This looks nice. It's slightly strange you can't create a builder directly, but that'd just have you do Uri.create("myscheme:").toBuilder(), which seems pretty clear and guarantees there is a scheme.

Thanks! Actually the Builder() ctor is public. My intention is for users to write code like:

Uri uri =
        new Uri.Builder()
            .setScheme("http")
            .setHost("grpc.io")
            .setPath("/index.html")
            .build();

Maybe there should be a public static newBuilder() method instead to match Attributes and CreateSubchannelArgs and MethodDescriptor ... ?

Ah, I see. Yeah, it would match our other APIs more to have a newBuilder(). You should probably add a note to the API that creates a builder that scheme must be set before build().