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Handly is a relatively new project and Java 8 is coming into mainstream. We can't afford to finalize Handly core APIs without giving careful consideration to potential impact of the new Java 8 features such as default interface methods and streams.
It is important to note that streams don't mix well with checked exceptions. See for example http://stackoverflow.com/questions/19757300/java-8-lambda-streams-filter-by-method-with-exception Since many Eclipse Platform APIs throw CoreException, this might become quite an issue for stream-oriented processing. This general problem calls for a general solution. I don't (yet) think we can and should address it separately in Handly.
Default interface methods would help with evolution of interfaces that clients may implement and could provide convenient "mini-implementations" that don't lock clients into a particular class hierarchy (see bug 474023 for an example of where they would be really useful in Handly).
Just a note that streams also don't mix especially well with iterables: http://stackoverflow.com/questions/23114015/why-does-iterablet-not-provide-stream-and-parallelstream-methods
A broad perspective on API design with Java 8: https://jarthorn.github.io/EclipseCon2014/java8/
To summarize, I think that default methods in Java 8 could be really useful for Handly. Streams look promising, but unfortunately don't mix well with checked exceptions such as the CoreException omnipresent in Eclipse. We need to adopt Java 8 as soon as it becomes feasible (bug 474391).
