Add groupByEachUniqueKey to RichIterable

[donraab]

RichIterable currently has groupBy / groupByEach but only has groupByUniqueKey without the corresponding groupByEachUniqueKey. The method groupByEachUniqueKey would index an item in a Map using multiple keys. Each key will only point to one value. An example of this might be mapping a Person to his/her aliases.

I ran into this problem recently trying to group an Object that contains a distinct range of numbers by each of the distinct numbers. I was able to accomplish this by using a nested injectInto, but it is somewhat awkward and may be difficult to comprehend. In my particular case, it is more optimal using injectInto as I am grouping into an IntObjectMap.

[pranukrish]

Hi @donraab I'd like to work on this issue. Could you please assign it to me. Thanks!

[donraab]

Hi @pranukrish, done!

[pranukrish]

Hi @donraab! So while trying to implement the functionality I realized that it has to be implemented in all the classes that inherit the RichIterable interface(and that's around a 100 classes) and I wanted to know if there is any work around to implementing this in all of these classes.

[donraab]

Hi @pranukrish, yes adding a new method to RichIterable can be very complicated and involve a lot of changes. There is an approach which can split the total amount of work into multiple pieces. If you look at adding groupByEachUniqueKey with a target map similar to groupByUniqueKey, it might be possible to add the implementation as a default method on RichIterable. There will still need to be overrides in a few places, but the return type is not covariant so will always return R extends MutableMapIterable<V, T>.

I think the signature of this version of the method with a target map would look as follows:

    <V, R extends MutableMapIterable<V, T>> R groupByEachUniqueKey(
            Function<? super T, ? extends Iterable<V>> function,
            R target);

Let me know if this makes sense or needs any clarification. Thanks!

[motlin]

The other thing which can make this kind of change easier is searching for similar methods and putting the new method in the same classes. For example, putting this right below each groupByUniqueKey implementation.

[pranukrish]

Thanks for the inputs @donraab and @motlin. I think I'll try a default implementation in RichIterable and see how that's working. And yes, I have implemented it with the same signature you mentioned.
@motlin Yes, I searched for the groupByUniqueKey implementation and that's how I found implementations of this method in around 100 classes. Will try implementing it in RichIterable but if it doesn't work out I will try implementing it in all the classes!