Why implement two interfaces that already extend each other?

Question

I am trying to understand something about ArrayObject and ArrayIterator classes

ArrayObject: This class implements the IteratorAggregate and Traversable interfaces. Since IteratorAggregate extends Traversable itself, why does ArrayObject implement both when it can just implement IteratorAggregate?

ArrayIterator: As above, this class implements both Iterator and Traversable which Iterator extends.

Why do these classes implement two interfaces when they could get away with implementing the one that extends Traversable?

I suspect those links are only there to make the manual clearer and more *traversable* (ha) - ironically in your case that seems to have had the total opposite effect. I think you are correct in that one would more correctly say that they implement the `IteratorAggregate` and `Iterator` interfaces respectively. — DaveRandom, Jul 06 '12 at 16:17
...although [it would seem](http://codepad.viper-7.com/r4FfNY) that you can write your code in this seemingly conflicting way and PHP will not complain about it. — DaveRandom, Jul 06 '12 at 16:24
I don't think Java would complain either but haven't tested it. It's like declaring unchecked exceptions: superfluous but not an error. — David Harkness, Jul 06 '12 at 16:26

score 1 · Accepted Answer · answered Jul 06 '12 at 16:27

1

According to the SPL source code, ArrayObject doesn't implement Traversable directly:

class ArrayObject implements IteratorAggregate, ArrayAccess, Countable
{
    ...
}

This matches the SPL documentation. I believe DaveRandom is correct.

answered Jul 06 '12 at 16:27

David Harkness

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I looked at the link and it looks like ArrayObject and ArrayIterator Classes both is not implements Traversable Interface directly, – Aviel Fedida Jul 06 '12 at 16:36

Why implement two interfaces that already extend each other?

1 Answers1