Change `Iterable<T>` to `List<T>` return types for Repository and Template API methods

[mp911de]

We should expose the most concrete return type on our repository and Template API implementation

[schauder]

Do you want the return types of JdbcAggregateOperations changed as well?

• Obviously it would be a breaking change. Although, I expect few will be affected.
• Otherwise we have to introduce a type check and a potential conversion for all the repository methods.

[mp911de]

Yes, as Iterable on its own is hard to use and in almost all cases, we return some List implementation.

In minor versions we can require calling code to be recompiled.

[mipo256]

Obviously it would be a breaking change. Although, I expect few will be affected.

@schauder I do not think it is going to be breaking, though. It is not a problem if the callee that expected Iterable gets List

[schauder]

JdbcAggregateOperations is an interface. Any implementation that actually returns an Iterable would break.

[mp911de]

While it is possible to follow that line of argument, we also could argue that each change to our interface would prevent us from introducing new methods.

Spring's …Operations interfaces are designated to be implemented by Spring projects. Anyone else implementing such an interface with real code is, with every release, at risk of suffering from catching up. Implementing by inheritance (subclassing …Template that implements …Operations) is the recommended approach. Alternatively, using JDK interface proxies.