? of Comparable<E>. The base type might or might not be compatible with E. So it wouldn't necessarily be safe to extract an element from source and use it against an E instance.

In the second, source contains elements guaranteed to be of type capture ? of E, which means the base type is compatible with E. It would be usable against an instance of E as long as the particular subtype doesn't matter.

In the third, the class time is weird. It says that E is comparable to its supertypes, which is not generally true for comparable types. It is likely to cause you heartache, and also there's no guarantee that the capture ? of Comparable<E> base type of source will work to compare to E or be otherwise compatible with E.

Generally you don't want to mess with captures of E in a type generic over E, but the full details are complex. I suggest Angelika Langer's excellent site, http://www.angelikalanger.com/, particularly http://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html, to obtain more knowledge, as well as the articles on Developerworks and in the Oracle Tutorials.

answered Jun 28, 2016 at 0:47

Lew Bloch

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1 Comment

user2259824 Over a year ago

One more detail please; does the second case guarantee that all collection elements are comparable with each other? Since I declared E to be that way in class declaration.

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