Hey @hsaputra - I'd think we should recommend "//" to users since it's the widely used way. Unfortunately URL naming is totally messed up and inconsistent not only in Hadoop but in the broader internet ecosystem. The recommended philosphy in the IETF is to be "liberal in what you accept, and conservative in what you send". I think we should be liberal in accepting weird URL's, but we should not advise users to make quirky URL's.

http://en.wikipedia.org/wiki/Robustness_principle

That said, this PR is mainly because java.net.URL (incorrectly) considers `hdfs` an invalid scheme. So we can't rely on java.net.URL to validate this.