[0] http://movingfulcrum.tumblr.com/post/97624791473/critique-of...
There's a tradeoff space here, as that tweet indicates. Doing things generically makes the library less idiomatic, but means developers don't need to churn their library dependency as often. Go's a different language from Java, and it's less common to have to deal with casting, etc. As that tweet conversation indicates (particularly https://twitter.com/JimDanz/status/511732603884294145), we're also considering changing the Java library implementation in the future.
I'm currently doing a project that takes json, with no predefined structure, or at least very varying struct and transforms it. Dealing with checking for keys and doing type conversion it's that much fun. The same project need to parse an XML file, with a clear and defined structure, it took something like five minutes and work on my first try.
I don't think you would need to update the Go library in the case of just adding new parameters, not if the parameter can be excluded. The json parser should just skip that value, it won't be available to you of cause, but it should break the parser. Given that I haven't touched Java in 10 years I don't know how Java deals with unexpected parameters.