Looking at the sample code makes me wonder if Java does not have type inference like auto" or "var" in C#/C++? LINQ like code (or STL in C++) is much nicer if you don't have to explicitly declare variable types.
Yes Java 10 now has local variable type inference so you can now use "var", this repository looks like it was last touched 2 years ago so probably why the examples aren't using it.
Java 11 in September is going to be the first stable version supporting inference for locals, and IMHO that's when it starts to be reasonable to write code that doesn't run on today's LTS.