If you were programming in Python or many other languages you could download documentation and have a local copy.

In both Python and Java doing a mini-project I frequently challenge myself to only use the standard library. It's good for practicing HackerRank-rank style programming (the fun of single-file Java)

When I am waiting for builds I sometimes hack on this

https://github.com/paulhoule/pidove

because I don't really like the Streams API and want to perfect my mastery of generics and internal DSLs.

Now that I think of it, standard-library only for node seems like a good challenge for me because I code a lot of front-end Javascript but just barely know the node API.

I've also hacked on things that are data-science and A.I. related on the bus where I've very deliberately set up a Jupyter environment with everything I need before the trip.