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Iterables is a utility class (it contains only static method and cannot be instantiated).

I guess that your com.google.common.collect.Iterables$4 is just some anonymous class implementing java.lang.Iterable.

To sum up, defining components as Iterable should work for you.

EDIT: To answer your follow-up questions:

1) You wrote that it does not look like Iterables implements Iterable, and you're right - it doesn't. In order to understand what com.google.common.collect.Iterables$4 means you need to understand the compilation naming rules of anonymous classes.

In short, com.google.common.collect.Iterables$4 means "4th anonymous class nested in com.google.common.collect.Iterables class".

2) As to how you find out the type without guessing - you simply track the API and what it returns:

• Repository.facet returns a Facet of given type (here: StorageFacet)
• StorageFacet.txSupplier returns Supplier
• StorageTx.browseComponents returns Iterable

Note that it's all interfaces above, so we still don't see from this how the returned Iterable is implemented.

To find out about this, we need to see into the implementation: StorageTxImpl. This, however, delegates the call further and I don't feel like tracing it any further (you can do it on your own if you wish; it'd much be easier to do if one opened this project in an IDE, though).

However, I know that what happens there is that a call to one of the methods in Guava's Iterables utility class is made, which in turn returns Iterable implemented by means of an anonymous class, and that's all.